766 lines
28 KiB
ReStructuredText
766 lines
28 KiB
ReStructuredText
|
|
.. _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:: source character set, encoding declarations (source file)
|
|
|
|
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).
|
|
|
|
(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.
|
|
|