| \chapter{Built-in Types \label{types}} |
| |
| The following sections describe the standard types that are built into |
| the interpreter. |
| \note{Historically (until release 2.2), Python's built-in types have |
| differed from user-defined types because it was not possible to use |
| the built-in types as the basis for object-oriented inheritance. |
| This limitation does not exist any longer.} |
| |
| The principal built-in types are numerics, sequences, mappings, files, |
| classes, instances and exceptions. |
| \indexii{built-in}{types} |
| |
| Some operations are supported by several object types; in particular, |
| practically all objects can be compared, tested for truth value, |
| and converted to a string (with |
| the \function{repr()} function or the slightly different |
| \function{str()} function). The latter |
| function is implicitly used when an object is written by the |
| \keyword{print}\stindex{print} statement. |
| (Information on the \ulink{\keyword{print} statement}{../ref/print.html} |
| and other language statements can be found in the |
| \citetitle[../ref/ref.html]{Python Reference Manual} and the |
| \citetitle[../tut/tut.html]{Python Tutorial}.) |
| |
| |
| \section{Truth Value Testing\label{truth}} |
| |
| Any object can be tested for truth value, for use in an \keyword{if} or |
| \keyword{while} condition or as operand of the Boolean operations below. |
| The following values are considered false: |
| \stindex{if} |
| \stindex{while} |
| \indexii{truth}{value} |
| \indexii{Boolean}{operations} |
| \index{false} |
| |
| \begin{itemize} |
| |
| \item \code{None} |
| \withsubitem{(Built-in object)}{\ttindex{None}} |
| |
| \item \code{False} |
| \withsubitem{(Built-in object)}{\ttindex{False}} |
| |
| \item zero of any numeric type, for example, \code{0}, \code{0L}, |
| \code{0.0}, \code{0j}. |
| |
| \item any empty sequence, for example, \code{''}, \code{()}, \code{[]}. |
| |
| \item any empty mapping, for example, \code{\{\}}. |
| |
| \item instances of user-defined classes, if the class defines a |
| \method{__bool__()} or \method{__len__()} method, when that |
| method returns the integer zero or \class{bool} value |
| \code{False}.\footnote{Additional |
| information on these special methods may be found in the |
| \citetitle[../ref/ref.html]{Python Reference Manual}.} |
| |
| \end{itemize} |
| |
| All other values are considered true --- so objects of many types are |
| always true. |
| \index{true} |
| |
| Operations and built-in functions that have a Boolean result always |
| return \code{0} or \code{False} for false and \code{1} or \code{True} |
| for true, unless otherwise stated. (Important exception: the Boolean |
| operations \samp{or}\opindex{or} and \samp{and}\opindex{and} always |
| return one of their operands.) |
| \index{False} |
| \index{True} |
| |
| \section{Boolean Operations --- |
| \keyword{and}, \keyword{or}, \keyword{not} |
| \label{boolean}} |
| |
| These are the Boolean operations, ordered by ascending priority: |
| \indexii{Boolean}{operations} |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{\var{x} or \var{y}} |
| {if \var{x} is false, then \var{y}, else \var{x}}{(1)} |
| \lineiii{\var{x} and \var{y}} |
| {if \var{x} is false, then \var{x}, else \var{y}}{(1)} |
| \hline |
| \lineiii{not \var{x}} |
| {if \var{x} is false, then \code{True}, else \code{False}}{(2)} |
| \end{tableiii} |
| \opindex{and} |
| \opindex{or} |
| \opindex{not} |
| |
| \noindent |
| Notes: |
| |
| \begin{description} |
| |
| \item[(1)] |
| These only evaluate their second argument if needed for their outcome. |
| |
| \item[(2)] |
| \samp{not} has a lower priority than non-Boolean operators, so |
| \code{not \var{a} == \var{b}} is interpreted as \code{not (\var{a} == |
| \var{b})}, and \code{\var{a} == not \var{b}} is a syntax error. |
| |
| \end{description} |
| |
| |
| \section{Comparisons \label{comparisons}} |
| |
| Comparison operations are supported by all objects. They all have the |
| same priority (which is higher than that of the Boolean operations). |
| Comparisons can be chained arbitrarily; for example, \code{\var{x} < |
| \var{y} <= \var{z}} is equivalent to \code{\var{x} < \var{y} and |
| \var{y} <= \var{z}}, except that \var{y} is evaluated only once (but |
| in both cases \var{z} is not evaluated at all when \code{\var{x} < |
| \var{y}} is found to be false). |
| \indexii{chaining}{comparisons} |
| |
| This table summarizes the comparison operations: |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Meaning}{Notes} |
| \lineiii{<}{strictly less than}{} |
| \lineiii{<=}{less than or equal}{} |
| \lineiii{>}{strictly greater than}{} |
| \lineiii{>=}{greater than or equal}{} |
| \lineiii{==}{equal}{} |
| \lineiii{!=}{not equal}{} |
| \lineiii{is}{object identity}{} |
| \lineiii{is not}{negated object identity}{} |
| \end{tableiii} |
| \indexii{operator}{comparison} |
| \opindex{==} % XXX *All* others have funny characters < ! > |
| \opindex{is} |
| \opindex{is not} |
| |
| Objects of different types, except different numeric types and different string types, never |
| compare equal; such objects are ordered consistently but arbitrarily |
| (so that sorting a heterogeneous array yields a consistent result). |
| Furthermore, some types (for example, file objects) support only a |
| degenerate notion of comparison where any two objects of that type are |
| unequal. Again, such objects are ordered arbitrarily but |
| consistently. The \code{<}, \code{<=}, \code{>} and \code{>=} |
| operators will raise a \exception{TypeError} exception when any operand |
| is a complex number. |
| \indexii{object}{numeric} |
| \indexii{objects}{comparing} |
| |
| Instances of a class normally compare as non-equal unless the class |
| \withsubitem{(instance method)}{\ttindex{__cmp__()}} |
| defines the \method{__cmp__()} method. Refer to the |
| \citetitle[../ref/customization.html]{Python Reference Manual} for |
| information on the use of this method to effect object comparisons. |
| |
| \strong{Implementation note:} Objects of different types except |
| numbers are ordered by their type names; objects of the same types |
| that don't support proper comparison are ordered by their address. |
| |
| Two more operations with the same syntactic priority, |
| \samp{in}\opindex{in} and \samp{not in}\opindex{not in}, are supported |
| only by sequence types (below). |
| |
| |
| \section{Numeric Types --- |
| \class{int}, \class{float}, \class{long}, \class{complex} |
| \label{typesnumeric}} |
| |
| There are four distinct numeric types: \dfn{plain integers}, |
| \dfn{long integers}, |
| \dfn{floating point numbers}, and \dfn{complex numbers}. |
| In addition, Booleans are a subtype of plain integers. |
| Plain integers (also just called \dfn{integers}) |
| are implemented using \ctype{long} in C, which gives them at least 32 |
| bits of precision (\code{sys.maxint} is always set to the maximum |
| plain integer value for the current platform, the minimum value is |
| \code{-sys.maxint - 1}). Long integers have unlimited precision. |
| Floating point numbers are implemented using \ctype{double} in C. |
| All bets on their precision are off unless you happen to know the |
| machine you are working with. |
| \obindex{numeric} |
| \obindex{Boolean} |
| \obindex{integer} |
| \obindex{long integer} |
| \obindex{floating point} |
| \obindex{complex number} |
| \indexii{C}{language} |
| |
| Complex numbers have a real and imaginary part, which are each |
| implemented using \ctype{double} in C. To extract these parts from |
| a complex number \var{z}, use \code{\var{z}.real} and \code{\var{z}.imag}. |
| |
| Numbers are created by numeric literals or as the result of built-in |
| functions and operators. Unadorned integer literals (including hex |
| and octal numbers) yield plain integers unless the value they denote |
| is too large to be represented as a plain integer, in which case |
| they yield a long integer. Integer literals with an |
| \character{L} or \character{l} suffix yield long integers |
| (\character{L} is preferred because \samp{1l} looks too much like |
| eleven!). Numeric literals containing a decimal point or an exponent |
| sign yield floating point numbers. Appending \character{j} or |
| \character{J} to a numeric literal yields a complex number with a |
| zero real part. A complex numeric literal is the sum of a real and |
| an imaginary part. |
| \indexii{numeric}{literals} |
| \indexii{integer}{literals} |
| \indexiii{long}{integer}{literals} |
| \indexii{floating point}{literals} |
| \indexii{complex number}{literals} |
| \indexii{hexadecimal}{literals} |
| \indexii{octal}{literals} |
| |
| Python fully supports mixed arithmetic: when a binary arithmetic |
| operator has operands of different numeric types, the operand with the |
| ``narrower'' type is widened to that of the other, where plain |
| integer is narrower than long integer is narrower than floating point is |
| narrower than complex. |
| Comparisons between numbers of mixed type use the same rule.\footnote{ |
| As a consequence, the list \code{[1, 2]} is considered equal |
| to \code{[1.0, 2.0]}, and similarly for tuples. |
| } The constructors \function{int()}, \function{long()}, \function{float()}, |
| and \function{complex()} can be used |
| to produce numbers of a specific type. |
| \index{arithmetic} |
| \bifuncindex{int} |
| \bifuncindex{long} |
| \bifuncindex{float} |
| \bifuncindex{complex} |
| |
| All numeric types (except complex) support the following operations, |
| sorted by ascending priority (operations in the same box have the same |
| priority; all numeric operations have a higher priority than |
| comparison operations): |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{} |
| \lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{} |
| \hline |
| \lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{} |
| \lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(1)} |
| \lineiii{\var{x} // \var{y}}{(floored) quotient of \var{x} and \var{y}}{(5)} |
| \lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{(4)} |
| \hline |
| \lineiii{-\var{x}}{\var{x} negated}{} |
| \lineiii{+\var{x}}{\var{x} unchanged}{} |
| \hline |
| \lineiii{abs(\var{x})}{absolute value or magnitude of \var{x}}{} |
| \lineiii{int(\var{x})}{\var{x} converted to integer}{(2)} |
| \lineiii{long(\var{x})}{\var{x} converted to long integer}{(2)} |
| \lineiii{float(\var{x})}{\var{x} converted to floating point}{} |
| \lineiii{complex(\var{re},\var{im})}{a complex number with real part \var{re}, imaginary part \var{im}. \var{im} defaults to zero.}{} |
| \lineiii{\var{c}.conjugate()}{conjugate of the complex number \var{c}}{} |
| \lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} // \var{y}, \var{x} \%{} \var{y})}}{(3)(4)} |
| \lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{} |
| \lineiii{\var{x} ** \var{y}}{\var{x} to the power \var{y}}{} |
| \end{tableiii} |
| \indexiii{operations on}{numeric}{types} |
| \withsubitem{(complex number method)}{\ttindex{conjugate()}} |
| |
| \noindent |
| Notes: |
| \begin{description} |
| |
| \item[(1)] |
| For (plain or long) integer division, the result is an integer. |
| The result is always rounded towards minus infinity: 1/2 is 0, |
| (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. Note that the result |
| is a long integer if either operand is a long integer, regardless of |
| the numeric value. |
| \indexii{integer}{division} |
| \indexiii{long}{integer}{division} |
