Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 1 | \section{Built-in Types} |
| 2 | |
| 3 | The following sections describe the standard types that are built into |
| 4 | the interpreter. These are the numeric types, sequence types, and |
| 5 | several others, including types themselves. There is no explicit |
| 6 | Boolean type; use integers instead. |
| 7 | \indexii{built-in}{types} |
| 8 | \indexii{Boolean}{type} |
| 9 | |
| 10 | Some operations are supported by several object types; in particular, |
| 11 | all objects can be compared, tested for truth value, and converted to |
| 12 | a string (with the \code{`{\rm \ldots}`} notation). The latter conversion is |
| 13 | implicitly used when an object is written by the \code{print} statement. |
| 14 | \stindex{print} |
| 15 | |
| 16 | \subsection{Truth Value Testing} |
| 17 | |
| 18 | Any object can be tested for truth value, for use in an \code{if} or |
| 19 | \code{while} condition or as operand of the Boolean operations below. |
| 20 | The following values are false: |
| 21 | \stindex{if} |
| 22 | \stindex{while} |
| 23 | \indexii{truth}{value} |
| 24 | \indexii{Boolean}{operations} |
| 25 | \index{false} |
| 26 | |
| 27 | \begin{itemize} |
| 28 | \renewcommand{\indexsubitem}{(Built-in object)} |
| 29 | |
| 30 | \item \code{None} |
| 31 | \ttindex{None} |
| 32 | |
| 33 | \item zero of any numeric type, e.g., \code{0}, \code{0L}, \code{0.0}. |
| 34 | |
| 35 | \item any empty sequence, e.g., \code{''}, \code{()}, \code{[]}. |
| 36 | |
| 37 | \item any empty mapping, e.g., \code{\{\}}. |
| 38 | |
| 39 | \end{itemize} |
| 40 | |
| 41 | \emph{All} other values are true --- so objects of many types are |
| 42 | always true. |
| 43 | \index{true} |
| 44 | |
| 45 | \subsection{Boolean Operations} |
| 46 | |
| 47 | These are the Boolean operations: |
| 48 | \indexii{Boolean}{operations} |
| 49 | |
| 50 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 51 | \lineiii{\var{x} or \var{y}}{if \var{x} is false, then \var{y}, else \var{x}}{(1)} |
| 52 | \lineiii{\var{x} and \var{y}}{if \var{x} is false, then \var{x}, else \var{y}}{(1)} |
| 53 | \lineiii{not \var{x}}{if \var{x} is false, then \code{1}, else \code{0}}{} |
| 54 | \end{tableiii} |
| 55 | \opindex{and} |
| 56 | \opindex{or} |
| 57 | \opindex{not} |
| 58 | |
| 59 | \noindent |
| 60 | Notes: |
| 61 | |
| 62 | \begin{description} |
| 63 | |
| 64 | \item[(1)] |
| 65 | These only evaluate their second argument if needed for their outcome. |
| 66 | |
| 67 | \end{description} |
| 68 | |
| 69 | \subsection{Comparisons} |
| 70 | |
| 71 | Comparison operations are supported by all objects: |
| 72 | |
| 73 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Meaning}{Notes} |
| 74 | \lineiii{<}{strictly less than}{} |
| 75 | \lineiii{<=}{less than or equal}{} |
| 76 | \lineiii{>}{strictly greater than}{} |
| 77 | \lineiii{>=}{greater than or equal}{} |
| 78 | \lineiii{==}{equal}{} |
| 79 | \lineiii{<>}{not equal}{(1)} |
| 80 | \lineiii{!=}{not equal}{(1)} |
| 81 | \lineiii{is}{object identity}{} |
| 82 | \lineiii{is not}{negated object identity}{} |
| 83 | \end{tableiii} |
| 84 | \indexii{operator}{comparison} |
| 85 | \opindex{==} % XXX *All* others have funny characters < ! > |
| 86 | \opindex{is} |
| 87 | \opindex{is not} |
| 88 | |
| 89 | \noindent |
| 90 | Notes: |
| 91 | |
| 92 | \begin{description} |
| 93 | |
| 94 | \item[(1)] |
| 95 | \code{<>} and \code{!=} are alternate spellings for the same operator. |
| 96 | (I couldn't choose between \ABC{} and \C{}! :-) |
| 97 | \indexii{\ABC{}}{language} |
| 98 | \indexii{\C{}}{language} |
| 99 | |
| 100 | \end{description} |
| 101 | |
| 102 | Objects of different types, except different numeric types, never |
| 103 | compare equal; such objects are ordered consistently but arbitrarily |
| 104 | (so that sorting a heterogeneous array yields a consistent result). |
