Doc/reference/lexical_analysis.rst

.. _lexical:

****************
Lexical analysis
****************

.. index:: lexical analysis, parser, token

A Python program is read by a *parser*.  Input to the parser is a stream of
*tokens*, generated by the *lexical analyzer*.  This chapter describes how the
lexical analyzer breaks a file into tokens.

Python reads program text as Unicode code points; the encoding of a source file
can be given by an encoding declaration and defaults to UTF-8, see :pep:`3120`
for details.  If the source file cannot be decoded, a :exc:`SyntaxError` is
raised.


.. _line-structure:

Line structure
==============

.. index:: line structure

A Python program is divided into a number of *logical lines*.


.. _logical-lines:

Logical lines
-------------

.. index:: logical line, physical line, line joining, NEWLINE token

The end of a logical line is represented by the token NEWLINE.  Statements
cannot cross logical line boundaries except where NEWLINE is allowed by the
syntax (e.g., between statements in compound statements). A logical line is
constructed from one or more *physical lines* by following the explicit or
implicit *line joining* rules.


.. _physical-lines:

Physical lines
--------------

A physical line is a sequence of characters terminated by an end-of-line
sequence.  In source files, any of the standard platform line termination
sequences can be used - the Unix form using ASCII LF (linefeed), the Windows
form using the ASCII sequence CR LF (return followed by linefeed), or the
Macintosh form using the ASCII CR (return) character.  All of these forms can be
used equally, regardless of platform.

When embedding Python, source code strings should be passed to Python APIs using
the standard C conventions for newline characters (the ``\n`` character,
representing ASCII LF, is the line terminator).


.. _comments:

Comments
--------

.. index:: comment, hash character

A comment starts with a hash character (``#``) that is not part of a string
literal, and ends at the end of the physical line.  A comment signifies the end
of the logical line unless the implicit line joining rules are invoked. Comments
are ignored by the syntax; they are not tokens.


.. _encodings:

Encoding declarations
---------------------

.. index:: source character set, encodings

If a comment in the first or second line of the Python script matches the
regular expression ``coding[=:]\s*([-\w.]+)``, this comment is processed as an
encoding declaration; the first group of this expression names the encoding of
the source code file. The recommended forms of this expression are ::

   # -*- coding: <encoding-name> -*-

which is recognized also by GNU Emacs, and ::

   # vim:fileencoding=<encoding-name>

which is recognized by Bram Moolenaar's VIM.

If no encoding declaration is found, the default encoding is UTF-8.  In
addition, if the first bytes of the file are the UTF-8 byte-order mark
(``b'\xef\xbb\xbf'``), the declared file encoding is UTF-8 (this is supported,
among others, by Microsoft's :program:`notepad`).

If an encoding is declared, the encoding name must be recognized by Python. The
encoding is used for all lexical analysis, including string literals, comments
and identifiers. The encoding declaration must appear on a line of its own.

.. XXX there should be a list of supported encodings.


.. _explicit-joining:

Explicit line joining
---------------------

.. index:: physical line, line joining, line continuation, backslash character

Two or more physical lines may be joined into logical lines using backslash
characters (``\``), as follows: when a physical line ends in a backslash that is
not part of a string literal or comment, it is joined with the following forming
a single logical line, deleting the backslash and the following end-of-line
character.  For example::

   if 1900 < year < 2100 and 1 <= month <= 12 \
      and 1 <= day <= 31 and 0 <= hour < 24 \
      and 0 <= minute < 60 and 0 <= second < 60:   # Looks like a valid date
           return 1

A line ending in a backslash cannot carry a comment.  A backslash does not
continue a comment.  A backslash does not continue a token except for string
literals (i.e., tokens other than string literals cannot be split across
physical lines using a backslash).  A backslash is illegal elsewhere on a line
outside a string literal.


.. _implicit-joining:

Implicit line joining
---------------------

Expressions in parentheses, square brackets or curly braces can be split over
more than one physical line without using backslashes. For example::

   month_names = ['Januari', 'Februari', 'Maart',      # These are the
                  'April',   'Mei',      'Juni',       # Dutch names
                  'Juli',    'Augustus', 'September',  # for the months
                  'Oktober', 'November', 'December']   # of the year

Implicitly continued lines can carry comments.  The indentation of the
continuation lines is not important.  Blank continuation lines are allowed.
There is no NEWLINE token between implicit continuation lines.  Implicitly
continued lines can also occur within triple-quoted strings (see below); in that
case they cannot carry comments.