| |
| \item[(2)] |
| Conversion from floating point to (long or plain) integer may round or |
| truncate as in C; see functions \function{floor()} and |
| \function{ceil()} in the \refmodule{math}\refbimodindex{math} module |
| for well-defined conversions. |
| \withsubitem{(in module math)}{\ttindex{floor()}\ttindex{ceil()}} |
| \indexii{numeric}{conversions} |
| \indexii{C}{language} |
| |
| \item[(3)] |
| See section \ref{built-in-funcs}, ``Built-in Functions,'' for a full |
| description. |
| |
| \item[(4)] |
| Complex floor division operator, modulo operator, and \function{divmod()}. |
| |
| \deprecated{2.3}{Instead convert to float using \function{abs()} |
| if appropriate.} |
| |
| \item[(5)] |
| Also referred to as integer division. The resultant value is a whole integer, |
| though the result's type is not necessarily int. |
| \end{description} |
| % XXXJH exceptions: overflow (when? what operations?) zerodivision |
| |
| \subsection{Bit-string Operations on Integer Types \label{bitstring-ops}} |
| \nodename{Bit-string Operations} |
| |
| Plain and long integer types support additional operations that make |
| sense only for bit-strings. Negative numbers are treated as their 2's |
| complement value (for long integers, this assumes a sufficiently large |
| number of bits that no overflow occurs during the operation). |
| |
| The priorities of the binary bit-wise operations are all lower than |
| the numeric operations and higher than the comparisons; the unary |
| operation \samp{\~} has the same priority as the other unary numeric |
| operations (\samp{+} and \samp{-}). |
| |
| This table lists the bit-string operations sorted in ascending |
| priority (operations in the same box have the same priority): |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{} |
| \lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{} |
| \lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{} |
| % The empty groups below prevent conversion to guillemets. |
| \lineiii{\var{x} <{}< \var{n}}{\var{x} shifted left by \var{n} bits}{(1), (2)} |
| \lineiii{\var{x} >{}> \var{n}}{\var{x} shifted right by \var{n} bits}{(1), (3)} |
| \hline |
| \lineiii{\~\var{x}}{the bits of \var{x} inverted}{} |
| \end{tableiii} |
| \indexiii{operations on}{integer}{types} |
| \indexii{bit-string}{operations} |
| \indexii{shifting}{operations} |
| \indexii{masking}{operations} |
| |
| \noindent |
| Notes: |
| \begin{description} |
| \item[(1)] Negative shift counts are illegal and cause a |
| \exception{ValueError} to be raised. |
| \item[(2)] A left shift by \var{n} bits is equivalent to |
| multiplication by \code{pow(2, \var{n})} without overflow check. |
| \item[(3)] A right shift by \var{n} bits is equivalent to |
| division by \code{pow(2, \var{n})} without overflow check. |
| \end{description} |
| |
| |
| \section{Iterator Types \label{typeiter}} |
| |
| \versionadded{2.2} |
| \index{iterator protocol} |
| \index{protocol!iterator} |
| \index{sequence!iteration} |
| \index{container!iteration over} |
| |
| Python supports a concept of iteration over containers. This is |
| implemented using two distinct methods; these are used to allow |
| user-defined classes to support iteration. Sequences, described below |
| in more detail, always support the iteration methods. |
| |
| One method needs to be defined for container objects to provide |
| iteration support: |
| |
| \begin{methoddesc}[container]{__iter__}{} |
| Return an iterator object. The object is required to support the |
| iterator protocol described below. If a container supports |
| different types of iteration, additional methods can be provided to |
| specifically request iterators for those iteration types. (An |
| example of an object supporting multiple forms of iteration would be |
| a tree structure which supports both breadth-first and depth-first |
| traversal.) This method corresponds to the \member{tp_iter} slot of |
| the type structure for Python objects in the Python/C API. |
| \end{methoddesc} |
| |
| The iterator objects themselves are required to support the following |
| two methods, which together form the \dfn{iterator protocol}: |
| |
| \begin{methoddesc}[iterator]{__iter__}{} |
| Return the iterator object itself. This is required to allow both |
| containers and iterators to be used with the \keyword{for} and |
| \keyword{in} statements. This method corresponds to the |
| \member{tp_iter} slot of the type structure for Python objects in |
| the Python/C API. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[iterator]{next}{} |
| Return the next item from the container. If there are no further |
| items, raise the \exception{StopIteration} exception. This method |
| corresponds to the \member{tp_iternext} slot of the type structure |
| for Python objects in the Python/C API. |
| \end{methoddesc} |
| |
| Python defines several iterator objects to support iteration over |
| general and specific sequence types, dictionaries, and other more |
| specialized forms. The specific types are not important beyond their |
| implementation of the iterator protocol. |
| |
| The intention of the protocol is that once an iterator's |
| \method{next()} method raises \exception{StopIteration}, it will |
| continue to do so on subsequent calls. Implementations that |
| do not obey this property are deemed broken. (This constraint |
| was added in Python 2.3; in Python 2.2, various iterators are |
| broken according to this rule.) |
| |
| Python's generators provide a convenient way to implement the |
| iterator protocol. If a container object's \method{__iter__()} |
| method is implemented as a generator, it will automatically |
| return an iterator object (technically, a generator object) |
| supplying the \method{__iter__()} and \method{next()} methods. |
| |
| |
| \section{Sequence Types --- |
| \class{str}, \class{unicode}, \class{list}, |
| \class{tuple}, \class{buffer}, \class{xrange} |
| \label{typesseq}} |
| |
| There are six sequence types: strings, Unicode strings, lists, |
| tuples, buffers, and xrange objects. |
| |
| String literals are written in single or double quotes: |
| \code{'xyzzy'}, \code{"frobozz"}. See chapter 2 of the |
| \citetitle[../ref/strings.html]{Python Reference Manual} for more about |
| string literals. Unicode strings are much like strings, but are |
| specified in the syntax using a preceding \character{u} character: |
| \code{u'abc'}, \code{u"def"}. Lists are constructed with square brackets, |
| separating items with commas: \code{[a, b, c]}. Tuples are |
| constructed by the comma operator (not within square brackets), with |
| or without enclosing parentheses, but an empty tuple must have the |
| enclosing parentheses, such as \code{a, b, c} or \code{()}. A single |
| item tuple must have a trailing comma, such as \code{(d,)}. |
| \obindex{sequence} |
| \obindex{string} |
| \obindex{Unicode} |
| \obindex{tuple} |
| \obindex{list} |
| |
| Buffer objects are not directly supported by Python syntax, but can be |
| created by calling the builtin function |
| \function{buffer()}.\bifuncindex{buffer} They don't support |
| concatenation or repetition. |
| \obindex{buffer} |
| |
| Xrange objects are similar to buffers in that there is no specific |
| syntax to create them, but they are created using the \function{xrange()} |
| function.\bifuncindex{xrange} They don't support slicing, |
| concatenation or repetition, and using \code{in}, \code{not in}, |
| \function{min()} or \function{max()} on them is inefficient. |
| \obindex{xrange} |
| |
| Most sequence types support the following operations. The \samp{in} and |
| \samp{not in} operations have the same priorities as the comparison |
| operations. The \samp{+} and \samp{*} operations have the same |
| priority as the corresponding numeric operations.\footnote{They must |
| have since the parser can't tell the type of the operands.} |
| |
| This table lists the sequence operations sorted in ascending priority |
| (operations in the same box have the same priority). In the table, |
| \var{s} and \var{t} are sequences of the same type; \var{n}, \var{i} |
| and \var{j} are integers: |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{\var{x} in \var{s}}{\code{True} if an item of \var{s} is equal to \var{x}, else \code{False}}{(1)} |
| \lineiii{\var{x} not in \var{s}}{\code{False} if an item of \var{s} is |
| equal to \var{x}, else \code{True}}{(1)} |
| \hline |
| \lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{(6)} |
| \lineiii{\var{s} * \var{n}\textrm{,} \var{n} * \var{s}}{\var{n} shallow copies of \var{s} concatenated}{(2)} |
| \hline |
| \lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(3)} |
| \lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(3), (4)} |
| \lineiii{\var{s}[\var{i}:\var{j}:\var{k}]}{slice of \var{s} from \var{i} to \var{j} with step \var{k}}{(3), (5)} |
| \hline |
| \lineiii{len(\var{s})}{length of \var{s}}{} |
| \lineiii{min(\var{s})}{smallest item of \var{s}}{} |
| \lineiii{max(\var{s})}{largest item of \var{s}}{} |
| \end{tableiii} |
| \indexiii{operations on}{sequence}{types} |
| \bifuncindex{len} |
| \bifuncindex{min} |
| \bifuncindex{max} |
| \indexii{concatenation}{operation} |
| \indexii{repetition}{operation} |
| \indexii{subscript}{operation} |
| \indexii{slice}{operation} |
| \indexii{extended slice}{operation} |
| \opindex{in} |
| \opindex{not in} |
| |
| \noindent |
| Notes: |
| |
| \begin{description} |
| \item[(1)] When \var{s} is a string or Unicode string object the |
| \code{in} and \code{not in} operations act like a substring test. In |
| Python versions before 2.3, \var{x} had to be a string of length 1. |
| In Python 2.3 and beyond, \var{x} may be a string of any length. |
| |
| \item[(2)] Values of \var{n} less than \code{0} are treated as |
| \code{0} (which yields an empty sequence of the same type as |
| \var{s}). Note also that the copies are shallow; nested structures |
| are not copied. This often haunts new Python programmers; consider: |
| |
| \begin{verbatim} |
| >>> lists = [[]] * 3 |
| >>> lists |
| [[], [], []] |
| >>> lists[0].append(3) |
| >>> lists |
| [[3], [3], [3]] |
| \end{verbatim} |
| |
| What has happened is that \code{[[]]} is a one-element list containing |
| an empty list, so all three elements of \code{[[]] * 3} are (pointers to) |
| this single empty list. Modifying any of the elements of \code{lists} |
| modifies this single list. You can create a list of different lists this |
| way: |
| |
| \begin{verbatim} |
| >>> lists = [[] for i in range(3)] |
| >>> lists[0].append(3) |
| >>> lists[1].append(5) |
| >>> lists[2].append(7) |
| >>> lists |
| [[3], [5], [7]] |
| \end{verbatim} |
| |
| \item[(3)] If \var{i} or \var{j} is negative, the index is relative to |
| the end of the string: \code{len(\var{s}) + \var{i}} or |
| \code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is |
| still \code{0}. |
| |
| \item[(4)] The slice of \var{s} from \var{i} to \var{j} is defined as |
| the sequence of items with index \var{k} such that \code{\var{i} <= |
| \var{k} < \var{j}}. If \var{i} or \var{j} is greater than |
| \code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted |
| or \code{None}, use \code{0}. If \var{j} is omitted or \code{None}, |
| use \code{len(\var{s})}. If \var{i} is greater than or equal to \var{j}, |
| the slice is empty. |
| |
| \item[(5)] The slice of \var{s} from \var{i} to \var{j} with step |
| \var{k} is defined as the sequence of items with index |
