
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 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 ``SyntaxError`` is raised.


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.

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 **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.


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 interactive interpreter, 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.

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.

Indentation is rejected as inconsistent if a source file mixes tabs
and spaces in a way that makes the meaning dependent on the worth of a
tab in spaces; a ``TabError`` is raised in that case.

**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.

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
``unicodedata`` module.

Identifiers are unlimited in length.  Case is significant.

   identifier   ::= xid_start xid_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>
   xid_start    ::= <all characters in id_start whose NFKC normalization is in "id_start xid_continue*">
   xid_continue ::= <all characters in id_continue whose NFKC normalization is in "id_continue*">

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

* *Other_ID_Start* - explicit list of characters in PropList.txt to
  support backwards compatibility

* *Other_ID_Continue* - likewise

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

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
--------

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


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 ``builtins`` 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 and Bytes literals
-------------------------

String literals are described by the following lexical definitions:

   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>

   bytesliteral   ::= bytesprefix(shortbytes | longbytes)
   bytesprefix    ::= "b" | "B" | "br" | "Br" | "bR" | "BR"
   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 ``stringprefix`` or
``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 *Encoding
declarations*.

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.

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

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

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``      | 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 *ooo*  | (1,3)   |
+-------------------+-----------------------------------+---------+
| ``\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, exactly two hex digits are required.

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.  Exactly four 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).  Exactly eight hex digits are required.

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 literal concatenation
----------------------------

Multiple adjacent string or bytes 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 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``.


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

Integer literals are described by the following lexical definitions:

   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 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 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 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:

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


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:

   $       ?       `