| 105 | Furthermore, some types (e.g., windows) support only a degenerate |
| 106 | notion of comparison where any two objects of that type are unequal. |
| 107 | Again, such objects are ordered arbitrarily but consistently. |
| 108 | \indexii{types}{numeric} |
| 109 | \indexii{objects}{comparing} |
| 110 | |
| 111 | (Implementation note: objects of different types except numbers are |
| 112 | ordered by their type names; objects of the same types that don't |
| 113 | support proper comparison are ordered by their address.) |
| 114 | |
| 115 | Two more operations with the same syntactic priority, \code{in} and |
| 116 | \code{not in}, are supported only by sequence types (below). |
| 117 | \opindex{in} |
| 118 | \opindex{not in} |
| 119 | |
| 120 | \subsection{Numeric Types} |
| 121 | |
| 122 | There are three numeric types: \dfn{plain integers}, \dfn{long integers}, and |
| 123 | \dfn{floating point numbers}. Plain integers (also just called \dfn{integers}) |
| 124 | are implemented using \code{long} in \C{}, which gives them at least 32 |
| 125 | bits of precision. Long integers have unlimited precision. Floating |
| 126 | point numbers are implemented using \code{double} in \C{}. All bets on |
| 127 | their precision are off unless you happen to know the machine you are |
| 128 | working with. |
| 129 | \indexii{numeric}{types} |
| 130 | \indexii{integer}{types} |
| 131 | \indexii{integer}{type} |
| 132 | \indexiii{long}{integer}{type} |
| 133 | \indexii{floating point}{type} |
| 134 | \indexii{\C{}}{language} |
| 135 | |
| 136 | Numbers are created by numeric literals or as the result of built-in |
| 137 | functions and operators. Unadorned integer literals (including hex |
| 138 | and octal numbers) yield plain integers. Integer literals with an \samp{L} |
| 139 | or \samp{l} suffix yield long integers |
| 140 | (\samp{L} is preferred because \code{1l} looks too much like eleven!). |
| 141 | Numeric literals containing a decimal point or an exponent sign yield |
| 142 | floating point numbers. |
| 143 | \indexii{numeric}{literals} |
| 144 | \indexii{integer}{literals} |
| 145 | \indexiii{long}{integer}{literals} |
| 146 | \indexii{floating point}{literals} |
| 147 | \indexii{hexadecimal}{literals} |
| 148 | \indexii{octal}{literals} |
| 149 | |
| 150 | Python fully supports mixed arithmetic: when a binary arithmetic |
| 151 | operator has operands of different numeric types, the operand with the |
| 152 | ``smaller'' type is converted to that of the other, where plain |
| 153 | integer is smaller than long integer is smaller than floating point. |
| 154 | Comparisons between numbers of mixed type use the same rule.% |
| 155 | \footnote{As a consequence, the list \code{[1, 2]} is considered equal |
| 156 | to \code{[1.0, 2.0]}, and similar for tuples.} |
| 157 | The functions \code{int()}, \code{long()} and \code{float()} can be used |
| 158 | to coerce numbers to a specific type. |
| 159 | \index{arithmetic} |
| 160 | \bifuncindex{int} |
| 161 | \bifuncindex{long} |
| 162 | \bifuncindex{float} |
| 163 | |
| 164 | All numeric types support the following operations: |
| 165 | |
| 166 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 167 | \lineiii{abs(\var{x})}{absolute value of \var{x}}{} |
| 168 | \lineiii{int(\var{x})}{\var{x} converted to integer}{(1)} |
| 169 | \lineiii{long(\var{x})}{\var{x} converted to long integer}{(1)} |
| 170 | \lineiii{float(\var{x})}{\var{x} converted to floating point}{} |
| 171 | \lineiii{-\var{x}}{\var{x} negated}{} |
| 172 | \lineiii{+\var{x}}{\var{x} unchanged}{} |
| 173 | \lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{} |
| 174 | \lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{} |
| 175 | \lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{} |
| 176 | \lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(2)} |
| 177 | \lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{} |