| \code{\var{x} = \var{i} + \var{n}*\var{k}} such that |
| $0 \leq n < \frac{j-i}{k}$. In other words, the indices |
| are \code{i}, \code{i+k}, \code{i+2*k}, \code{i+3*k} and so on, stopping when |
| \var{j} is reached (but never including \var{j}). If \var{i} or \var{j} |
| is greater than \code{len(\var{s})}, use \code{len(\var{s})}. If |
| \var{i} or \var{j} are omitted or \code{None}, they become ``end'' values |
| (which end depends on the sign of \var{k}). Note, \var{k} cannot |
| be zero. If \var{k} is \code{None}, it is treated like \code{1}. |
| |
| \item[(6)] If \var{s} and \var{t} are both strings, some Python |
| implementations such as CPython can usually perform an in-place optimization |
| for assignments of the form \code{\var{s}=\var{s}+\var{t}} or |
| \code{\var{s}+=\var{t}}. When applicable, this optimization makes |
| quadratic run-time much less likely. This optimization is both version |
| and implementation dependent. For performance sensitive code, it is |
| preferable to use the \method{str.join()} method which assures consistent |
| linear concatenation performance across versions and implementations. |
| \versionchanged[Formerly, string concatenation never occurred in-place]{2.4} |
| |
| \end{description} |
| |
| |
| \subsection{String Methods \label{string-methods}} |
| \indexii{string}{methods} |
| |
| These are the string methods which both 8-bit strings and Unicode |
| objects support: |
| |
| \begin{methoddesc}[string]{capitalize}{} |
| Return a copy of the string with only its first character capitalized. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{center}{width\optional{, fillchar}} |
| Return centered in a string of length \var{width}. Padding is done |
| using the specified \var{fillchar} (default is a space). |
| \versionchanged[Support for the \var{fillchar} argument]{2.4} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{count}{sub\optional{, start\optional{, end}}} |
| Return the number of occurrences of substring \var{sub} in string |
| S\code{[\var{start}:\var{end}]}. Optional arguments \var{start} and |
| \var{end} are interpreted as in slice notation. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{decode}{\optional{encoding\optional{, errors}}} |
| Decodes the string using the codec registered for \var{encoding}. |
| \var{encoding} defaults to the default string encoding. \var{errors} |
| may be given to set a different error handling scheme. The default is |
| \code{'strict'}, meaning that encoding errors raise |
| \exception{UnicodeError}. Other possible values are \code{'ignore'}, |
| \code{'replace'} and any other name registered via |
| \function{codecs.register_error}, see section~\ref{codec-base-classes}. |
| \versionadded{2.2} |
| \versionchanged[Support for other error handling schemes added]{2.3} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{encode}{\optional{encoding\optional{,errors}}} |
| Return an encoded version of the string. Default encoding is the current |
| default string encoding. \var{errors} may be given to set a different |
| error handling scheme. The default for \var{errors} is |
| \code{'strict'}, meaning that encoding errors raise a |
| \exception{UnicodeError}. Other possible values are \code{'ignore'}, |
| \code{'replace'}, \code{'xmlcharrefreplace'}, \code{'backslashreplace'} |
| and any other name registered via \function{codecs.register_error}, |
| see section~\ref{codec-base-classes}. |
| For a list of possible encodings, see section~\ref{standard-encodings}. |
| \versionadded{2.0} |
| \versionchanged[Support for \code{'xmlcharrefreplace'} and |
| \code{'backslashreplace'} and other error handling schemes added]{2.3} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{endswith}{suffix\optional{, start\optional{, end}}} |
| Return \code{True} if the string ends with the specified \var{suffix}, |
| otherwise return \code{False}. \var{suffix} can also be a tuple of |
| suffixes to look for. With optional \var{start}, test beginning at |
| that position. With optional \var{end}, stop comparing at that position. |
| |
| \versionchanged[Accept tuples as \var{suffix}]{2.5} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{expandtabs}{\optional{tabsize}} |
| Return a copy of the string where all tab characters are expanded |
| using spaces. If \var{tabsize} is not given, a tab size of \code{8} |
| characters is assumed. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{find}{sub\optional{, start\optional{, end}}} |
| Return the lowest index in the string where substring \var{sub} is |
| found, such that \var{sub} is contained in the range [\var{start}, |
| \var{end}]. Optional arguments \var{start} and \var{end} are |
| interpreted as in slice notation. Return \code{-1} if \var{sub} is |
| not found. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{index}{sub\optional{, start\optional{, end}}} |
| Like \method{find()}, but raise \exception{ValueError} when the |
| substring is not found. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{isalnum}{} |
| Return true if all characters in the string are alphanumeric and there |
| is at least one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{isalpha}{} |
| Return true if all characters in the string are alphabetic and there |
| is at least one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{isdigit}{} |
| Return true if all characters in the string are digits and there |
| is at least one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{islower}{} |
| Return true if all cased characters in the string are lowercase and |
| there is at least one cased character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{isspace}{} |
| Return true if there are only whitespace characters in the string and |
| there is at least one character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{istitle}{} |
| Return true if the string is a titlecased string and there is at least one |
| character, for example uppercase characters may only follow uncased |
| characters and lowercase characters only cased ones. Return false |
| otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{isupper}{} |
| Return true if all cased characters in the string are uppercase and |
| there is at least one cased character, false otherwise. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{join}{seq} |
| Return a string which is the concatenation of the strings in the |
| sequence \var{seq}. The separator between elements is the string |
| providing this method. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{ljust}{width\optional{, fillchar}} |
| Return the string left justified in a string of length \var{width}. |
| Padding is done using the specified \var{fillchar} (default is a |
| space). The original string is returned if |
| \var{width} is less than \code{len(\var{s})}. |
| \versionchanged[Support for the \var{fillchar} argument]{2.4} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{lower}{} |
| Return a copy of the string converted to lowercase. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{lstrip}{\optional{chars}} |
| Return a copy of the string with leading characters removed. The |
| \var{chars} argument is a string specifying the set of characters |
| to be removed. If omitted or \code{None}, the \var{chars} argument |
| defaults to removing whitespace. The \var{chars} argument is not |
| a prefix; rather, all combinations of its values are stripped: |
| \begin{verbatim} |
| >>> ' spacious '.lstrip() |
| 'spacious ' |
| >>> 'www.example.com'.lstrip('cmowz.') |
| 'example.com' |
| \end{verbatim} |
| \versionchanged[Support for the \var{chars} argument]{2.2.2} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{partition}{sep} |
| Split the string at the first occurrence of \var{sep}, and return |
| a 3-tuple containing the part before the separator, the separator |
| itself, and the part after the separator. If the separator is not |
| found, return a 3-tuple containing the string itself, followed by |
| two empty strings. |
| \versionadded{2.5} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{replace}{old, new\optional{, count}} |
| Return a copy of the string with all occurrences of substring |
| \var{old} replaced by \var{new}. If the optional argument |
| \var{count} is given, only the first \var{count} occurrences are |
| replaced. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rfind}{sub \optional{,start \optional{,end}}} |
| Return the highest index in the string where substring \var{sub} is |
| found, such that \var{sub} is contained within s[start,end]. Optional |
| arguments \var{start} and \var{end} are interpreted as in slice |
| notation. Return \code{-1} on failure. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rindex}{sub\optional{, start\optional{, end}}} |
| Like \method{rfind()} but raises \exception{ValueError} when the |
| substring \var{sub} is not found. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rjust}{width\optional{, fillchar}} |
| Return the string right justified in a string of length \var{width}. |
| Padding is done using the specified \var{fillchar} (default is a space). |
| The original string is returned if |
| \var{width} is less than \code{len(\var{s})}. |
| \versionchanged[Support for the \var{fillchar} argument]{2.4} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rpartition}{sep} |
| Split the string at the last occurrence of \var{sep}, and return |
| a 3-tuple containing the part before the separator, the separator |
| itself, and the part after the separator. If the separator is not |
| found, return a 3-tuple containing two empty strings, followed by |
| the string itself. |
| \versionadded{2.5} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rsplit}{\optional{sep \optional{,maxsplit}}} |
| Return a list of the words in the string, using \var{sep} as the |
| delimiter string. If \var{maxsplit} is given, at most \var{maxsplit} |
| splits are done, the \emph{rightmost} ones. If \var{sep} is not specified |
| or \code{None}, any whitespace string is a separator. Except for splitting |
| from the right, \method{rsplit()} behaves like \method{split()} which |
| is described in detail below. |
| \versionadded{2.4} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{rstrip}{\optional{chars}} |
| Return a copy of the string with trailing characters removed. The |
| \var{chars} argument is a string specifying the set of characters |
| to be removed. If omitted or \code{None}, the \var{chars} argument |
| defaults to removing whitespace. The \var{chars} argument is not |
| a suffix; rather, all combinations of its values are stripped: |
| \begin{verbatim} |
| >>> ' spacious '.rstrip() |
| ' spacious' |
| >>> 'mississippi'.rstrip('ipz') |
| 'mississ' |
| \end{verbatim} |
| \versionchanged[Support for the \var{chars} argument]{2.2.2} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{split}{\optional{sep \optional{,maxsplit}}} |
| Return a list of the words in the string, using \var{sep} as the |
| delimiter string. If \var{maxsplit} is given, at most \var{maxsplit} |
| splits are done. (thus, the list will have at most \code{\var{maxsplit}+1} |
| elements). If \var{maxsplit} is not specified, then there |
| is no limit on the number of splits (all possible splits are made). |
| Consecutive delimiters are not grouped together and are |
| deemed to delimit empty strings (for example, \samp{'1,,2'.split(',')} |
| returns \samp{['1', '', '2']}). The \var{sep} argument may consist of |
