.. _lexical:
****************
Lexical analysis
****************
.. index:: lexical analysis, parser, token
A Python program is read by a *parser*. Input to the parser is a stream of
*tokens*, generated by the *lexical analyzer*. This chapter describes how the
lexical analyzer breaks a file into tokens.
Python reads program text as Unicode code points; the encoding of a source file
can be given by an encoding declaration and defaults to UTF-8, see :pep:`3120`
for details. If the source file cannot be decoded, a :exc:`SyntaxError` is
raised.
.. _line-structure:
Line structure
==============
.. index:: line structure
A Python program is divided into a number of *logical lines*.
.. _logical-lines:
Logical lines
-------------
.. index:: logical line, physical line, line joining, NEWLINE token
The end of a logical line is represented by the token NEWLINE. Statements
cannot cross logical line boundaries except where NEWLINE is allowed by the
syntax (e.g., between statements in compound statements). A logical line is
constructed from one or more *physical lines* by following the explicit or
implicit *line joining* rules.
.. _physical-lines:
Physical lines
--------------
A physical line is a sequence of characters terminated by an end-of-line
sequence. In source files and strings, 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. The end of input also serves
as an implicit terminator for the final physical line.
When embedding Python, source code strings should be passed to Python APIs using
the standard C conventions for newline characters (the ``\n`` character,
representing ASCII LF, is the line terminator).
.. _comments:
Comments
--------
.. index:: comment, hash character
single: # (hash); comment
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.
.. _encodings:
Encoding declarations
---------------------
.. index:: source character set, encoding declarations (source file)
single: # (hash); source encoding declaration
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 encoding declaration must appear on a line of its
own. If it is the second line, the first line must also be a comment-only line.
The recommended forms of an encoding 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. If the
implicit or explicit encoding of a file is UTF-8, an initial UTF-8 byte-order
mark (b'\xef\xbb\xbf') is ignored rather than being a syntax error.
If an encoding is declared, the encoding name must be recognized by Python
(see :ref:`standard-encodings`). The
encoding is used for all lexical analysis, including string literals, comments
and identifiers.
.. _explicit-joining:
Explicit line joining
---------------------
.. index:: physical line, line joining, line continuation, backslash character
Two or more physical lines may be joined into logical lines using backslash
characters (``\``), as follows: when a physical line ends in a backslash that is
not part of a string literal or comment, it is joined with the following forming
a single logical line, deleting the backslash and the following end-of-line
character. For example::
if 1900 < year < 2100 and 1 <= month <= 12 \
and 1 <= day <= 31 and 0 <= hour < 24 \
and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
return 1
A line ending in a backslash cannot carry a comment. A backslash does not
continue a comment. A backslash does not continue a token except for string
literals (i.e., tokens other than string literals cannot be split across
physical lines using a backslash). A backslash is illegal elsewhere on a line
outside a string literal.
.. _implicit-joining:
Implicit line joining
---------------------
Expressions in parentheses, square brackets or curly braces can be split over
more than one physical line without using backslashes. For example::
month_names = ['Januari', 'Februari', 'Maart', # These are the
'April', 'Mei', 'Juni', # Dutch names
'Juli', 'Augustus', 'September', # for the months
'Oktober', 'November', 'December'] # of the year
Implicitly continued lines can carry comments. The indentation of the
continuation lines is not important. Blank continuation lines are allowed.
There is no NEWLINE token between implicit continuation lines. Implicitly
continued lines can also occur within triple-quoted strings (see below); in that
case they cannot carry comments.
.. _blank-lines:
Blank lines
-----------
.. index:: single: blank line
A logical line that contains only spaces, tabs, formfeeds and possibly a
comment, is ignored (i.e., no NEWLINE token is generated). During interactive
input of statements, handling of a blank line may differ depending on the
implementation of the read-eval-print loop. In the standard interactive
interpreter, an entirely blank logical line (i.e. one containing not even
whitespace or a comment) terminates a multi-line statement.