| 178 | \lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} / \var{y}, \var{x} \%{} \var{y})}}{(3)} |
| 179 | \lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{} |
| 180 | \end{tableiii} |
| 181 | \indexiii{operations on}{numeric}{types} |
| 182 | |
| 183 | \noindent |
| 184 | Notes: |
| 185 | \begin{description} |
| 186 | \item[(1)] |
| 187 | Conversion from floating point to (long or plain) integer may round or |
| 188 | % XXXJH xref here |
| 189 | truncate as in \C{}; see functions \code{floor} and \code{ceil} in module |
| 190 | \code{math} for well-defined conversions. |
| 191 | \indexii{numeric}{conversions} |
| 192 | \ttindex{math} |
| 193 | \indexii{\C{}}{language} |
| 194 | |
| 195 | \item[(2)] |
| 196 | For (plain or long) integer division, the result is an integer; it |
| 197 | always truncates towards zero. |
| 198 | % XXXJH integer division is better defined nowadays |
| 199 | \indexii{integer}{division} |
| 200 | \indexiii{long}{integer}{division} |
| 201 | |
| 202 | \item[(3)] |
| 203 | See the section on built-in functions for an exact definition. |
| 204 | |
| 205 | \end{description} |
| 206 | % XXXJH exceptions: overflow (when? what operations?) zerodivision |
| 207 | |
| 208 | \subsubsection{Bit-string Operations on Integer Types.} |
| 209 | |
| 210 | Plain and long integer types support additional operations that make |
| 211 | sense only for bit-strings. Negative numbers are treated as their 2's |
| 212 | complement value: |
| 213 | |
| 214 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 215 | \lineiii{\~\var{x}}{the bits of \var{x} inverted}{} |
| 216 | \lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{} |
| 217 | \lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{} |
| 218 | \lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{} |
| 219 | \lineiii{\var{x} << \var{n}}{\var{x} shifted left by \var{n} bits}{} |
| 220 | \lineiii{\var{x} >> \var{n}}{\var{x} shifted right by \var{n} bits}{} |
| 221 | \end{tableiii} |
| 222 | % XXXJH what's `left'? `right'? maybe better use lsb or msb or something |
| 223 | \indexiii{operations on}{integer}{types} |
| 224 | \indexii{bit-string}{operations} |
| 225 | \indexii{shifting}{operations} |
| 226 | \indexii{masking}{operations} |
| 227 | |
| 228 | \subsection{Sequence Types} |
| 229 | |
| 230 | There are three sequence types: strings, lists and tuples. |
| 231 | Strings literals are written in single quotes: \code{'xyzzy'}. |
| 232 | Lists are constructed with square brackets, |
| 233 | separating items with commas: |
| 234 | \code{[a, b, c]}. |
| 235 | Tuples are constructed by the comma operator |
| 236 | (not within square brackets), with or without enclosing parentheses, |
| 237 | but an empty tuple must have the enclosing parentheses, e.g., |
| 238 | \code{a, b, c} or \code{()}. A single item tuple must have a trailing comma, |
| 239 | e.g., \code{(d,)}. |
| 240 | \indexii{sequence}{types} |
| 241 | \indexii{string}{type} |
| 242 | \indexii{tuple}{type} |
| 243 | \indexii{list}{type} |
| 244 | |
| 245 | Sequence types support the following operations (\var{s} and \var{t} are |
| 246 | sequences of the same type; \var{n}, \var{i} and \var{j} are integers): |
| 247 | |
| 248 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 249 | \lineiii{len(\var{s})}{length of \var{s}}{} |
| 250 | \lineiii{min(\var{s})}{smallest item of \var{s}}{} |
| 251 | \lineiii{max(\var{s})}{largest item of \var{s}}{} |
| 252 | \lineiii{\var{x} in \var{s}}{\code{1} if an item of \var{s} is equal to \var{x}, else \code{0}}{} |
| 253 | \lineiii{\var{x} not in \var{s}}{\code{0} if an item of \var{s} is equal to \var{x}, else \code{1}}{} |
| 254 | \lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{} |
| 255 | \lineiii{\var{s} * \var{n}{\rm ,} \var{n} * \var{s}}{\var{n} copies of \var{s} concatenated}{} |
| 256 | \lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(1)} |