| multiple characters (for example, \samp{'1, 2, 3'.split(', ')} returns |
| \samp{['1', '2', '3']}). Splitting an empty string with a specified |
| separator returns \samp{['']}. |
| |
| If \var{sep} is not specified or is \code{None}, a different splitting |
| algorithm is applied. First, whitespace characters (spaces, tabs, |
| newlines, returns, and formfeeds) are stripped from both ends. Then, |
| words are separated by arbitrary length strings of whitespace |
| characters. Consecutive whitespace delimiters are treated as a single |
| delimiter (\samp{'1 2 3'.split()} returns \samp{['1', '2', '3']}). |
| Splitting an empty string or a string consisting of just whitespace |
| returns an empty list. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{splitlines}{\optional{keepends}} |
| Return a list of the lines in the string, breaking at line |
| boundaries. Line breaks are not included in the resulting list unless |
| \var{keepends} is given and true. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{startswith}{prefix\optional{, |
| start\optional{, end}}} |
| Return \code{True} if string starts with the \var{prefix}, otherwise |
| return \code{False}. \var{prefix} can also be a tuple of |
| prefixes to look for. With optional \var{start}, test string beginning at |
| that position. With optional \var{end}, stop comparing string at that |
| position. |
| |
| \versionchanged[Accept tuples as \var{prefix}]{2.5} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{strip}{\optional{chars}} |
| Return a copy of the string with the leading and trailing characters |
| removed. The \var{chars} argument is a string specifying the set of |
| characters to be removed. If omitted or \code{None}, the \var{chars} |
| argument defaults to removing whitespace. The \var{chars} argument is not |
| a prefix or suffix; rather, all combinations of its values are stripped: |
| \begin{verbatim} |
| >>> ' spacious '.strip() |
| 'spacious' |
| >>> 'www.example.com'.strip('cmowz.') |
| 'example' |
| \end{verbatim} |
| \versionchanged[Support for the \var{chars} argument]{2.2.2} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{swapcase}{} |
| Return a copy of the string with uppercase characters converted to |
| lowercase and vice versa. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{title}{} |
| Return a titlecased version of the string: words start with uppercase |
| characters, all remaining cased characters are lowercase. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{translate}{table\optional{, deletechars}} |
| Return a copy of the string where all characters occurring in the |
| optional argument \var{deletechars} are removed, and the remaining |
| characters have been mapped through the given translation table, which |
| must be a string of length 256. |
| |
| You can use the \function{maketrans()} helper function in the |
| \refmodule{string} module to create a translation table. |
| |
| For Unicode objects, the \method{translate()} method does not |
| accept the optional \var{deletechars} argument. Instead, it |
| returns a copy of the \var{s} where all characters have been mapped |
| through the given translation table which must be a mapping of |
| Unicode ordinals to Unicode ordinals, Unicode strings or \code{None}. |
| Unmapped characters are left untouched. Characters mapped to \code{None} |
| are deleted. Note, a more flexible approach is to create a custom |
| character mapping codec using the \refmodule{codecs} module (see |
| \module{encodings.cp1251} for an example). |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{upper}{} |
| Return a copy of the string converted to uppercase. |
| |
| For 8-bit strings, this method is locale-dependent. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[string]{zfill}{width} |
| Return the numeric string left filled with zeros in a string |
| of length \var{width}. The original string is returned if |
| \var{width} is less than \code{len(\var{s})}. |
| \versionadded{2.2.2} |
| \end{methoddesc} |
| |
| |
| \subsection{String Formatting Operations \label{typesseq-strings}} |
| |
| \index{formatting, string (\%{})} |
| \index{interpolation, string (\%{})} |
| \index{string!formatting} |
| \index{string!interpolation} |
| \index{printf-style formatting} |
| \index{sprintf-style formatting} |
| \index{\protect\%{} formatting} |
| \index{\protect\%{} interpolation} |
| |
| String and Unicode objects have one unique built-in operation: the |
| \code{\%} operator (modulo). This is also known as the string |
| \emph{formatting} or \emph{interpolation} operator. Given |
| \code{\var{format} \% \var{values}} (where \var{format} is a string or |
| Unicode object), \code{\%} conversion specifications in \var{format} |
| are replaced with zero or more elements of \var{values}. The effect |
| is similar to the using \cfunction{sprintf()} in the C language. If |
| \var{format} is a Unicode object, or if any of the objects being |
| converted using the \code{\%s} conversion are Unicode objects, the |
| result will also be a Unicode object. |
| |
| If \var{format} requires a single argument, \var{values} may be a |
| single non-tuple object.\footnote{To format only a tuple you |
| should therefore provide a singleton tuple whose only element |
| is the tuple to be formatted.} Otherwise, \var{values} must be a tuple with |
| exactly the number of items specified by the format string, or a |
| single mapping object (for example, a dictionary). |
| |
| A conversion specifier contains two or more characters and has the |
| following components, which must occur in this order: |
| |
| \begin{enumerate} |
| \item The \character{\%} character, which marks the start of the |
| specifier. |
| \item Mapping key (optional), consisting of a parenthesised sequence |
| of characters (for example, \code{(somename)}). |
| \item Conversion flags (optional), which affect the result of some |
| conversion types. |
| \item Minimum field width (optional). If specified as an |
| \character{*} (asterisk), the actual width is read from the |
| next element of the tuple in \var{values}, and the object to |
| convert comes after the minimum field width and optional |
| precision. |
| \item Precision (optional), given as a \character{.} (dot) followed |
| by the precision. If specified as \character{*} (an |
| asterisk), the actual width is read from the next element of |
| the tuple in \var{values}, and the value to convert comes after |
| the precision. |
| \item Length modifier (optional). |
| \item Conversion type. |
| \end{enumerate} |
| |
| When the right argument is a dictionary (or other mapping type), then |
| the formats in the string \emph{must} include a parenthesised mapping key into |
| that dictionary inserted immediately after the \character{\%} |
| character. The mapping key selects the value to be formatted from the |
| mapping. For example: |
| |
| \begin{verbatim} |
| >>> print '%(language)s has %(#)03d quote types.' % \ |
| {'language': "Python", "#": 2} |
| Python has 002 quote types. |
| \end{verbatim} |
| |
| In this case no \code{*} specifiers may occur in a format (since they |
| require a sequential parameter list). |
| |
| The conversion flag characters are: |
| |
| \begin{tableii}{c|l}{character}{Flag}{Meaning} |
| \lineii{\#}{The value conversion will use the ``alternate form'' |
| (where defined below).} |
| \lineii{0}{The conversion will be zero padded for numeric values.} |
| \lineii{-}{The converted value is left adjusted (overrides |
| the \character{0} conversion if both are given).} |
| \lineii{{~}}{(a space) A blank should be left before a positive number |
| (or empty string) produced by a signed conversion.} |
| \lineii{+}{A sign character (\character{+} or \character{-}) will |
| precede the conversion (overrides a "space" flag).} |
| \end{tableii} |
| |
| A length modifier (\code{h}, \code{l}, or \code{L}) may be |
| present, but is ignored as it is not necessary for Python. |
| |
| The conversion types are: |
| |
| \begin{tableiii}{c|l|c}{character}{Conversion}{Meaning}{Notes} |
| \lineiii{d}{Signed integer decimal.}{} |
| \lineiii{i}{Signed integer decimal.}{} |
| \lineiii{o}{Unsigned octal.}{(1)} |
| \lineiii{u}{Unsigned decimal.}{} |
| \lineiii{x}{Unsigned hexadecimal (lowercase).}{(2)} |
| \lineiii{X}{Unsigned hexadecimal (uppercase).}{(2)} |
| \lineiii{e}{Floating point exponential format (lowercase).}{(3)} |
| \lineiii{E}{Floating point exponential format (uppercase).}{(3)} |
| \lineiii{f}{Floating point decimal format.}{(3)} |
| \lineiii{F}{Floating point decimal format.}{(3)} |
| \lineiii{g}{Floating point format. Uses exponential format |
| if exponent is greater than -4 or less than precision, |
| decimal format otherwise.}{(4)} |
| \lineiii{G}{Floating point format. Uses exponential format |
| if exponent is greater than -4 or less than precision, |
| decimal format otherwise.}{(4)} |
| \lineiii{c}{Single character (accepts integer or single character |
| string).}{} |
| \lineiii{r}{String (converts any python object using |
| \function{repr()}).}{(5)} |
| \lineiii{s}{String (converts any python object using |
| \function{str()}).}{(6)} |
| \lineiii{\%}{No argument is converted, results in a \character{\%} |
| character in the result.}{} |
| \end{tableiii} |
| |
| \noindent |
| Notes: |
| \begin{description} |
| \item[(1)] |
| The alternate form causes a leading zero (\character{0}) to be |
| inserted between left-hand padding and the formatting of the |
| number if the leading character of the result is not already a |
| zero. |
| \item[(2)] |
| The alternate form causes a leading \code{'0x'} or \code{'0X'} |
| (depending on whether the \character{x} or \character{X} format |
| was used) to be inserted between left-hand padding and the |
| formatting of the number if the leading character of the result is |
| not already a zero. |
| \item[(3)] |
| The alternate form causes the result to always contain a decimal |
| point, even if no digits follow it. |
| |
| The precision determines the number of digits after the decimal |
| point and defaults to 6. |
| \item[(4)] |
| The alternate form causes the result to always contain a decimal |
| point, and trailing zeroes are not removed as they would |
| otherwise be. |
| |
| The precision determines the number of significant digits before |
| and after the decimal point and defaults to 6. |
| \item[(5)] |
| The \code{\%r} conversion was added in Python 2.0. |
| |
| The precision determines the maximal number of characters used. |
| \item[(6)] |
| If the object or format provided is a \class{unicode} string, |
| the resulting string will also be \class{unicode}. |
| |
| The precision determines the maximal number of characters used. |
| \end{description} |
| |
| % XXX Examples? |
| |
| Since Python strings have an explicit length, \code{\%s} conversions |
| do not assume that \code{'\e0'} is the end of the string. |
| |
| For safety reasons, floating point precisions are clipped to 50; |
| \code{\%f} conversions for numbers whose absolute value is over 1e25 |
| are replaced by \code{\%g} conversions.\footnote{ |
| These numbers are fairly arbitrary. They are intended to |
| avoid printing endless strings of meaningless digits without hampering |
| correct use and without having to know the exact precision of floating |
| point values on a particular machine. |
| } All other errors raise exceptions. |
| |
| Additional string operations are defined in standard modules |
| \refmodule{string}\refstmodindex{string}\ and |
| \refmodule{re}.\refstmodindex{re} |
