Python-2.7.5/Doc/reference/lexical_analysis.rst

.. _lexical:

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

.. index::
   single: lexical analysis
   single: parser
   single: 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 uses the 7-bit ASCII character set for program text.

.. versionadded:: 2.3
   An encoding declaration can be used to indicate that  string literals and
   comments use an encoding different from ASCII.

For compatibility with older versions, Python only warns if it finds 8-bit
characters; those warnings should be corrected by either declaring an explicit
encoding, or using escape sequences if those bytes are binary data, instead of
characters.

The run-time character set depends on the I/O devices connected to the program
but is generally a superset of ASCII.

**Future compatibility note:** It may be tempting to assume that the character
set for 8-bit characters is ISO Latin-1 (an ASCII superset that covers most
western languages that use the Latin alphabet), but it is possible that in the
future Unicode text editors will become common.  These generally use the UTF-8
encoding, which is also an ASCII superset, but with very different use for the
characters with ordinals 128-255.  While there is no consensus on this subject
yet, it is unwise to assume either Latin-1 or UTF-8, even though the current
implementation appears to favor Latin-1.  This applies both to the source
character set and the run-time character set.


.. _line-structure:

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

.. index:: single: line structure

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


.. _logical:

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

.. index::
   single: logical line
   single: physical line
   single: line joining
   single: 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:

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 old
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::
   single: comment
   single: 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::
   single: source character set
   single: 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. In addition, if the first bytes of
the file are the UTF-8 byte-order mark (``'\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, in particular to find the end of a
string, and to interpret the contents of Unicode literals. String literals are
converted to Unicode for syntactical analysis, then converted back to their
original encoding before interpretation starts. 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::
   single: physical line
   single: line joining
   single: line continuation
   single: 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 implementation, an
entirely blank logical line (i.e. one containing not even whitespace or a
comment) terminates a multi-line statement.


.. _indentation:

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

.. index::
   single: indentation
   single: whitespace
   single: leading whitespace
   single: space
   single: tab
   single: grouping
   single: 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::
   single: INDENT token
   single: 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::
   single: identifier
   single: name

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

.. productionlist::
   identifier: (`letter`|"_") (`letter` | `digit` | "_")*
   letter: `lowercase` | `uppercase`
   lowercase: "a"..."z"
   uppercase: "A"..."Z"
   digit: "0"..."9"

Identifiers are unlimited in length.  Case is significant.


.. _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:

.. sourcecode:: text

   and       del       from      not       while
   as        elif      global    or        with
   assert    else      if        pass      yield
   break     except    import    print
   class     exec      in        raise
   continue  finally   is        return
   def       for       lambda    try

.. versionchanged:: 2.4
   :const:`None` became a constant and is now recognized by the compiler as a name
   for the built-in object :const:`None`.  Although it is not a keyword, you cannot
   assign a different object to it.

.. versionchanged:: 2.5
   Using :keyword:`as` and :keyword:`with` as identifiers triggers a warning.  To
   use them as keywords, enable the ``with_statement`` future feature .

.. versionchanged:: 2.6
    :keyword:`as` and :keyword:`with` are full keywords.


.. _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:`__builtin__` 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).  Current system names are
   discussed in the :ref:`specialnames` section and elsewhere.  More will likely
   be defined in future versions of Python.  *Any* use of ``__*__`` names, in
   any context, that does not follow explicitly documented use, is subject to
   breakage without warning.

``__*``
   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::
   single: literal
   single: constant

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


.. _strings:

String literals
---------------

.. index:: single: string literal

String literals are described by the following lexical definitions:

.. index:: single: ASCII@ASCII

.. productionlist::
   stringliteral: [`stringprefix`](`shortstring` | `longstring`)
   stringprefix: "r" | "u" | "ur" | "R" | "U" | "UR" | "Ur" | "uR"
               : | "b" | "B" | "br" | "Br" | "bR" | "BR"
   shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"'
   longstring: "'''" `longstringitem`* "'''"
             : | '"""' `longstringitem`* '"""'
   shortstringitem: `shortstringchar` | `escapeseq`
   longstringitem: `longstringchar` | `escapeseq`
   shortstringchar: <any source character except "\" or newline or the quote>
   longstringchar: <any source character except "\">
   escapeseq: "\" <any ASCII character>

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

.. index::
   single: triple-quoted string
   single: Unicode Consortium
   single: string; Unicode
   single: raw string

In plain English: String 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 use
different rules for interpreting backslash escape sequences.  A prefix of
``'u'`` or ``'U'`` makes the string a Unicode string.  Unicode strings use the
Unicode character set as defined by the Unicode Consortium and ISO 10646.  Some
additional escape sequences, described below, are available in Unicode strings.
A prefix of ``'b'`` or ``'B'`` is ignored in Python 2; it indicates that the
literal should become a bytes literal in Python 3 (e.g. when code is
automatically converted with 2to3).  A ``'u'`` or ``'b'`` prefix may be followed
by an ``'r'`` prefix.