| 257 | \lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(1), (2)} |
| 258 | \end{tableiii} |
| 259 | \indexiii{operations on}{sequence}{types} |
| 260 | \bifuncindex{len} |
| 261 | \bifuncindex{min} |
| 262 | \bifuncindex{max} |
| 263 | \indexii{concatenation}{operation} |
| 264 | \indexii{repetition}{operation} |
| 265 | \indexii{subscript}{operation} |
| 266 | \indexii{slice}{operation} |
| 267 | \opindex{in} |
| 268 | \opindex{not in} |
| 269 | |
| 270 | \noindent |
| 271 | Notes: |
| 272 | |
| 273 | % XXXJH all TeX-math expressions replaced by python-syntax expressions |
| 274 | \begin{description} |
| 275 | |
| 276 | \item[(1)] If \var{i} or \var{j} is negative, the index is relative to |
| 277 | the end of the string, i.e., \code{len(\var{s}) + \var{i}} or |
| 278 | \code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is |
| 279 | still \code{0}. |
| 280 | |
| 281 | \item[(2)] The slice of \var{s} from \var{i} to \var{j} is defined as |
| 282 | the sequence of items with index \var{k} such that \code{\var{i} <= |
| 283 | \var{k} < \var{j}}. If \var{i} or \var{j} is greater than |
| 284 | \code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted, |
| 285 | use \code{0}. If \var{j} is omitted, use \code{len(\var{s})}. If |
| 286 | \var{i} is greater than or equal to \var{j}, the slice is empty. |
| 287 | |
| 288 | \end{description} |
| 289 | |
| 290 | \subsubsection{More String Operations.} |
| 291 | |
| 292 | String objects have one unique built-in operation: the \code{\%} |
| 293 | operator (modulo) with a string left argument interprets this string |
| 294 | as a C sprintf format string to be applied to the right argument, and |
| 295 | returns the string resulting from this formatting operation. |
| 296 | |
Guido van Rossum | 8b605eb | 1994-06-23 12:14:07 +0000 | [diff] [blame] | 297 | The right argument should be a tuple with one item for each argument |
| 298 | required by the format string; if the string requires a single |
| 299 | argument, the right argument may also be a single non-tuple object.% |
| 300 | \footnote{A tuple object in this case should be a singleton.} |
| 301 | The following format characters are understood: |
| 302 | \%, c, s, i, d, u, o, x, X, e, E, f, g, G. |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 303 | Width and precision may be a * to specify that an integer argument |
| 304 | specifies the actual width or precision. The flag characters -, +, |
| 305 | blank, \# and 0 are understood. The size specifiers h, l or L may be |
Guido van Rossum | 1738311 | 1994-04-21 10:32:28 +0000 | [diff] [blame] | 306 | present but are ignored. The \code{\%s} conversion takes any Python |
| 307 | object and converts it to a string using \code{str()} before |
| 308 | formatting it. The ANSI features \code{\%p} and \code{\%n} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 309 | are not supported. Since Python strings have an explicit length, |
| 310 | \code{\%s} conversions don't assume that \code{'\\0'} is the end of |
| 311 | the string. |
| 312 | |
Guido van Rossum | e6ef032 | 1994-05-09 14:54:24 +0000 | [diff] [blame] | 313 | For safety reasons, floating point precisions are clipped to 50; |
| 314 | \code{\%f} conversions for numbers whose absolute value is over 1e25 |
| 315 | are replaced by \code{\%g} conversions.% |
| 316 | \footnote{These numbers are fairly arbitrary. They are intended to |
| 317 | avoid printing endless strings of meaningless digits without hampering |
| 318 | correct use and without having to know the exact precision of floating |
| 319 | point values on a particular machine.} |
| 320 | All other errors raise exceptions. |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 321 | |
Guido van Rossum | 1738311 | 1994-04-21 10:32:28 +0000 | [diff] [blame] | 322 | If the right argument is a dictionary (or any kind of mapping), then |