.. _blank-lines:

Blank lines
-----------

.. index:: single: blank line

A logical line that contains only spaces, tabs, formfeeds and possibly a
comment, is ignored (i.e., no NEWLINE token is generated).  During interactive
input of statements, handling of a blank line may differ depending on the
implementation of the read-eval-print loop.  In the standard interactive
interpreter, an entirely blank logical line (i.e. one containing not even
whitespace or a comment) terminates a multi-line statement.


.. _indentation:

Indentation
-----------

.. index:: indentation, leading whitespace, space, tab, grouping, statement grouping

Leading whitespace (spaces and tabs) at the beginning of a logical line is used
to compute the indentation level of the line, which in turn is used to determine
the grouping of statements.

First, tabs are replaced (from left to right) by one to eight spaces such that
the total number of characters up to and including the replacement is a multiple
of eight (this is intended to be the same rule as used by Unix).  The total
number of spaces preceding the first non-blank character then determines the
line's indentation.  Indentation cannot be split over multiple physical lines
using backslashes; the whitespace up to the first backslash determines the
indentation.

**Cross-platform compatibility note:** because of the nature of text editors on
non-UNIX platforms, it is unwise to use a mixture of spaces and tabs for the
indentation in a single source file.  It should also be noted that different
platforms may explicitly limit the maximum indentation level.

A formfeed character may be present at the start of the line; it will be ignored
for the indentation calculations above.  Formfeed characters occurring elsewhere
in the leading whitespace have an undefined effect (for instance, they may reset
the space count to zero).

.. index:: INDENT token, DEDENT token

The indentation levels of consecutive lines are used to generate INDENT and
DEDENT tokens, using a stack, as follows.

Before the first line of the file is read, a single zero is pushed on the stack;
this will never be popped off again.  The numbers pushed on the stack will
always be strictly increasing from bottom to top.  At the beginning of each
logical line, the line's indentation level is compared to the top of the stack.
If it is equal, nothing happens. If it is larger, it is pushed on the stack, and
one INDENT token is generated.  If it is smaller, it *must* be one of the
numbers occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is generated.  At the
end of the file, a DEDENT token is generated for each number remaining on the
stack that is larger than zero.

Here is an example of a correctly (though confusingly) indented piece of Python
code::

   def perm(l):
           # Compute the list of all permutations of l
       if len(l) <= 1:
                     return [l]
       r = []
       for i in range(len(l)):
                s = l[:i] + l[i+1:]
                p = perm(s)
                for x in p:
                 r.append(l[i:i+1] + x)
       return r

The following example shows various indentation errors::

    def perm(l):                       # error: first line indented
   for i in range(len(l)):             # error: not indented
       s = l[:i] + l[i+1:]
           p = perm(l[:i] + l[i+1:])   # error: unexpected indent
           for x in p:
                   r.append(l[i:i+1] + x)
               return r                # error: inconsistent dedent

(Actually, the first three errors are detected by the parser; only the last
error is found by the lexical analyzer --- the indentation of ``return r`` does
not match a level popped off the stack.)


.. _whitespace:

Whitespace between tokens
-------------------------

Except at the beginning of a logical line or in string literals, the whitespace
characters space, tab and formfeed can be used interchangeably to separate
tokens.  Whitespace is needed between two tokens only if their concatenation
could otherwise be interpreted as a different token (e.g., ab is one token, but
a b is two tokens).


.. _other-tokens:

Other tokens
============

Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist:
*identifiers*, *keywords*, *literals*, *operators*, and *delimiters*. Whitespace
characters (other than line terminators, discussed earlier) are not tokens, but
serve to delimit tokens. Where ambiguity exists, a token comprises the longest
possible string that forms a legal token, when read from left to right.


.. _identifiers:

Identifiers and keywords
========================

.. index:: identifier, name

Identifiers (also referred to as *names*) are described by the following lexical
definitions.

The syntax of identifiers in Python is based on the Unicode standard annex
UAX-31, with elaboration and changes as defined below; see also :pep:`3131` for
further details.

Within the ASCII range (U+0001..U+007F), the valid characters for identifiers
are the same as in Python 2.x: the uppercase and lowercase letters ``A`` through
``Z``, the underscore ``_`` and, except for the first character, the digits
``0`` through ``9``.

Python 3.0 introduces additional characters from outside the ASCII range (see
:pep:`3131`).  For these characters, the classification uses the version of the
Unicode Character Database as included in the :mod:`unicodedata` module.

Identifiers are unlimited in length.  Case is significant.