| |
| |
| \subsection{XRange Type \label{typesseq-xrange}} |
| |
| The \class{xrange}\obindex{xrange} type is an immutable sequence which |
| is commonly used for looping. The advantage of the \class{xrange} |
| type is that an \class{xrange} object will always take the same amount |
| of memory, no matter the size of the range it represents. There are |
| no consistent performance advantages. |
| |
| XRange objects have very little behavior: they only support indexing, |
| iteration, and the \function{len()} function. |
| |
| |
| \subsection{Mutable Sequence Types \label{typesseq-mutable}} |
| |
| List objects support additional operations that allow in-place |
| modification of the object. |
| Other mutable sequence types (when added to the language) should |
| also support these operations. |
| Strings and tuples are immutable sequence types: such objects cannot |
| be modified once created. |
| The following operations are defined on mutable sequence types (where |
| \var{x} is an arbitrary object): |
| \indexiii{mutable}{sequence}{types} |
| \obindex{list} |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{\var{s}[\var{i}] = \var{x}} |
| {item \var{i} of \var{s} is replaced by \var{x}}{} |
| \lineiii{\var{s}[\var{i}:\var{j}] = \var{t}} |
| {slice of \var{s} from \var{i} to \var{j} |
| is replaced by the contents of the iterable \var{t}}{} |
| \lineiii{del \var{s}[\var{i}:\var{j}]} |
| {same as \code{\var{s}[\var{i}:\var{j}] = []}}{} |
| \lineiii{\var{s}[\var{i}:\var{j}:\var{k}] = \var{t}} |
| {the elements of \code{\var{s}[\var{i}:\var{j}:\var{k}]} are replaced by those of \var{t}}{(1)} |
| \lineiii{del \var{s}[\var{i}:\var{j}:\var{k}]} |
| {removes the elements of \code{\var{s}[\var{i}:\var{j}:\var{k}]} from the list}{} |
| \lineiii{\var{s}.append(\var{x})} |
| {same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{(2)} |
| \lineiii{\var{s}.extend(\var{x})} |
| {same as \code{\var{s}[len(\var{s}):len(\var{s})] = \var{x}}}{(3)} |
| \lineiii{\var{s}.count(\var{x})} |
| {return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{} |
| \lineiii{\var{s}.index(\var{x}\optional{, \var{i}\optional{, \var{j}}})} |
| {return smallest \var{k} such that \code{\var{s}[\var{k}] == \var{x}} and |
| \code{\var{i} <= \var{k} < \var{j}}}{(4)} |
| \lineiii{\var{s}.insert(\var{i}, \var{x})} |
| {same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}}{(5)} |
| \lineiii{\var{s}.pop(\optional{\var{i}})} |
| {same as \code{\var{x} = \var{s}[\var{i}]; del \var{s}[\var{i}]; return \var{x}}}{(6)} |
| \lineiii{\var{s}.remove(\var{x})} |
| {same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(4)} |
| \lineiii{\var{s}.reverse()} |
| {reverses the items of \var{s} in place}{(7)} |
| \lineiii{\var{s}.sort(\optional{\var{cmp}\optional{, |
| \var{key}\optional{, \var{reverse}}}})} |
| {sort the items of \var{s} in place}{(7), (8), (9), (10)} |
| \end{tableiii} |
| \indexiv{operations on}{mutable}{sequence}{types} |
| \indexiii{operations on}{sequence}{types} |
| \indexiii{operations on}{list}{type} |
| \indexii{subscript}{assignment} |
| \indexii{slice}{assignment} |
| \indexii{extended slice}{assignment} |
| \stindex{del} |
| \withsubitem{(list method)}{ |
| \ttindex{append()}\ttindex{extend()}\ttindex{count()}\ttindex{index()} |
| \ttindex{insert()}\ttindex{pop()}\ttindex{remove()}\ttindex{reverse()} |
| \ttindex{sort()}} |
| \noindent |
| Notes: |
| \begin{description} |
| \item[(1)] \var{t} must have the same length as the slice it is |
| replacing. |
| |
| \item[(2)] The C implementation of Python has historically accepted |
| multiple parameters and implicitly joined them into a tuple; this |
| no longer works in Python 2.0. Use of this misfeature has been |
| deprecated since Python 1.4. |
| |
| \item[(3)] \var{x} can be any iterable object. |
| |
| \item[(4)] Raises \exception{ValueError} when \var{x} is not found in |
| \var{s}. When a negative index is passed as the second or third parameter |
| to the \method{index()} method, the list length is added, as for slice |
| indices. If it is still negative, it is truncated to zero, as for |
| slice indices. \versionchanged[Previously, \method{index()} didn't |
| have arguments for specifying start and stop positions]{2.3} |
| |
| \item[(5)] When a negative index is passed as the first parameter to |
| the \method{insert()} method, the list length is added, as for slice |
| indices. If it is still negative, it is truncated to zero, as for |
| slice indices. \versionchanged[Previously, all negative indices |
| were truncated to zero]{2.3} |
| |
| \item[(6)] The \method{pop()} method is only supported by the list and |
| array types. The optional argument \var{i} defaults to \code{-1}, |
| so that by default the last item is removed and returned. |
| |
| \item[(7)] The \method{sort()} and \method{reverse()} methods modify the |
| list in place for economy of space when sorting or reversing a large |
| list. To remind you that they operate by side effect, they don't return |
| the sorted or reversed list. |
| |
| \item[(8)] The \method{sort()} method takes optional arguments for |
| controlling the comparisons. |
| |
| \var{cmp} specifies a custom comparison function of two arguments |
| (list items) which should return a negative, zero or positive number |
| depending on whether the first argument is considered smaller than, |
| equal to, or larger than the second argument: |
| \samp{\var{cmp}=\keyword{lambda} \var{x},\var{y}: |
| \function{cmp}(x.lower(), y.lower())} |
| |
| \var{key} specifies a function of one argument that is used to |
| extract a comparison key from each list element: |
| \samp{\var{key}=\function{str.lower}} |
| |
| \var{reverse} is a boolean value. If set to \code{True}, then the |
| list elements are sorted as if each comparison were reversed. |
| |
| In general, the \var{key} and \var{reverse} conversion processes are |
| much faster than specifying an equivalent \var{cmp} function. This is |
| because \var{cmp} is called multiple times for each list element while |
| \var{key} and \var{reverse} touch each element only once. |
| |
| \versionchanged[Support for \code{None} as an equivalent to omitting |
| \var{cmp} was added]{2.3} |
| |
| \versionchanged[Support for \var{key} and \var{reverse} was added]{2.4} |
| |
| \item[(9)] Starting with Python 2.3, the \method{sort()} method is |
| guaranteed to be stable. A sort is stable if it guarantees not to |
| change the relative order of elements that compare equal --- this is |
| helpful for sorting in multiple passes (for example, sort by |
| department, then by salary grade). |
| |
| \item[(10)] While a list is being sorted, the effect of attempting to |
| mutate, or even inspect, the list is undefined. The C |
| implementation of Python 2.3 and newer makes the list appear empty |
| for the duration, and raises \exception{ValueError} if it can detect |
| that the list has been mutated during a sort. |
| \end{description} |
| |
| \section{Set Types --- |
| \class{set}, \class{frozenset} |
| \label{types-set}} |
| \obindex{set} |
| |
| A \dfn{set} object is an unordered collection of immutable values. |
| Common uses include membership testing, removing duplicates from a sequence, |
| and computing mathematical operations such as intersection, union, difference, |
| and symmetric difference. |
| \versionadded{2.4} |
| |
| Like other collections, sets support \code{\var{x} in \var{set}}, |
| \code{len(\var{set})}, and \code{for \var{x} in \var{set}}. Being an |
| unordered collection, sets do not record element position or order of |
| insertion. Accordingly, sets do not support indexing, slicing, or |
| other sequence-like behavior. |
| |
| There are currently two builtin set types, \class{set} and \class{frozenset}. |
| The \class{set} type is mutable --- the contents can be changed using methods |
| like \method{add()} and \method{remove()}. Since it is mutable, it has no |
| hash value and cannot be used as either a dictionary key or as an element of |
| another set. The \class{frozenset} type is immutable and hashable --- its |
| contents cannot be altered after is created; however, it can be used as |
| a dictionary key or as an element of another set. |
| |
| Instances of \class{set} and \class{frozenset} provide the following operations: |
| |
| \begin{tableiii}{c|c|l}{code}{Operation}{Equivalent}{Result} |
| \lineiii{len(\var{s})}{}{cardinality of set \var{s}} |
| |
| \hline |
| \lineiii{\var{x} in \var{s}}{} |
| {test \var{x} for membership in \var{s}} |
| \lineiii{\var{x} not in \var{s}}{} |
| {test \var{x} for non-membership in \var{s}} |
| \lineiii{\var{s}.issubset(\var{t})}{\code{\var{s} <= \var{t}}} |
| {test whether every element in \var{s} is in \var{t}} |
| \lineiii{\var{s}.issuperset(\var{t})}{\code{\var{s} >= \var{t}}} |
| {test whether every element in \var{t} is in \var{s}} |
| |
| \hline |
| \lineiii{\var{s}.union(\var{t})}{\var{s} | \var{t}} |
| {new set with elements from both \var{s} and \var{t}} |
| \lineiii{\var{s}.intersection(\var{t})}{\var{s} \&\ \var{t}} |
| {new set with elements common to \var{s} and \var{t}} |
| \lineiii{\var{s}.difference(\var{t})}{\var{s} - \var{t}} |
| {new set with elements in \var{s} but not in \var{t}} |
| \lineiii{\var{s}.symmetric_difference(\var{t})}{\var{s} \^\ \var{t}} |
| {new set with elements in either \var{s} or \var{t} but not both} |
| \lineiii{\var{s}.copy()}{} |
| {new set with a shallow copy of \var{s}} |
| \end{tableiii} |
| |
| Note, the non-operator versions of \method{union()}, \method{intersection()}, |
| \method{difference()}, and \method{symmetric_difference()}, |
| \method{issubset()}, and \method{issuperset()} methods will accept any |
| iterable as an argument. In contrast, their operator based counterparts |
| require their arguments to be sets. This precludes error-prone constructions |
| like \code{set('abc') \&\ 'cbs'} in favor of the more readable |
| \code{set('abc').intersection('cbs')}. |
| |
| Both \class{set} and \class{frozenset} support set to set comparisons. |
| Two sets are equal if and only if every element of each set is contained in |
| the other (each is a subset of the other). |
| A set is less than another set if and only if the first set is a proper |
| subset of the second set (is a subset, but is not equal). |
| A set is greater than another set if and only if the first set is a proper |
| superset of the second set (is a superset, but is not equal). |
| |
| Instances of \class{set} are compared to instances of \class{frozenset} based |
| on their members. For example, \samp{set('abc') == frozenset('abc')} returns |
| \code{True}. |
| |
| The subset and equality comparisons do not generalize to a complete |
| ordering function. For example, any two disjoint sets are not equal and |
| are not subsets of each other, so \emph{all} of the following return |
| \code{False}: \code{\var{a}<\var{b}}, \code{\var{a}==\var{b}}, or |
| \code{\var{a}>\var{b}}. |
| Accordingly, sets do not implement the \method{__cmp__} method. |
| |
| Since sets only define partial ordering (subset relationships), the output |
| of the \method{list.sort()} method is undefined for lists of sets. |
| |
| Set elements are like dictionary keys; they need to define both |
| \method{__hash__} and \method{__eq__} methods. |
| |
| Binary operations that mix \class{set} instances with \class{frozenset} |
| return the type of the first operand. For example: |
| \samp{frozenset('ab') | set('bc')} returns an instance of \class{frozenset}. |