| 323 | the formats in the string must have a parenthesized key into that |
| 324 | dictionary inserted immediately after the \code{\%} character, and |
| 325 | each format formats the corresponding entry from the mapping. E.g. |
| 326 | \begin{verbatim} |
| 327 | >>> count = 2 |
| 328 | >>> language = 'Python' |
| 329 | >>> print '%(language)s has %(count)03d quote types.' % vars() |
| 330 | Python has 002 quote types. |
| 331 | >>> |
| 332 | \end{verbatim} |
| 333 | In this case no * specifiers may occur in a format. |
| 334 | |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 335 | Additional string operations are defined in standard module |
| 336 | \code{string} and in built-in module \code{regex}. |
| 337 | \index{string} |
| 338 | \index{regex} |
| 339 | |
| 340 | \subsubsection{Mutable Sequence Types.} |
| 341 | |
| 342 | List objects support additional operations that allow in-place |
| 343 | modification of the object. |
| 344 | These operations would be supported by other mutable sequence types |
| 345 | (when added to the language) as well. |
| 346 | Strings and tuples are immutable sequence types and such objects cannot |
| 347 | be modified once created. |
| 348 | The following operations are defined on mutable sequence types (where |
| 349 | \var{x} is an arbitrary object): |
| 350 | \indexiii{mutable}{sequence}{types} |
| 351 | \indexii{list}{type} |
| 352 | |
| 353 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 354 | \lineiii{\var{s}[\var{i}] = \var{x}} |
| 355 | {item \var{i} of \var{s} is replaced by \var{x}}{} |
| 356 | \lineiii{\var{s}[\var{i}:\var{j}] = \var{t}} |
| 357 | {slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{} |
| 358 | \lineiii{del \var{s}[\var{i}:\var{j}]} |
| 359 | {same as \code{\var{s}[\var{i}:\var{j}] = []}}{} |
| 360 | \lineiii{\var{s}.append(\var{x})} |
Guido van Rossum | e6ef032 | 1994-05-09 14:54:24 +0000 | [diff] [blame] | 361 | {same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 362 | \lineiii{\var{s}.count(\var{x})} |
| 363 | {return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{} |
| 364 | \lineiii{\var{s}.index(\var{x})} |
| 365 | {return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(1)} |
| 366 | \lineiii{\var{s}.insert(\var{i}, \var{x})} |
| 367 | {same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}}{} |
| 368 | \lineiii{\var{s}.remove(\var{x})} |
| 369 | {same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(1)} |
| 370 | \lineiii{\var{s}.reverse()} |
| 371 | {reverses the items of \var{s} in place}{} |
| 372 | \lineiii{\var{s}.sort()} |
| 373 | {permutes the items of \var{s} to satisfy |
| 374 | \code{\var{s}[\var{i}] <= \var{s}[\var{j}]}, |
| 375 | for \code{\var{i} < \var{j}}}{(2)} |
| 376 | \end{tableiii} |
| 377 | \indexiv{operations on}{mutable}{sequence}{types} |
| 378 | \indexiii{operations on}{sequence}{types} |
| 379 | \indexiii{operations on}{list}{type} |
| 380 | \indexii{subscript}{assignment} |
| 381 | \indexii{slice}{assignment} |
| 382 | \stindex{del} |
| 383 | \renewcommand{\indexsubitem}{(list method)} |
| 384 | \ttindex{append} |
| 385 | \ttindex{count} |
| 386 | \ttindex{index} |
| 387 | \ttindex{insert} |
| 388 | \ttindex{remove} |
| 389 | \ttindex{reverse} |
| 390 | \ttindex{sort} |
| 391 | |
| 392 | \noindent |
| 393 | Notes: |
| 394 | \begin{description} |
| 395 | \item[(1)] Raises an exception when \var{x} is not found in \var{s}. |
| 396 | |
| 397 | \item[(2)] The \code{sort()} method takes an optional argument |
| 398 | specifying a comparison function of two arguments (list items) which |
| 399 | should return \code{-1}, \code{0} or \code{1} depending on whether the |
| 400 | first argument is considered smaller than, equal to, or larger than the |
| 401 | second argument. Note that this slows the sorting process down |
| 402 | considerably; e.g. to sort an array in reverse order it is much faster |