.. _indentation:
Indentation
-----------
.. index:: indentation, leading whitespace, space, tab, grouping, statement grouping
Leading whitespace (spaces and tabs) at the beginning of a logical line is used
to compute the indentation level of the line, which in turn is used to determine
the grouping of statements.
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
:exc:`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).
.. index:: INDENT token, DEDENT token
The indentation levels of consecutive lines are used to generate INDENT and
DEDENT tokens, using a stack, as follows.
Before the first line of the file is read, a single zero is pushed on the stack;
this will never be popped off again. The numbers pushed on the stack will
always be strictly increasing from bottom to top. At the beginning of each
logical line, the line's indentation level is compared to the top of the stack.
If it is equal, nothing happens. If it is larger, it is pushed on the stack, and
one INDENT token is generated. If it is smaller, it *must* be one of the
numbers occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is generated. At the
end of the file, a DEDENT token is generated for each number remaining on the
stack that is larger than zero.
Here is an example of a correctly (though confusingly) indented piece of Python
code::
def perm(l):
# Compute the list of all permutations of l
if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
The following example shows various indentation errors::
def perm(l): # error: first line indented
for i in range(len(l)): # error: not indented
s = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:]) # error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r # error: inconsistent dedent
(Actually, the first three errors are detected by the parser; only the last
error is found by the lexical analyzer --- the indentation of ``return r`` does
not match a level popped off the stack.)
.. _whitespace:
Whitespace between tokens
-------------------------
Except at the beginning of a logical line or in string literals, the whitespace
characters space, tab and formfeed can be used interchangeably to separate
tokens. Whitespace is needed between two tokens only if their concatenation
could otherwise be interpreted as a different token (e.g., ab is one token, but
a b is two tokens).
.. _other-tokens:
Other tokens
============
Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist:
*identifiers*, *keywords*, *literals*, *operators*, and *delimiters*. Whitespace
characters (other than line terminators, discussed earlier) are not tokens, but
serve to delimit tokens. Where ambiguity exists, a token comprises the longest
possible string that forms a legal token, when read from left to right.
.. _identifiers:
Identifiers and keywords
========================
.. index:: identifier, name
Identifiers (also referred to as *names*) are described by the following lexical
definitions.
The syntax of identifiers in Python is based on the Unicode standard annex
UAX-31, with elaboration and changes as defined below; see also :pep:`3131` for
further details.
Within the ASCII range (U+0001..U+007F), the valid characters for identifiers
include the uppercase and lowercase letters ``A`` through
``Z``, the underscore ``_`` and, except for the first character, the digits
``0`` through ``9``.
Python 3.0 introduced additional characters from outside the ASCII range (see
:pep:`3131`). For these characters, the classification uses the version of the
Unicode Character Database as included in the :mod:`unicodedata` module.
Identifiers are unlimited in length. Case is significant.
.. productionlist:: python-grammar
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
<https://www.unicode.org/Public/16.0.0/ucd/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
16.0.0 can be found at
https://www.unicode.org/Public/16.0.0/ucd/DerivedCoreProperties.txt
.. _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
False await else import pass
None break except in raise
True class finally is return
and continue for lambda try
as def from nonlocal while
assert del global not with
async elif if or yield
.. _soft-keywords:
Soft Keywords
-------------
.. index:: soft keyword, keyword
.. versionadded:: 3.10
Some identifiers are only reserved under specific contexts. These are known as
*soft keywords*. The identifiers ``match``, ``case``, ``type`` and ``_`` can
syntactically act as keywords in certain contexts,
but this distinction is done at the parser level, not when tokenizing.
As soft keywords, their use in the grammar is possible while still
preserving compatibility with existing code that uses these names as
identifier names.
``match``, ``case``, and ``_`` are used in the :keyword:`match` statement.
``type`` is used in the :keyword:`type` statement.