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::
   single: physical line
   single: escape sequence
   single: Standard C
   single: 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``    | Ignored                         |       |
+-----------------+---------------------------------+-------+
| ``\\``          | Backslash (``\``)               |       |
+-----------------+---------------------------------+-------+
| ``\'``          | Single quote (``'``)            |       |
+-----------------+---------------------------------+-------+
| ``\"``          | Double quote (``"``)            |       |
+-----------------+---------------------------------+-------+
| ``\a``          | ASCII Bell (BEL)                |       |
+-----------------+---------------------------------+-------+
| ``\b``          | ASCII Backspace (BS)            |       |
+-----------------+---------------------------------+-------+
| ``\f``          | ASCII Formfeed (FF)             |       |
+-----------------+---------------------------------+-------+
| ``\n``          | ASCII Linefeed (LF)             |       |
+-----------------+---------------------------------+-------+
| ``\N{name}``    | Character named *name* in the   |       |
|                 | Unicode database (Unicode only) |       |
+-----------------+---------------------------------+-------+
| ``\r``          | ASCII Carriage Return (CR)      |       |
+-----------------+---------------------------------+-------+
| ``\t``          | ASCII Horizontal Tab (TAB)      |       |
+-----------------+---------------------------------+-------+
| ``\uxxxx``      | Character with 16-bit hex value | \(1)  |
|                 | *xxxx* (Unicode only)           |       |
+-----------------+---------------------------------+-------+
| ``\Uxxxxxxxx``  | Character with 32-bit hex value | \(2)  |
|                 | *xxxxxxxx* (Unicode only)       |       |
+-----------------+---------------------------------+-------+
| ``\v``          | ASCII Vertical Tab (VT)         |       |
+-----------------+---------------------------------+-------+
| ``\ooo``        | Character with octal value      | (3,5) |
|                 | *ooo*                           |       |
+-----------------+---------------------------------+-------+
| ``\xhh``        | Character with hex value *hh*   | (4,5) |
+-----------------+---------------------------------+-------+

.. index:: single: ASCII@ASCII

Notes:

(1)
   Individual code units which form parts of a surrogate pair can be encoded using
   this escape sequence.

(2)
   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.

(3)
   As in Standard C, up to three octal digits are accepted.

(4)
   Unlike in Standard C, exactly two hex digits are required.

(5)
   In a string literal, hexadecimal and octal escapes denote the byte with the
   given value; it is not necessary that the byte encodes a character in the source
   character set. In a Unicode literal, these escapes denote a Unicode character
   with the given value.

.. index:: single: 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 marked as "(Unicode only)" in the table above fall into the
category of unrecognized escapes for non-Unicode string literals.

When an ``'r'`` or ``'R'`` prefix is present, a character following a backslash
is included in the string without change, and *all backslashes are left in the
string*.  For example, the string literal ``r"\n"`` consists of two characters:
a backslash and a lowercase ``'n'``.  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.

When an ``'r'`` or ``'R'`` prefix is used in conjunction with a ``'u'`` or
``'U'`` prefix, then the ``\uXXXX`` and ``\UXXXXXXXX`` escape sequences are
processed while  *all other backslashes are left in the string*. For example,
the string literal ``ur"\u0062\n"`` consists of three Unicode characters: 'LATIN
SMALL LETTER B', 'REVERSE SOLIDUS', and 'LATIN SMALL LETTER N'. Backslashes can
be escaped with a preceding backslash; however, both remain in the string.  As a
result, ``\uXXXX`` escape sequences are only recognized when there are an odd
number of backslashes.


.. _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::
   single: number
   single: numeric literal
   single: integer literal
   single: plain integer literal
   single: long integer literal
   single: floating point literal
   single: hexadecimal literal
   single: binary literal
   single: octal literal
   single: decimal literal
   single: imaginary literal
   single: complex; literal

There are four types of numeric literals: plain integers, long 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 and long integer literals
---------------------------------

Integer and long integer literals are described by the following lexical
definitions:

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

Although both lower case ``'l'`` and upper case ``'L'`` are allowed as suffix
for long integers, it is strongly recommended to always use ``'L'``, since the
letter ``'l'`` looks too much like the digit ``'1'``.

Plain integer literals that are above the largest representable plain integer
(e.g., 2147483647 when using 32-bit arithmetic) are accepted as if they were
long integers instead. [#]_  There is no limit for long integer literals apart
from what can be stored in available memory.

Some examples of plain integer literals (first row) and long integer literals
(second and third rows)::

   7     2147483647                        0177
   3L    79228162514264337593543950336L    0377L   0x100000000L
         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 of floating point numbers can look like
octal integers, but are 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::

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

The comparison operators ``<>`` and ``!=`` are alternate spellings of the same
operator.  ``!=`` is the preferred spelling; ``<>`` is obsolescent.


.. _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 in slices. 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::

   '       "       #       \

.. index:: single: ASCII@ASCII

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

   $       ?

.. rubric:: Footnotes

.. [#] In versions of Python prior to 2.4, octal and hexadecimal literals in the range
   just above the largest representable plain integer but below the largest
   unsigned 32-bit number (on a machine using 32-bit arithmetic), 4294967296, were
   taken as the negative plain integer obtained by subtracting 4294967296 from
   their unsigned value.