| 403 | to use calls to \code{sort()} and \code{reverse()} than to use |
| 404 | \code{sort()} with a comparison function that reverses the ordering of |
| 405 | the elements. |
| 406 | \end{description} |
| 407 | |
| 408 | \subsection{Mapping Types} |
| 409 | |
| 410 | A \dfn{mapping} object maps values of one type (the key type) to |
| 411 | arbitrary objects. Mappings are mutable objects. There is currently |
| 412 | only one mapping type, the \dfn{dictionary}. A dictionary's keys are |
| 413 | almost arbitrary values. The only types of values not acceptable as |
| 414 | keys are values containing lists or dictionaries or other mutable |
| 415 | types that are compared by value rather than by object identity. |
| 416 | Numeric types used for keys obey the normal rules for numeric |
| 417 | comparison: if two numbers compare equal (e.g. 1 and 1.0) then they |
| 418 | can be used interchangeably to index the same dictionary entry. |
| 419 | |
| 420 | \indexii{mapping}{types} |
| 421 | \indexii{dictionary}{type} |
| 422 | |
| 423 | Dictionaries are created by placing a comma-separated list of |
| 424 | \code{\var{key}: \var{value}} pairs within braces, for example: |
| 425 | \code{\{'jack': 4098, 'sjoerd: 4127\}} or |
| 426 | \code{\{4098: 'jack', 4127: 'sjoerd\}}. |
| 427 | |
| 428 | The following operations are defined on mappings (where \var{a} is a |
| 429 | mapping, \var{k} is a key and \var{x} is an arbitrary object): |
| 430 | |
| 431 | \begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes} |
| 432 | \lineiii{len(\var{a})}{the number of items in \var{a}}{} |
| 433 | \lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1)} |
| 434 | \lineiii{\var{a}[\var{k}] = \var{x}}{set \code{\var{a}[\var{k}]} to \var{x}}{} |
| 435 | \lineiii{del \var{a}[\var{k}]}{remove \code{\var{a}[\var{k}]} from \var{a}}{(1)} |
| 436 | \lineiii{\var{a}.items()}{a copy of \var{a}'s list of (key, item) pairs}{(2)} |
| 437 | \lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(2)} |
| 438 | \lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(2)} |
| 439 | \lineiii{\var{a}.has_key(\var{k})}{\code{1} if \var{a} has a key \var{k}, else \code{0}}{} |
| 440 | \end{tableiii} |
| 441 | \indexiii{operations on}{mapping}{types} |
| 442 | \indexiii{operations on}{dictionary}{type} |
| 443 | \stindex{del} |
| 444 | \bifuncindex{len} |
| 445 | \renewcommand{\indexsubitem}{(dictionary method)} |
| 446 | \ttindex{keys} |
| 447 | \ttindex{has_key} |
| 448 | |
| 449 | % XXXJH some lines above, you talk about `true', elsewhere you |
| 450 | % explicitely states \code{0} or \code{1}. |
| 451 | \noindent |
| 452 | Notes: |
| 453 | \begin{description} |
| 454 | \item[(1)] Raises an exception if \var{k} is not in the map. |
| 455 | |
| 456 | \item[(2)] Keys and values are listed in random order, but at any |
| 457 | moment the ordering of the \code{keys()}, \code{values()} and |
| 458 | \code{items()} lists is the consistent with each other. |
| 459 | \end{description} |
| 460 | |
| 461 | \subsection{Other Built-in Types} |
| 462 | |
| 463 | The interpreter supports several other kinds of objects. |
| 464 | Most of these support only one or two operations. |
| 465 | |
| 466 | \subsubsection{Modules.} |
| 467 | |
| 468 | The only special operation on a module is attribute access: |
| 469 | \code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} accesses |
| 470 | a name defined in \var{m}'s symbol table. Module attributes can be |
| 471 | assigned to. (Note that the \code{import} statement is not, strictly |
| 472 | spoken, an operation on a module object; \code{import \var{foo}} does not |
| 473 | require a module object named \var{foo} to exist, rather it requires |
| 474 | an (external) \emph{definition} for a module named \var{foo} |
| 475 | somewhere.) |
| 476 | |
| 477 | A special member of every module is \code{__dict__}. |