.. versionchanged:: 3.12
``type`` is now a soft keyword.
.. index::
single: _, identifiers
single: __, identifiers
.. _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 *``.
``_``
In a ``case`` pattern within a :keyword:`match` statement, ``_`` is a
:ref:`soft keyword <soft-keywords>` that denotes a
:ref:`wildcard <wildcard-patterns>`.
Separately, the interactive interpreter makes the result of the last evaluation
available in the variable ``_``.
(It is stored in the :mod:`builtins` module, alongside built-in
functions like ``print``.)
Elsewhere, ``_`` is a regular identifier. It is often used to name
"special" items, but it is not special to Python itself.
.. 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.
It is also commonly used for unused variables.
``__*__``
System-defined names, informally known as "dunder" 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:: literal, constant
Literals are notations for constant values of some built-in types.
.. index:: string literal, bytes literal, ASCII
single: ' (single quote); string literal
single: " (double quote); string literal
single: u'; string literal
single: u"; string literal
.. _strings:
String and Bytes literals
-------------------------
String literals are described by the following lexical definitions:
.. productionlist:: python-grammar
stringliteral: [`stringprefix`](`shortstring` | `longstring`)
stringprefix: "r" | "u" | "R" | "U" | "f" | "F"
: | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
shortstring: "'" `shortstringitem`* "'" | '"' `shortstringitem`* '"'
longstring: "'''" `longstringitem`* "'''" | '"""' `longstringitem`* '"""'
shortstringitem: `shortstringchar` | `stringescapeseq`
longstringitem: `longstringchar` | `stringescapeseq`
shortstringchar: <any source character except "\" or newline or the quote>
longstringchar: <any source character except "\">
stringescapeseq: "\" <any source character>
.. productionlist:: python-grammar
bytesliteral: `bytesprefix`(`shortbytes` | `longbytes`)
bytesprefix: "b" | "B" | "br" | "Br" | "bR" | "BR" | "rb" | "rB" | "Rb" | "RB"
shortbytes: "'" `shortbytesitem`* "'" | '"' `shortbytesitem`* '"'
longbytes: "'''" `longbytesitem`* "'''" | '"""' `longbytesitem`* '"""'
shortbytesitem: `shortbyteschar` | `bytesescapeseq`
longbytesitem: `longbyteschar` | `bytesescapeseq`
shortbyteschar: <any ASCII character except "\" or newline or the quote>
longbyteschar: <any ASCII character except "\">
bytesescapeseq: "\" <any ASCII character>
One syntactic restriction not indicated by these productions is that whitespace
is not allowed between the :token:`~python-grammar:stringprefix` or
:token:`~python-grammar:bytesprefix` and the rest of the literal. The source
character set is defined by the encoding declaration; it is UTF-8 if no encoding
declaration is given in the source file; see section :ref:`encodings`.
.. index:: triple-quoted string, Unicode Consortium, raw string
single: """; string literal
single: '''; string literal
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 give special
meaning to otherwise ordinary characters like ``n``, which means 'newline' when
escaped (``\n``). It can also be used to escape characters that otherwise have a
special meaning, such as newline, backslash itself, or the quote character.
See :ref:`escape sequences <escape-sequences>` below for examples.
.. index::
single: b'; bytes literal
single: b"; bytes literal
Bytes literals are always prefixed with ``'b'`` or ``'B'``; they produce an
instance of the :class:`bytes` type instead of the :class:`str` type. They
may only contain ASCII characters; bytes with a numeric value of 128 or greater
must be expressed with escapes.
.. index::
single: r'; raw string literal
single: r"; raw string literal
Both string and bytes literals may optionally be prefixed with a letter ``'r'``
or ``'R'``; such constructs are called :dfn:`raw string literals`
and :dfn:`raw bytes literals` respectively and treat backslashes as
literal characters. As a result, in raw string literals, ``'\U'`` and ``'\u'``
escapes are not treated specially.