.. productionlist::
   identifier: `id_start` `id_continue`*
   id_start: <all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property>
   id_continue: <all characters in `id_start`, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property>

The Unicode category codes mentioned above stand for:

* *Lu* - uppercase letters
* *Ll* - lowercase letters
* *Lt* - titlecase letters
* *Lm* - modifier letters
* *Lo* - other letters
* *Nl* - letter numbers
* *Mn* - nonspacing marks
* *Mc* - spacing combining marks
* *Nd* - decimal numbers
* *Pc* - connector punctuations

All identifiers are converted into the normal form NFC while parsing; comparison
of identifiers is based on NFC.

A non-normative HTML file listing all valid identifier characters for Unicode
4.1 can be found at
http://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html.


.. _keywords:

Keywords
--------

.. index::
   single: keyword
   single: reserved word

The following identifiers are used as reserved words, or *keywords* of the
language, and cannot be used as ordinary identifiers.  They must be spelled
exactly as written here::

   False      class      finally    is         return
   None       continue   for        lambda     try
   True       def        from       nonlocal   while
   and        del        global     not        with
   as         elif       if         or         yield
   assert     else       import     pass
   break      except     in         raise

.. _id-classes:

Reserved classes of identifiers
-------------------------------

Certain classes of identifiers (besides keywords) have special meanings.  These
classes are identified by the patterns of leading and trailing underscore
characters:

``_*``
   Not imported by ``from module import *``.  The special identifier ``_`` is used
   in the interactive interpreter to store the result of the last evaluation; it is
   stored in the :mod:`builtins` module.  When not in interactive mode, ``_``
   has no special meaning and is not defined. See section :ref:`import`.

   .. note::

      The name ``_`` is often used in conjunction with internationalization;
      refer to the documentation for the :mod:`gettext` module for more
      information on this convention.

``__*__``
   System-defined names.  These names are defined by the interpreter and its
   implementation (including the standard library); applications should not expect
   to define additional names using this convention.  The set of names of this
   class defined by Python may be extended in future versions. See section
   :ref:`specialnames`.

``__*``
   Class-private names.  Names in this category, when used within the context of a
   class definition, are re-written to use a mangled form to help avoid name
   clashes between "private" attributes of base and derived classes. See section
   :ref:`atom-identifiers`.


.. _literals:

Literals
========

.. index:: literal, constant

Literals are notations for constant values of some built-in types.


.. _strings:

String and Bytes literals
-------------------------

.. index:: string literal, bytes literal, ASCII

String literals are described by the following lexical definitions:

.. productionlist::
   stringliteral: [`stringprefix`](`shortstring` | `longstring`)
   stringprefix: "r" | "R"
   shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"'
   longstring: "'''" `longstringitem`* "'''" | '"""' `longstringitem`* '"""'
   shortstringitem: `shortstringchar` | `stringescapeseq`
   longstringitem: `longstringchar` | `stringescapeseq`
   shortstringchar: <any source character except "\" or newline or the quote>
   longstringchar: <any source character except "\">
   stringescapeseq: "\" <any source character>

.. productionlist::
   bytesliteral: `bytesprefix`(`shortbytes` | `longbytes`)
   bytesprefix: "b" | "B"
   shortbytes: "'" `shortbytesitem`* "'" | '"' `shortbytesitem`* '"'
   longbytes: "'''" `longbytesitem`* "'''" | '"""' `longbytesitem`* '"""'
   shortbytesitem: `shortbyteschar` | `bytesescapeseq`
   longbytesitem: `longbyteschar` | `bytesescapeseq`
   shortbyteschar: <any ASCII character except "\" or newline or the quote>
   longbyteschar: <any ASCII character except "\">
   bytesescapeseq: "\" <any ASCII character>

One syntactic restriction not indicated by these productions is that whitespace
is not allowed between the :token:`stringprefix` or :token:`bytesprefix` and the
rest of the literal. The source character set is defined by the encoding
declaration; it is UTF-8 if no encoding declaration is given in the source file;
see section :ref:`encodings`.

.. index:: triple-quoted string, Unicode Consortium, raw string

In plain English: Both types of literals can be enclosed in matching single quotes
(``'``) or double quotes (``"``).  They can also be enclosed in matching groups
of three single or double quotes (these are generally referred to as
*triple-quoted strings*).  The backslash (``\``) character is used to escape
characters that otherwise have a special meaning, such as newline, backslash
itself, or the quote character.

String literals may optionally be prefixed with a letter ``'r'`` or ``'R'``;
such strings are called :dfn:`raw strings` and treat backslashes as literal
characters.  As a result, ``'\U'`` and ``'\u'`` escapes in raw strings are not
treated specially.