| 478 | This is the dictionary containing the module's symbol table. |
| 479 | Modifying this dictionary will actually change the module's symbol |
| 480 | table, but direct assignment to the \code{__dict__} attribute is not |
| 481 | possible (i.e., you can write \code{\var{m}.__dict__['a'] = 1}, which |
| 482 | defines \code{\var{m}.a} to be \code{1}, but you can't write \code{\var{m}.__dict__ = \{\}}. |
| 483 | |
| 484 | Modules are written like this: \code{<module 'sys'>}. |
| 485 | |
| 486 | \subsubsection{Classes and Class Instances.} |
| 487 | % XXXJH cross ref here |
| 488 | (See the Python Reference Manual for these.) |
| 489 | |
| 490 | \subsubsection{Functions.} |
| 491 | |
| 492 | Function objects are created by function definitions. The only |
| 493 | operation on a function object is to call it: |
| 494 | \code{\var{func}(\var{argument-list})}. |
| 495 | |
| 496 | There are really two flavors of function objects: built-in functions |
| 497 | and user-defined functions. Both support the same operation (to call |
| 498 | the function), but the implementation is different, hence the |
| 499 | different object types. |
| 500 | |
| 501 | The implementation adds two special read-only attributes: |
| 502 | \code{\var{f}.func_code} is a function's \dfn{code object} (see below) and |
| 503 | \code{\var{f}.func_globals} is the dictionary used as the function's |
| 504 | global name space (this is the same as \code{\var{m}.__dict__} where |
| 505 | \var{m} is the module in which the function \var{f} was defined). |
| 506 | |
| 507 | \subsubsection{Methods.} |
| 508 | |
| 509 | Methods are functions that are called using the attribute notation. |
| 510 | There are two flavors: built-in methods (such as \code{append()} on |
| 511 | lists) and class instance methods. Built-in methods are described |
| 512 | with the types that support them. |
| 513 | |
| 514 | The implementation adds two special read-only attributes to class |
| 515 | instance methods: \code{\var{m}.im_self} is the object whose method this |
| 516 | is, and \code{\var{m}.im_func} is the function implementing the method. |
| 517 | Calling \code{\var{m}(\var{arg-1}, \var{arg-2}, {\rm \ldots}, |
| 518 | \var{arg-n})} is completely equivalent to calling |
| 519 | \code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, \var{arg-2}, {\rm |
| 520 | \ldots}, \var{arg-n})}. |
| 521 | |
| 522 | (See the Python Reference Manual for more info.) |
| 523 | |
| 524 | \subsubsection{Type Objects.} |
| 525 | |
| 526 | Type objects represent the various object types. An object's type is |
| 527 | % XXXJH xref here |
| 528 | accessed by the built-in function \code{type()}. There are no special |
| 529 | operations on types. |
| 530 | |
| 531 | Types are written like this: \code{<type 'int'>}. |
| 532 | |
| 533 | \subsubsection{The Null Object.} |
| 534 | |
| 535 | This object is returned by functions that don't explicitly return a |
| 536 | value. It supports no special operations. There is exactly one null |
| 537 | object, named \code{None} (a built-in name). |
| 538 | |
| 539 | It is written as \code{None}. |
| 540 | |
| 541 | \subsubsection{File Objects.} |
| 542 | |
| 543 | File objects are implemented using \C{}'s \code{stdio} package and can be |
| 544 | % XXXJH xref here |
| 545 | created with the built-in function \code{open()} described under |
| 546 | Built-in Functions below. |
| 547 | |
| 548 | When a file operation fails for an I/O-related reason, the exception |
| 549 | \code{IOError} is raised. This includes situations where the |
| 550 | operation is not defined for some reason, like \code{seek()} on a tty |
| 551 | device or writing a file opened for reading. |
| 552 | |
| 553 | Files have the following methods: |
| 554 | |
| 555 | |
| 556 | \renewcommand{\indexsubitem}{(file method)} |
| 557 | |
| 558 | \begin{funcdesc}{close}{} |