.. versionadded:: 3.3
The ``'rb'`` prefix of raw bytes literals has been added as a synonym
of ``'br'``.
Support for the unicode legacy literal (``u'value'``) was reintroduced
to simplify the maintenance of dual Python 2.x and 3.x codebases.
See :pep:`414` for more information.
.. index::
single: f'; formatted string literal
single: f"; formatted string literal
A string literal with ``'f'`` or ``'F'`` in its prefix is a
:dfn:`formatted string literal`; see :ref:`f-strings`. The ``'f'`` may be
combined with ``'r'``, but not with ``'b'`` or ``'u'``, therefore raw
formatted strings are possible, but formatted bytes literals are not.
In triple-quoted literals, unescaped newlines and quotes are allowed (and are
retained), except that three unescaped quotes in a row terminate the literal. (A
"quote" is the character used to open the literal, i.e. either ``'`` or ``"``.)
.. index:: physical line, escape sequence, Standard C, C
single: \ (backslash); escape sequence
single: \\; escape sequence
single: \a; escape sequence
single: \b; escape sequence
single: \f; escape sequence
single: \n; escape sequence
single: \r; escape sequence
single: \t; escape sequence
single: \v; escape sequence
single: \x; escape sequence
single: \N; escape sequence
single: \u; escape sequence
single: \U; escape sequence
.. _escape-sequences:
Escape sequences
^^^^^^^^^^^^^^^^
Unless an ``'r'`` or ``'R'`` prefix is present, escape sequences in string and
bytes literals 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 | \(1) |
+-------------------------+---------------------------------+-------+
| ``\\`` | 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) | |
+-------------------------+---------------------------------+-------+
| :samp:`\\\\{ooo}` | Character with octal value | (2,4) |
| | *ooo* | |
+-------------------------+---------------------------------+-------+
| :samp:`\\x{hh}` | Character with hex value *hh* | (3,4) |
+-------------------------+---------------------------------+-------+
Escape sequences only recognized in string literals are:
+-------------------------+---------------------------------+-------+
| Escape Sequence | Meaning | Notes |
+=========================+=================================+=======+
| :samp:`\\N\\{{name}\\}` | Character named *name* in the | \(5) |
| | Unicode database | |
+-------------------------+---------------------------------+-------+
| :samp:`\\u{xxxx}` | Character with 16-bit hex value | \(6) |
| | *xxxx* | |
+-------------------------+---------------------------------+-------+
| :samp:`\\U{xxxxxxxx}` | Character with 32-bit hex value | \(7) |
| | *xxxxxxxx* | |
+-------------------------+---------------------------------+-------+
Notes:
(1)
A backslash can be added at the end of a line to ignore the newline::
>>> 'This string will not include \
... backslashes or newline characters.'
'This string will not include backslashes or newline characters.'
The same result can be achieved using :ref:`triple-quoted strings <strings>`,
or parentheses and :ref:`string literal concatenation <string-concatenation>`.
(2)
As in Standard C, up to three octal digits are accepted.
.. versionchanged:: 3.11
Octal escapes with value larger than ``0o377`` produce a
:exc:`DeprecationWarning`.
.. versionchanged:: 3.12
Octal escapes with value larger than ``0o377`` produce a
:exc:`SyntaxWarning`. In a future Python version they will be eventually
a :exc:`SyntaxError`.
(3)
Unlike in Standard C, exactly two hex digits are required.
(4)
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.
(5)
.. versionchanged:: 3.3
Support for name aliases [#]_ has been added.
(6)
Exactly four hex digits are required.
(7)
Any Unicode character can be encoded this way. Exactly eight hex digits
are required.
.. index:: unrecognized escape sequence
Unlike Standard C, all unrecognized escape sequences are left in the string
unchanged, i.e., *the backslash is left in the result*. (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.