Bytes literals are always prefixed with ``'b'`` or ``'B'``; they produce an
instance of the :class:`bytes` type instead of the :class:`str` type.  They
may only contain ASCII characters; bytes with a numeric value of 128 or greater
must be expressed with escapes.

In triple-quoted strings, unescaped newlines and quotes are allowed (and are
retained), except that three unescaped quotes in a row terminate the string.  (A
"quote" is the character used to open the string, i.e. either ``'`` or ``"``.)

.. index:: physical line, escape sequence, Standard C, C

Unless an ``'r'`` or ``'R'`` prefix is present, escape sequences in strings are
interpreted according to rules similar to those used by Standard C.  The
recognized escape sequences are:

+-----------------+---------------------------------+-------+
| Escape Sequence | Meaning                         | Notes |
+=================+=================================+=======+
| ``\newline``    | Backslash and newline ignored   |       |
+-----------------+---------------------------------+-------+
| ``\\``          | Backslash (``\``)               |       |
+-----------------+---------------------------------+-------+
| ``\'``          | Single quote (``'``)            |       |
+-----------------+---------------------------------+-------+
| ``\"``          | Double quote (``"``)            |       |
+-----------------+---------------------------------+-------+
| ``\a``          | ASCII Bell (BEL)                |       |
+-----------------+---------------------------------+-------+
| ``\b``          | ASCII Backspace (BS)            |       |
+-----------------+---------------------------------+-------+
| ``\f``          | ASCII Formfeed (FF)             |       |
+-----------------+---------------------------------+-------+
| ``\n``          | ASCII Linefeed (LF)             |       |
+-----------------+---------------------------------+-------+
| ``\r``          | ASCII Carriage Return (CR)      |       |
+-----------------+---------------------------------+-------+
| ``\t``          | ASCII Horizontal Tab (TAB)      |       |
+-----------------+---------------------------------+-------+
| ``\v``          | ASCII Vertical Tab (VT)         |       |
+-----------------+---------------------------------+-------+
| ``\ooo``        | Character with octal value      | (1,3) |
|                 | *ooo*                           |       |
+-----------------+---------------------------------+-------+
| ``\xhh``        | Character with hex value *hh*   | (2,3) |
+-----------------+---------------------------------+-------+

Escape sequences only recognized in string literals are:

+-----------------+---------------------------------+-------+
| Escape Sequence | Meaning                         | Notes |
+=================+=================================+=======+
| ``\N{name}``    | Character named *name* in the   |       |
|                 | Unicode database                |       |
+-----------------+---------------------------------+-------+
| ``\uxxxx``      | Character with 16-bit hex value | \(4)  |
|                 | *xxxx*                          |       |
+-----------------+---------------------------------+-------+
| ``\Uxxxxxxxx``  | Character with 32-bit hex value | \(5)  |
|                 | *xxxxxxxx*                      |       |
+-----------------+---------------------------------+-------+

Notes:

(1)
   As in Standard C, up to three octal digits are accepted.

(2)
   Unlike in Standard C, at most two hex digits are accepted.

(3)
   In a bytes literal, hexadecimal and octal escapes denote the byte with the
   given value. In a string literal, these escapes denote a Unicode character
   with the given value.

(4)
   Individual code units which form parts of a surrogate pair can be encoded using
   this escape sequence. Unlike in Standard C, exactly two hex digits are required.

(5)
   Any Unicode character can be encoded this way, but characters outside the Basic
   Multilingual Plane (BMP) will be encoded using a surrogate pair if Python is
   compiled to use 16-bit code units (the default).  Individual code units which
   form parts of a surrogate pair can be encoded using this escape sequence.


.. index:: unrecognized escape sequence

Unlike Standard C, all unrecognized escape sequences are left in the string
unchanged, i.e., *the backslash is left in the string*.  (This behavior is
useful when debugging: if an escape sequence is mistyped, the resulting output
is more easily recognized as broken.)  It is also important to note that the
escape sequences only recognized in string literals fall into the category of
unrecognized escapes for bytes literals.

Even in a raw string, string quotes can be escaped with a backslash, but the
backslash remains in the string; for example, ``r"\""`` is a valid string
literal consisting of two characters: a backslash and a double quote; ``r"\"``
is not a valid string literal (even a raw string cannot end in an odd number of
backslashes).  Specifically, *a raw string cannot end in a single backslash*
(since the backslash would escape the following quote character).  Note also
that a single backslash followed by a newline is interpreted as those two
characters as part of the string, *not* as a line continuation.