| |
| The following table lists operations available for \class{set} |
| that do not apply to immutable instances of \class{frozenset}: |
| |
| \begin{tableiii}{c|c|l}{code}{Operation}{Equivalent}{Result} |
| \lineiii{\var{s}.update(\var{t})} |
| {\var{s} |= \var{t}} |
| {update set \var{s}, adding elements from \var{t}} |
| \lineiii{\var{s}.intersection_update(\var{t})} |
| {\var{s} \&= \var{t}} |
| {update set \var{s}, keeping only elements found in both \var{s} and \var{t}} |
| \lineiii{\var{s}.difference_update(\var{t})} |
| {\var{s} -= \var{t}} |
| {update set \var{s}, removing elements found in \var{t}} |
| \lineiii{\var{s}.symmetric_difference_update(\var{t})} |
| {\var{s} \textasciicircum= \var{t}} |
| {update set \var{s}, keeping only elements found in either \var{s} or \var{t} |
| but not in both} |
| |
| \hline |
| \lineiii{\var{s}.add(\var{x})}{} |
| {add element \var{x} to set \var{s}} |
| \lineiii{\var{s}.remove(\var{x})}{} |
| {remove \var{x} from set \var{s}; raises \exception{KeyError} |
| if not present} |
| \lineiii{\var{s}.discard(\var{x})}{} |
| {removes \var{x} from set \var{s} if present} |
| \lineiii{\var{s}.pop()}{} |
| {remove and return an arbitrary element from \var{s}; raises |
| \exception{KeyError} if empty} |
| \lineiii{\var{s}.clear()}{} |
| {remove all elements from set \var{s}} |
| \end{tableiii} |
| |
| Note, the non-operator versions of the \method{update()}, |
| \method{intersection_update()}, \method{difference_update()}, and |
| \method{symmetric_difference_update()} methods will accept any iterable |
| as an argument. |
| |
| The design of the set types was based on lessons learned from the |
| \module{sets} module. |
| |
| \begin{seealso} |
| \seelink{comparison-to-builtin-set.html} |
| {Comparison to the built-in set types} |
| {Differences between the \module{sets} module and the |
| built-in set types.} |
| \end{seealso} |
| |
| |
| \section{Mapping Types --- \class{dict} \label{typesmapping}} |
| \obindex{mapping} |
| \obindex{dictionary} |
| |
| A \dfn{mapping} object maps immutable values to |
| arbitrary objects. Mappings are mutable objects. There is currently |
| only one standard mapping type, the \dfn{dictionary}. A dictionary's keys are |
| almost arbitrary values. Only values containing lists, dictionaries |
| or other mutable types (that are compared by value rather than by |
| object identity) may not be used as keys. |
| Numeric types used for keys obey the normal rules for numeric |
| comparison: if two numbers compare equal (such as \code{1} and |
| \code{1.0}) then they can be used interchangeably to index the same |
| dictionary entry. |
| |
| Dictionaries are created by placing a comma-separated list of |
| \code{\var{key}: \var{value}} pairs within braces, for example: |
| \code{\{'jack': 4098, 'sjoerd': 4127\}} or |
| \code{\{4098: 'jack', 4127: 'sjoerd'\}}. |
| |
| The following operations are defined on mappings (where \var{a} and |
| \var{b} are mappings, \var{k} is a key, and \var{v} and \var{x} are |
| arbitrary objects): |
| \indexiii{operations on}{mapping}{types} |
| \indexiii{operations on}{dictionary}{type} |
| \stindex{del} |
| \bifuncindex{len} |
| \withsubitem{(dictionary method)}{ |
| \ttindex{clear()} |
| \ttindex{copy()} |
| \ttindex{has_key()} |
| \ttindex{fromkeys()} |
| \ttindex{items()} |
| \ttindex{keys()} |
| \ttindex{update()} |
| \ttindex{values()} |
| \ttindex{get()} |
| \ttindex{setdefault()} |
| \ttindex{pop()} |
| \ttindex{popitem()} |
| \ttindex{iteritems()} |
| \ttindex{iterkeys()} |
| \ttindex{itervalues()}} |
| |
| \begin{tableiii}{c|l|c}{code}{Operation}{Result}{Notes} |
| \lineiii{len(\var{a})}{the number of items in \var{a}}{} |
| \lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1), (10)} |
| \lineiii{\var{a}[\var{k}] = \var{v}} |
| {set \code{\var{a}[\var{k}]} to \var{v}} |
| {} |
| \lineiii{del \var{a}[\var{k}]} |
| {remove \code{\var{a}[\var{k}]} from \var{a}} |
| {(1)} |
| \lineiii{\var{a}.clear()}{remove all items from \code{a}}{} |
| \lineiii{\var{a}.copy()}{a (shallow) copy of \code{a}}{} |
| \lineiii{\var{k} in \var{a}} |
| {\code{True} if \var{a} has a key \var{k}, else \code{False}} |
| {(2)} |
| \lineiii{\var{k} not in \var{a}} |
| {Equivalent to \code{not} \var{k} in \var{a}} |
| {(2)} |
| \lineiii{\var{a}.has_key(\var{k})} |
| {Equivalent to \var{k} \code{in} \var{a}, use that form in new code} |
| {} |
| \lineiii{\var{a}.items()} |
| {a copy of \var{a}'s list of (\var{key}, \var{value}) pairs} |
| {(3)} |
| \lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(3)} |
| \lineiii{\var{a}.update(\optional{\var{b}})} |
| {updates \var{a} with key/value pairs from \var{b}, overwriting |
| existing keys, returns \code{None}} |
| {(9)} |
| \lineiii{\var{a}.fromkeys(\var{seq}\optional{, \var{value}})} |
| {Creates a new dictionary with keys from \var{seq} and values set to \var{value}} |
| {(7)} |
| \lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(3)} |
| \lineiii{\var{a}.get(\var{k}\optional{, \var{x}})} |
| {\code{\var{a}[\var{k}]} if \code{\var{k} in \var{a}}, |
| else \var{x}} |
| {(4)} |
| \lineiii{\var{a}.setdefault(\var{k}\optional{, \var{x}})} |
| {\code{\var{a}[\var{k}]} if \code{\var{k} in \var{a}}, |
| else \var{x} (also setting it)} |
| {(5)} |
| \lineiii{\var{a}.pop(\var{k}\optional{, \var{x}})} |
| {\code{\var{a}[\var{k}]} if \code{\var{k} in \var{a}}, |
| else \var{x} (and remove k)} |
| {(8)} |
| \lineiii{\var{a}.popitem()} |
| {remove and return an arbitrary (\var{key}, \var{value}) pair} |
| {(6)} |
| \lineiii{\var{a}.iteritems()} |
| {return an iterator over (\var{key}, \var{value}) pairs} |
| {(2), (3)} |
| \lineiii{\var{a}.iterkeys()} |
| {return an iterator over the mapping's keys} |
| {(2), (3)} |
| \lineiii{\var{a}.itervalues()} |
| {return an iterator over the mapping's values} |
| {(2), (3)} |
| \end{tableiii} |
| |
| \noindent |
| Notes: |
| \begin{description} |
| \item[(1)] Raises a \exception{KeyError} exception if \var{k} is not |
| in the map. |
| |
| \item[(2)] \versionadded{2.2} |
| |
| \item[(3)] Keys and values are listed in an arbitrary order which is |
| non-random, varies across Python implementations, and depends on the |
| dictionary's history of insertions and deletions. |
| If \method{items()}, \method{keys()}, \method{values()}, |
| \method{iteritems()}, \method{iterkeys()}, and \method{itervalues()} |
| are called with no intervening modifications to the dictionary, the |
| lists will directly correspond. This allows the creation of |
| \code{(\var{value}, \var{key})} pairs using \function{zip()}: |
| \samp{pairs = zip(\var{a}.values(), \var{a}.keys())}. The same |
| relationship holds for the \method{iterkeys()} and |
| \method{itervalues()} methods: \samp{pairs = zip(\var{a}.itervalues(), |
| \var{a}.iterkeys())} provides the same value for \code{pairs}. |
| Another way to create the same list is \samp{pairs = [(v, k) for (k, |
| v) in \var{a}.iteritems()]}. |
| |
| \item[(4)] Never raises an exception if \var{k} is not in the map, |
| instead it returns \var{x}. \var{x} is optional; when \var{x} is not |
| provided and \var{k} is not in the map, \code{None} is returned. |
| |
| \item[(5)] \function{setdefault()} is like \function{get()}, except |
| that if \var{k} is missing, \var{x} is both returned and inserted into |
| the dictionary as the value of \var{k}. \var{x} defaults to \var{None}. |
| |
| \item[(6)] \function{popitem()} is useful to destructively iterate |
| over a dictionary, as often used in set algorithms. If the dictionary |
| is empty, calling \function{popitem()} raises a \exception{KeyError}. |
| |
| \item[(7)] \function{fromkeys()} is a class method that returns a |
| new dictionary. \var{value} defaults to \code{None}. \versionadded{2.3} |
| |
| \item[(8)] \function{pop()} raises a \exception{KeyError} when no default |
| value is given and the key is not found. \versionadded{2.3} |
| |
| \item[(9)] \function{update()} accepts either another mapping object |
| or an iterable of key/value pairs (as a tuple or other iterable of |
| length two). If keyword arguments are specified, the mapping is |
| then is updated with those key/value pairs: |
| \samp{d.update(red=1, blue=2)}. |
| \versionchanged[Allowed the argument to be an iterable of key/value |
| pairs and allowed keyword arguments]{2.4} |
| |
| \item[(10)] If a subclass of dict defines a method \method{__missing__}, |
| if the key \var{k} is not present, the \var{a}[\var{k}] operation calls |
| that method with the key \var{k} as argument. The \var{a}[\var{k}] |
| operation then returns or raises whatever is returned or raised by the |
| \function{__missing__}(\var{k}) call if the key is not present. |
| No other operations or methods invoke \method{__missing__}(). |
| If \method{__missing__} is not defined, \exception{KeyError} is raised. |
| \method{__missing__} must be a method; it cannot be an instance variable. |
| For an example, see \module{collections}.\class{defaultdict}. |
| \versionadded{2.5} |
| |
| \end{description} |
| |
| \section{File Objects |
| \label{bltin-file-objects}} |
| |
| File objects\obindex{file} are implemented using C's \code{stdio} |
| package and can be created with the built-in constructor |
| \function{file()}\bifuncindex{file} described in section |
| \ref{built-in-funcs}, ``Built-in Functions.''\footnote{\function{file()} |
| is new in Python 2.2. The older built-in \function{open()} is an |
| alias for \function{file()}.} File objects are also returned |
| by some other built-in functions and methods, such as |
| \function{os.popen()} and \function{os.fdopen()} and the |
| \method{makefile()} method of socket objects. |
| \refstmodindex{os} |
| \refbimodindex{socket} |
| |
| When a file operation fails for an I/O-related reason, the exception |
| \exception{IOError} is raised. This includes situations where the |
| operation is not defined for some reason, like \method{seek()} on a tty |
| device or writing a file opened for reading. |
| |
| Files have the following methods: |
| |
| |
| \begin{methoddesc}[file]{close}{} |
| Close the file. A closed file cannot be read or written any more. |
| Any operation which requires that the file be open will raise a |
| \exception{ValueError} after the file has been closed. Calling |
| \method{close()} more than once is allowed. |
| |
| As of Python 2.5, you can avoid having to call this method explicitly |
| if you use the \keyword{with} statement. For example, the following |
| code will automatically close \code{f} when the \keyword{with} block |
| is exited: |
| |
| \begin{verbatim} |
| from __future__ import with_statement |
| |
| with open("hello.txt") as f: |
| for line in f: |
| print line |
| \end{verbatim} |
| |
| In older versions of Python, you would have needed to do this to get |
| the same effect: |
| |
| \begin{verbatim} |
| f = open("hello.txt") |
| try: |
| for line in f: |
| print line |
| finally: |
| f.close() |
| \end{verbatim} |
| |
| \note{Not all ``file-like'' types in Python support use as a context |
| manager for the \keyword{with} statement. If your code is intended to |
| work with any file-like object, you can use the \function{closing()} |
| function in the \module{contextlib} module instead of using the object |
| directly. See section~\ref{context-closing} for details.} |
| |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{flush}{} |
| Flush the internal buffer, like \code{stdio}'s |
| \cfunction{fflush()}. This may be a no-op on some file-like |
