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

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.

New in version 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
==============

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


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

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
--------------

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
--------

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.


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

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 **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.


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

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 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
-----------

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
-----------

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).

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 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
============

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 and keywords
========================

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

   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
--------

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:

   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

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

Changed in version 2.5: Both ``as`` and ``with`` are only recognized
when the ``with_statement`` future feature has been enabled. It will
always be enabled in Python 2.6.  See section *The with statement* for
details.  Note that using ``as`` and ``with`` as identifiers will
always issue a warning, even when the ``with_statement`` future
directive is not in effect.


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 ``__builtin__`` module.
   When not in interactive mode, ``_`` has no special meaning and is
   not defined. See section *The import statement*.

   Note: The name ``_`` is often used in conjunction with
     internationalization; refer to the documentation for the
     ``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 *Special method names* 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 *Identifiers (Names)*.


Literals
========

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


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

String literals are described by the following lexical definitions:

   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 **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 *Encoding declarations*.

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 *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 ``"``.)

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 *ooo*  | (3,5)   |
+-------------------+-----------------------------------+---------+
| ``\xhh``          | Character with hex value *hh*     | (4,5)   |
+-------------------+-----------------------------------+---------+

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.

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 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).


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

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``.


Integer and long integer literals
---------------------------------

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

   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. [1]  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 point literals
-----------------------

Floating point literals are described by the following lexical
definitions:

   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 literals
------------------

Imaginary literals are described by the following lexical definitions:

   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
=========

The following tokens are operators:

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

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


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:

   '       "       #       \

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

   $       ?

-[ Footnotes ]-

[1] 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.