.. _string-catenation:

String literal concatenation
----------------------------

Multiple adjacent string literals (delimited by whitespace), possibly using
different quoting conventions, are allowed, and their meaning is the same as
their concatenation.  Thus, ``"hello" 'world'`` is equivalent to
``"helloworld"``.  This feature can be used to reduce the number of backslashes
needed, to split long strings conveniently across long lines, or even to add
comments to parts of strings, for example::

   re.compile("[A-Za-z_]"       # letter or underscore
              "[A-Za-z0-9_]*"   # letter, digit or underscore
             )

Note that this feature is defined at the syntactical level, but implemented at
compile time.  The '+' operator must be used to concatenate string expressions
at run time.  Also note that literal concatenation can use different quoting
styles for each component (even mixing raw strings and triple quoted strings).


.. _numbers:

Numeric literals
----------------

.. index:: number, numeric literal, integer literal
   floating point literal, hexadecimal literal
   octal literal, binary literal, decimal literal, imaginary literal, complex literal

There are three types of numeric literals: integers, floating point numbers, and
imaginary numbers.  There are no complex literals (complex numbers can be formed
by adding a real number and an imaginary number).

Note that numeric literals do not include a sign; a phrase like ``-1`` is
actually an expression composed of the unary operator '``-``' and the literal
``1``.


.. _integers:

Integer literals
----------------

Integer literals are described by the following lexical definitions:

.. productionlist::
   integer: `decimalinteger` | `octinteger` | `hexinteger` | `bininteger`
   decimalinteger: `nonzerodigit` `digit`* | "0"+
   nonzerodigit: "1"..."9"
   digit: "0"..."9"
   octinteger: "0" ("o" | "O") `octdigit`+
   hexinteger: "0" ("x" | "X") `hexdigit`+
   bininteger: "0" ("b" | "B") `bindigit`+
   octdigit: "0"..."7"
   hexdigit: `digit` | "a"..."f" | "A"..."F"
   bindigit: "0" | "1"

There is no limit for the length of integer literals apart from what can be
stored in available memory.

Note that leading zeros in a non-zero decimal number are not allowed. This is
for disambiguation with C-style octal literals, which Python used before version
3.0.

Some examples of integer literals::

   7     2147483647                        0o177    0b100110111
   3     79228162514264337593543950336     0o377    0x100000000
         79228162514264337593543950336              0xdeadbeef						    


.. _floating:

Floating point literals
-----------------------

Floating point literals are described by the following lexical definitions:

.. productionlist::
   floatnumber: `pointfloat` | `exponentfloat`
   pointfloat: [`intpart`] `fraction` | `intpart` "."
   exponentfloat: (`intpart` | `pointfloat`) `exponent`
   intpart: `digit`+
   fraction: "." `digit`+
   exponent: ("e" | "E") ["+" | "-"] `digit`+

Note that the integer and exponent parts are always interpreted using radix 10.
For example, ``077e010`` is legal, and denotes the same number as ``77e10``. The
allowed range of floating point literals is implementation-dependent. Some
examples of floating point literals::

   3.14    10.    .001    1e100    3.14e-10    0e0

Note that numeric literals do not include a sign; a phrase like ``-1`` is
actually an expression composed of the unary operator ``-`` and the literal
``1``.


.. _imaginary:

Imaginary literals
------------------

Imaginary literals are described by the following lexical definitions:

.. productionlist::
   imagnumber: (`floatnumber` | `intpart`) ("j" | "J")

An imaginary literal yields a complex number with a real part of 0.0.  Complex
numbers are represented as a pair of floating point numbers and have the same
restrictions on their range.  To create a complex number with a nonzero real
part, add a floating point number to it, e.g., ``(3+4j)``.  Some examples of
imaginary literals::

   3.14j   10.j    10j     .001j   1e100j  3.14e-10j 


.. _operators:

Operators
=========

.. index:: single: operators

The following tokens are operators::

   +       -       *       **      /       //      %
   <<      >>      &       |       ^       ~
   <       >       <=      >=      ==      !=


.. _delimiters:

Delimiters
==========

.. index:: single: delimiters

The following tokens serve as delimiters in the grammar::

   (       )       [       ]       {       }      @
   ,       :       .       `       =       ;
   +=      -=      *=      /=      //=     %=
   &=      |=      ^=      >>=     <<=     **=

The period can also occur in floating-point and imaginary literals.  A sequence
of three periods has a special meaning as an ellipsis literal. The second half
of the list, the augmented assignment operators, serve lexically as delimiters,
but also perform an operation.

The following printing ASCII characters have special meaning as part of other
tokens or are otherwise significant to the lexical analyzer::

   '       "       #       \

The following printing ASCII characters are not used in Python.  Their
occurrence outside string literals and comments is an unconditional error::

   $       ?