| objects. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{fileno}{} |
| \index{file descriptor} |
| \index{descriptor, file} |
| Return the integer ``file descriptor'' that is used by the |
| underlying implementation to request I/O operations from the |
| operating system. This can be useful for other, lower level |
| interfaces that use file descriptors, such as the |
| \refmodule{fcntl}\refbimodindex{fcntl} module or |
| \function{os.read()} and friends. \note{File-like objects |
| which do not have a real file descriptor should \emph{not} provide |
| this method!} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{isatty}{} |
| Return \code{True} if the file is connected to a tty(-like) device, else |
| \code{False}. \note{If a file-like object is not associated |
| with a real file, this method should \emph{not} be implemented.} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{next}{} |
| A file object is its own iterator, for example \code{iter(\var{f})} returns |
| \var{f} (unless \var{f} is closed). When a file is used as an |
| iterator, typically in a \keyword{for} loop (for example, |
| \code{for line in f: print line}), the \method{next()} method is |
| called repeatedly. This method returns the next input line, or raises |
| \exception{StopIteration} when \EOF{} is hit. In order to make a |
| \keyword{for} loop the most efficient way of looping over the lines of |
| a file (a very common operation), the \method{next()} method uses a |
| hidden read-ahead buffer. As a consequence of using a read-ahead |
| buffer, combining \method{next()} with other file methods (like |
| \method{readline()}) does not work right. However, using |
| \method{seek()} to reposition the file to an absolute position will |
| flush the read-ahead buffer. |
| \versionadded{2.3} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{read}{\optional{size}} |
| Read at most \var{size} bytes from the file (less if the read hits |
| \EOF{} before obtaining \var{size} bytes). If the \var{size} |
| argument is negative or omitted, read all data until \EOF{} is |
| reached. The bytes are returned as a string object. An empty |
| string is returned when \EOF{} is encountered immediately. (For |
| certain files, like ttys, it makes sense to continue reading after |
| an \EOF{} is hit.) Note that this method may call the underlying |
| C function \cfunction{fread()} more than once in an effort to |
| acquire as close to \var{size} bytes as possible. Also note that |
| when in non-blocking mode, less data than what was requested may |
| be returned, even if no \var{size} parameter was given. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{readline}{\optional{size}} |
| Read one entire line from the file. A trailing newline character is |
| kept in the string (but may be absent when a file ends with an |
| incomplete line).\footnote{ |
| The advantage of leaving the newline on is that |
| returning an empty string is then an unambiguous \EOF{} |
| indication. It is also possible (in cases where it might |
| matter, for example, if you |
| want to make an exact copy of a file while scanning its lines) |
| to tell whether the last line of a file ended in a newline |
| or not (yes this happens!). |
| } If the \var{size} argument is present and |
| non-negative, it is a maximum byte count (including the trailing |
| newline) and an incomplete line may be returned. |
| An empty string is returned \emph{only} when \EOF{} is encountered |
| immediately. \note{Unlike \code{stdio}'s \cfunction{fgets()}, the |
| returned string contains null characters (\code{'\e 0'}) if they |
| occurred in the input.} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{readlines}{\optional{sizehint}} |
| Read until \EOF{} using \method{readline()} and return a list containing |
| the lines thus read. If the optional \var{sizehint} argument is |
| present, instead of reading up to \EOF, whole lines totalling |
| approximately \var{sizehint} bytes (possibly after rounding up to an |
| internal buffer size) are read. Objects implementing a file-like |
| interface may choose to ignore \var{sizehint} if it cannot be |
| implemented, or cannot be implemented efficiently. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{seek}{offset\optional{, whence}} |
| Set the file's current position, like \code{stdio}'s \cfunction{fseek()}. |
| The \var{whence} argument is optional and defaults to \code{0} |
| (absolute file positioning); other values are \code{1} (seek |
| relative to the current position) and \code{2} (seek relative to the |
| file's end). There is no return value. Note that if the file is |
| opened for appending (mode \code{'a'} or \code{'a+'}), any |
| \method{seek()} operations will be undone at the next write. If the |
| file is only opened for writing in append mode (mode \code{'a'}), |
| this method is essentially a no-op, but it remains useful for files |
| opened in append mode with reading enabled (mode \code{'a+'}). If the |
| file is opened in text mode (without \code{'b'}), only offsets returned |
| by \method{tell()} are legal. Use of other offsets causes undefined |
| behavior. |
| |
| Note that not all file objects are seekable. |
| \versionchanged{Passing float values as offset has been deprecated}[2.6] |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{tell}{} |
| Return the file's current position, like \code{stdio}'s |
| \cfunction{ftell()}. |
| |
| \note{On Windows, \method{tell()} can return illegal values (after an |
| \cfunction{fgets()}) when reading files with \UNIX{}-style line-endings. |
| Use binary mode (\code{'rb'}) to circumvent this problem.} |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{truncate}{\optional{size}} |
| Truncate the file's size. If the optional \var{size} argument is |
| present, the file is truncated to (at most) that size. The size |
| defaults to the current position. The current file position is |
| not changed. Note that if a specified size exceeds the file's |
| current size, the result is platform-dependent: possibilities |
| include that the file may remain unchanged, increase to the specified |
| size as if zero-filled, or increase to the specified size with |
| undefined new content. |
| Availability: Windows, many \UNIX{} variants. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{write}{str} |
| Write a string to the file. There is no return value. Due to |
| buffering, the string may not actually show up in the file until |
| the \method{flush()} or \method{close()} method is called. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[file]{writelines}{sequence} |
| Write a sequence of strings to the file. The sequence can be any |
| iterable object producing strings, typically a list of strings. |
| There is no return value. |
| (The name is intended to match \method{readlines()}; |
| \method{writelines()} does not add line separators.) |
| \end{methoddesc} |
| |
| |
| Files support the iterator protocol. Each iteration returns the same |
| result as \code{\var{file}.readline()}, and iteration ends when the |
| \method{readline()} method returns an empty string. |
| |
| |
| File objects also offer a number of other interesting attributes. |
| These are not required for file-like objects, but should be |
| implemented if they make sense for the particular object. |
| |
| \begin{memberdesc}[file]{closed} |
| bool indicating the current state of the file object. This is a |
| read-only attribute; the \method{close()} method changes the value. |
| It may not be available on all file-like objects. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[file]{encoding} |
| The encoding that this file uses. When Unicode strings are written |
| to a file, they will be converted to byte strings using this encoding. |
| In addition, when the file is connected to a terminal, the attribute |
| gives the encoding that the terminal is likely to use (that |
| information might be incorrect if the user has misconfigured the |
| terminal). The attribute is read-only and may not be present on |
| all file-like objects. It may also be \code{None}, in which case |
| the file uses the system default encoding for converting Unicode |
| strings. |
| |
| \versionadded{2.3} |
| \end{memberdesc} |
| |
| \begin{memberdesc}[file]{mode} |
| The I/O mode for the file. If the file was created using the |
| \function{open()} built-in function, this will be the value of the |
| \var{mode} parameter. This is a read-only attribute and may not be |
| present on all file-like objects. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[file]{name} |
| If the file object was created using \function{open()}, the name of |
| the file. Otherwise, some string that indicates the source of the |
| file object, of the form \samp{<\mbox{\ldots}>}. This is a read-only |
| attribute and may not be present on all file-like objects. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[file]{newlines} |
| If Python was built with the \longprogramopt{with-universal-newlines} |
| option to \program{configure} (the default) this read-only attribute |
| exists, and for files opened in |
| universal newline read mode it keeps track of the types of newlines |
| encountered while reading the file. The values it can take are |
| \code{'\e r'}, \code{'\e n'}, \code{'\e r\e n'}, \code{None} (unknown, |
| no newlines read yet) or a tuple containing all the newline |
| types seen, to indicate that multiple |
| newline conventions were encountered. For files not opened in universal |
| newline read mode the value of this attribute will be \code{None}. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[file]{softspace} |
| Boolean that indicates whether a space character needs to be printed |
| before another value when using the \keyword{print} statement. |
| Classes that are trying to simulate a file object should also have a |
| writable \member{softspace} attribute, which should be initialized to |
| zero. This will be automatic for most classes implemented in Python |
| (care may be needed for objects that override attribute access); types |
| implemented in C will have to provide a writable |
| \member{softspace} attribute. |
| \note{This attribute is not used to control the |
| \keyword{print} statement, but to allow the implementation of |
| \keyword{print} to keep track of its internal state.} |
| \end{memberdesc} |
| |
| |
| \section{Context Manager Types \label{typecontextmanager}} |
| |
| \versionadded{2.5} |
| \index{context manager} |
| \index{context management protocol} |
| \index{protocol!context management} |
| |
| Python's \keyword{with} statement supports the concept of a runtime |
| context defined by a context manager. This is implemented using |
| two separate methods that allow user-defined classes to define |
| a runtime context that is entered before the statement body is |
| executed and exited when the statement ends. |
| |
| The \dfn{context management protocol} consists of a pair of |
| methods that need to be provided for a context manager object to |
| define a runtime context: |
| |
| \begin{methoddesc}[context manager]{__enter__}{} |
| Enter the runtime context and return either this object or another |
| object related to the runtime context. The value returned by this |
| method is bound to the identifier in the \keyword{as} clause of |
| \keyword{with} statements using this context manager. |
| |
| An example of a context manager that returns itself is a file object. |
| File objects return themselves from __enter__() to allow |