| 559 | Close the file. A closed file cannot be read or written anymore. |
| 560 | \end{funcdesc} |
| 561 | |
| 562 | \begin{funcdesc}{flush}{} |
| 563 | Flush the internal buffer, like \code{stdio}'s \code{fflush()}. |
| 564 | \end{funcdesc} |
| 565 | |
| 566 | \begin{funcdesc}{isatty}{} |
| 567 | Return \code{1} if the file is connected to a tty(-like) device, else |
| 568 | \code{0}. |
| 569 | \end{funcdesc} |
| 570 | |
| 571 | \begin{funcdesc}{read}{size} |
| 572 | Read at most \var{size} bytes from the file (less if the read hits |
| 573 | \EOF{} or no more data is immediately available on a pipe, tty or |
| 574 | similar device). If the \var{size} argument is omitted, read all |
| 575 | data until \EOF{} is reached. The bytes are returned as a string |
| 576 | object. An empty string is returned when \EOF{} is encountered |
| 577 | immediately. (For certain files, like ttys, it makes sense to |
| 578 | continue reading after an \EOF{} is hit.) |
| 579 | \end{funcdesc} |
| 580 | |
| 581 | \begin{funcdesc}{readline}{} |
| 582 | Read one entire line from the file. A trailing newline character is |
| 583 | kept in the string (but may be absent when a file ends with an |
| 584 | incomplete line). An empty string is returned when \EOF{} is hit |
| 585 | immediately. Note: unlike \code{stdio}'s \code{fgets()}, the returned |
| 586 | string contains null characters (\code{'\e 0'}) if they occurred in the |
| 587 | input. |
| 588 | \end{funcdesc} |
| 589 | |
| 590 | \begin{funcdesc}{readlines}{} |
| 591 | Read until \EOF{} using \code{readline()} and return a list containing |
| 592 | the lines thus read. |
| 593 | \end{funcdesc} |
| 594 | |
| 595 | \begin{funcdesc}{seek}{offset\, whence} |
| 596 | Set the file's current position, like \code{stdio}'s \code{fseek()}. |
| 597 | The \var{whence} argument is optional and defaults to \code{0} |
| 598 | (absolute file positioning); other values are \code{1} (seek |
| 599 | relative to the current position) and \code{2} (seek relative to the |
| 600 | file's end). There is no return value. |
| 601 | \end{funcdesc} |
| 602 | |
| 603 | \begin{funcdesc}{tell}{} |
| 604 | Return the file's current position, like \code{stdio}'s \code{ftell()}. |
| 605 | \end{funcdesc} |
| 606 | |
| 607 | \begin{funcdesc}{write}{str} |
| 608 | Write a string to the file. There is no return value. |
| 609 | \end{funcdesc} |
| 610 | |
Guido van Rossum | 8b605eb | 1994-06-23 12:14:07 +0000 | [diff] [blame] | 611 | \begin{funcdesc}{writelines}{list} |
| 612 | Write a list of strings to the file. There is no return value. |
| 613 | (The name is intended to match \code{readlines}; \code{writelines} |
| 614 | does not add line separators.) |
| 615 | \end{funcdesc} |
| 616 | |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 617 | \subsubsection{Internal Objects.} |
| 618 | |
| 619 | (See the Python Reference Manual for these.) |
| 620 | |
| 621 | \subsection{Special Attributes} |
| 622 | |
| 623 | The implementation adds a few special read-only attributes to several |
| 624 | object types, where they are relevant: |
| 625 | |
| 626 | \begin{itemize} |
| 627 | |
| 628 | \item |
| 629 | \code{\var{x}.__dict__} is a dictionary of some sort used to store an |
| 630 | object's (writable) attributes; |
| 631 | |
| 632 | \item |
| 633 | \code{\var{x}.__methods__} lists the methods of many built-in object types, |
| 634 | e.g., \code{[].__methods__} is |
| 635 | % XXXJH results in?, yields?, written down as an example |
| 636 | \code{['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']}; |
| 637 | |
| 638 | \item |
| 639 | \code{\var{x}.__members__} lists data attributes; |
| 640 | |
| 641 | \item |
| 642 | \code{\var{x}.__class__} is the class to which a class instance belongs; |
| 643 | |
| 644 | \item |
| 645 | \code{\var{x}.__bases__} is the tuple of base classes of a class object. |
| 646 | |
| 647 | \end{itemize} |