| \function{open()} to be used as the context expression in a |
| \keyword{with} statement. |
| |
| An example of a context manager that returns a related |
| object is the one returned by \code{decimal.Context.get_manager()}. |
| These managers set the active decimal context to a copy of the |
| original decimal context and then return the copy. This allows |
| changes to be made to the current decimal context in the body of |
| the \keyword{with} statement without affecting code outside |
| the \keyword{with} statement. |
| \end{methoddesc} |
| |
| \begin{methoddesc}[context manager]{__exit__}{exc_type, exc_val, exc_tb} |
| Exit the runtime context and return a Boolean flag indicating if any |
| expection that occurred should be suppressed. If an exception |
| occurred while executing the body of the \keyword{with} statement, the |
| arguments contain the exception type, value and traceback information. |
| Otherwise, all three arguments are \var{None}. |
| |
| Returning a true value from this method will cause the \keyword{with} |
| statement to suppress the exception and continue execution with the |
| statement immediately following the \keyword{with} statement. Otherwise |
| the exception continues propagating after this method has finished |
| executing. Exceptions that occur during execution of this method will |
| replace any exception that occurred in the body of the \keyword{with} |
| statement. |
| |
| The exception passed in should never be reraised explicitly - instead, |
| this method should return a false value to indicate that the method |
| completed successfully and does not want to suppress the raised |
| exception. This allows context management code (such as |
| \code{contextlib.nested}) to easily detect whether or not an |
| \method{__exit__()} method has actually failed. |
| \end{methoddesc} |
| |
| Python defines several context managers to support easy thread |
| synchronisation, prompt closure of files or other objects, and |
| simpler manipulation of the active decimal arithmetic |
| context. The specific types are not treated specially beyond |
| their implementation of the context management protocol. |
| |
| Python's generators and the \code{contextlib.contextfactory} decorator |
| provide a convenient way to implement these protocols. If a generator |
| function is decorated with the \code{contextlib.contextfactory} |
| decorator, it will return a context manager implementing the necessary |
| \method{__enter__()} and \method{__exit__()} methods, rather than the |
| iterator produced by an undecorated generator function. |
| |
| Note that there is no specific slot for any of these methods in the |
| type structure for Python objects in the Python/C API. Extension |
| types wanting to define these methods must provide them as a normal |
| Python accessible method. Compared to the overhead of setting up the |
| runtime context, the overhead of a single class dictionary lookup |
| is negligible. |
| |
| |
| \section{Other Built-in Types \label{typesother}} |
| |
| The interpreter supports several other kinds of objects. |
| Most of these support only one or two operations. |
| |
| |
| \subsection{Modules \label{typesmodules}} |
| |
| The only special operation on a module is attribute access: |
| \code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} |
| accesses a name defined in \var{m}'s symbol table. Module attributes |
| can be assigned to. (Note that the \keyword{import} statement is not, |
| strictly speaking, an operation on a module object; \code{import |
| \var{foo}} does not require a module object named \var{foo} to exist, |
| rather it requires an (external) \emph{definition} for a module named |
| \var{foo} somewhere.) |
| |
| A special member of every module is \member{__dict__}. |
| This is the dictionary containing the module's symbol table. |
| Modifying this dictionary will actually change the module's symbol |
| table, but direct assignment to the \member{__dict__} attribute is not |
| possible (you can write \code{\var{m}.__dict__['a'] = 1}, which |
| defines \code{\var{m}.a} to be \code{1}, but you can't write |
| \code{\var{m}.__dict__ = \{\}}). Modifying \member{__dict__} directly |
| is not recommended. |
| |
| Modules built into the interpreter are written like this: |
| \code{<module 'sys' (built-in)>}. If loaded from a file, they are |
| written as \code{<module 'os' from |
| '/usr/local/lib/python\shortversion/os.pyc'>}. |
| |
| |
| \subsection{Classes and Class Instances \label{typesobjects}} |
| \nodename{Classes and Instances} |
| |
| See chapters 3 and 7 of the \citetitle[../ref/ref.html]{Python |
| Reference Manual} for these. |
| |
| |
| \subsection{Functions \label{typesfunctions}} |
| |
| Function objects are created by function definitions. The only |
| operation on a function object is to call it: |
| \code{\var{func}(\var{argument-list})}. |
| |
| There are really two flavors of function objects: built-in functions |
| and user-defined functions. Both support the same operation (to call |
| the function), but the implementation is different, hence the |
| different object types. |
| |
| See the \citetitle[../ref/ref.html]{Python Reference Manual} for more |
| information. |
| |
| \subsection{Methods \label{typesmethods}} |
| \obindex{method} |
| |
| Methods are functions that are called using the attribute notation. |
| There are two flavors: built-in methods (such as \method{append()} on |
| lists) and class instance methods. Built-in methods are described |
| with the types that support them. |
| |
| The implementation adds two special read-only attributes to class |
| instance methods: \code{\var{m}.im_self} is the object on which the |
| method operates, and \code{\var{m}.im_func} is the function |
| implementing the method. Calling \code{\var{m}(\var{arg-1}, |
| \var{arg-2}, \textrm{\ldots}, \var{arg-n})} is completely equivalent to |
| calling \code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, |
| \var{arg-2}, \textrm{\ldots}, \var{arg-n})}. |
| |
| Class instance methods are either \emph{bound} or \emph{unbound}, |
| referring to whether the method was accessed through an instance or a |
| class, respectively. When a method is unbound, its \code{im_self} |
| attribute will be \code{None} and if called, an explicit \code{self} |
| object must be passed as the first argument. In this case, |
| \code{self} must be an instance of the unbound method's class (or a |
| subclass of that class), otherwise a \exception{TypeError} is raised. |
| |
| Like function objects, methods objects support getting |
| arbitrary attributes. However, since method attributes are actually |
| stored on the underlying function object (\code{meth.im_func}), |
| setting method attributes on either bound or unbound methods is |
| disallowed. Attempting to set a method attribute results in a |
| \exception{TypeError} being raised. In order to set a method attribute, |
| you need to explicitly set it on the underlying function object: |
| |
| \begin{verbatim} |
| class C: |
| def method(self): |
| pass |
| |
| c = C() |
| c.method.im_func.whoami = 'my name is c' |
| \end{verbatim} |
| |
| See the \citetitle[../ref/ref.html]{Python Reference Manual} for more |
| information. |
| |
| |
| \subsection{Code Objects \label{bltin-code-objects}} |
| \obindex{code} |
| |
| Code objects are used by the implementation to represent |
| ``pseudo-compiled'' executable Python code such as a function body. |
| They differ from function objects because they don't contain a |
| reference to their global execution environment. Code objects are |
| returned by the built-in \function{compile()} function and can be |
| extracted from function objects through their \member{func_code} |
| attribute. |
| \bifuncindex{compile} |
| \withsubitem{(function object attribute)}{\ttindex{func_code}} |
| |
| A code object can be executed or evaluated by passing it (instead of a |
| source string) to the \function{exec()} or \function{eval()} |
| built-in functions. |
| \bifuncindex{exec} |
| \bifuncindex{eval} |
| |
| See the \citetitle[../ref/ref.html]{Python Reference Manual} for more |
| information. |
| |
| |
| \subsection{Type Objects \label{bltin-type-objects}} |
| |
| Type objects represent the various object types. An object's type is |
| accessed by the built-in function \function{type()}. There are no special |
| operations on types. The standard module \refmodule{types} defines names |
| for all standard built-in types. |
| \bifuncindex{type} |
| \refstmodindex{types} |
| |
| Types are written like this: \code{<type 'int'>}. |
| |
| |
| \subsection{The Null Object \label{bltin-null-object}} |
| |
| This object is returned by functions that don't explicitly return a |
| value. It supports no special operations. There is exactly one null |
| object, named \code{None} (a built-in name). |
| |
| It is written as \code{None}. |
| |
| |
| \subsection{The Ellipsis Object \label{bltin-ellipsis-object}} |
| |
| This object is mostly used by extended slice notation (see the |
| \citetitle[../ref/ref.html]{Python Reference Manual}). It supports no |
| special operations. There is exactly one ellipsis object, named |
| \constant{Ellipsis} (a built-in name). |
| |
| It is written as \code{Ellipsis} or \code{...}. |
| |
| \subsection{Boolean Values} |
| |
| Boolean values are the two constant objects \code{False} and |
| \code{True}. They are used to represent truth values (although other |
| values can also be considered false or true). In numeric contexts |
| (for example when used as the argument to an arithmetic operator), |
| they behave like the integers 0 and 1, respectively. The built-in |
| function \function{bool()} can be used to cast any value to a Boolean, |
| if the value can be interpreted as a truth value (see section Truth |
| Value Testing above). |
| |
| They are written as \code{False} and \code{True}, respectively. |
| \index{False} |
| \index{True} |
| \indexii{Boolean}{values} |
| |
| |
| \subsection{Internal Objects \label{typesinternal}} |
| |
| See the \citetitle[../ref/ref.html]{Python Reference Manual} for this |
| information. It describes stack frame objects, traceback objects, and |
| slice objects. |
| |
| |
| \section{Special Attributes \label{specialattrs}} |
| |
| The implementation adds a few special read-only attributes to several |
| object types, where they are relevant. Some of these are not reported |
| by the \function{dir()} built-in function. |
| |
| \begin{memberdesc}[object]{__dict__} |
| A dictionary or other mapping object used to store an |
| object's (writable) attributes. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[object]{__methods__} |
| \deprecated{2.2}{Use the built-in function \function{dir()} to get a |
| list of an object's attributes. This attribute is no longer available.} |
| \end{memberdesc} |
| |
| \begin{memberdesc}[object]{__members__} |
| \deprecated{2.2}{Use the built-in function \function{dir()} to get a |
| list of an object's attributes. This attribute is no longer available.} |
| \end{memberdesc} |
| |
| \begin{memberdesc}[instance]{__class__} |
| The class to which a class instance belongs. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[class]{__bases__} |
| The tuple of base classes of a class object. If there are no base |
| classes, this will be an empty tuple. |
| \end{memberdesc} |
| |
| \begin{memberdesc}[class]{__name__} |
| The name of the class or type. |
| \end{memberdesc} |