.. versionchanged:: 3.6
Unrecognized escape sequences produce a :exc:`DeprecationWarning`.
.. versionchanged:: 3.12
Unrecognized escape sequences produce a :exc:`SyntaxWarning`. In a future
Python version they will be eventually a :exc:`SyntaxError`.
Even in a raw literal, quotes can be escaped with a backslash, but the
backslash remains in the result; 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 literal 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 literal, *not* as a line continuation.
.. _string-concatenation:
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),
and formatted string literals may be concatenated with plain string literals.
.. index::
single: formatted string literal
single: interpolated string literal
single: string; formatted literal
single: string; interpolated literal
single: f-string
single: fstring
single: {} (curly brackets); in formatted string literal
single: ! (exclamation); in formatted string literal
single: : (colon); in formatted string literal
single: = (equals); for help in debugging using string literals
.. _f-strings:
.. _formatted-string-literals:
f-strings
---------
.. versionadded:: 3.6
A :dfn:`formatted string literal` or :dfn:`f-string` is a string literal
that is prefixed with ``'f'`` or ``'F'``. These strings may contain
replacement fields, which are expressions delimited by curly braces ``{}``.
While other string literals always have a constant value, formatted strings
are really expressions evaluated at run time.
Escape sequences are decoded like in ordinary string literals (except when
a literal is also marked as a raw string). After decoding, the grammar
for the contents of the string is:
.. productionlist:: python-grammar
f_string: (`literal_char` | "{{" | "}}" | `replacement_field`)*
replacement_field: "{" `f_expression` ["="] ["!" `conversion`] [":" `format_spec`] "}"
f_expression: (`conditional_expression` | "*" `or_expr`)
: ("," `conditional_expression` | "," "*" `or_expr`)* [","]
: | `yield_expression`
conversion: "s" | "r" | "a"
format_spec: (`literal_char` | `replacement_field`)*
literal_char: <any code point except "{", "}" or NULL>
The parts of the string outside curly braces are treated literally,
except that any doubled curly braces ``'{{'`` or ``'}}'`` are replaced
with the corresponding single curly brace. A single opening curly
bracket ``'{'`` marks a replacement field, which starts with a
Python expression. To display both the expression text and its value after
evaluation, (useful in debugging), an equal sign ``'='`` may be added after the
expression. A conversion field, introduced by an exclamation point ``'!'`` may
follow. A format specifier may also be appended, introduced by a colon ``':'``.
A replacement field ends with a closing curly bracket ``'}'``.
Expressions in formatted string literals are treated like regular
Python expressions surrounded by parentheses, with a few exceptions.
An empty expression is not allowed, and both :keyword:`lambda` and
assignment expressions ``:=`` must be surrounded by explicit parentheses.
Each expression is evaluated in the context where the formatted string literal
appears, in order from left to right. Replacement expressions can contain
newlines in both single-quoted and triple-quoted f-strings and they can contain
comments. Everything that comes after a ``#`` inside a replacement field
is a comment (even closing braces and quotes). In that case, replacement fields
must be closed in a different line.
.. code-block:: text
>>> f"abc{a # This is a comment }"
... + 3}"
'abc5'
.. versionchanged:: 3.7
Prior to Python 3.7, an :keyword:`await` expression and comprehensions
containing an :keyword:`async for` clause were illegal in the expressions
in formatted string literals due to a problem with the implementation.
.. versionchanged:: 3.12
Prior to Python 3.12, comments were not allowed inside f-string replacement
fields.
When the equal sign ``'='`` is provided, the output will have the expression
text, the ``'='`` and the evaluated value. Spaces after the opening brace
``'{'``, within the expression and after the ``'='`` are all retained in the
output. By default, the ``'='`` causes the :func:`repr` of the expression to be
provided, unless there is a format specified. When a format is specified it
defaults to the :func:`str` of the expression unless a conversion ``'!r'`` is
declared.
.. versionadded:: 3.8
The equal sign ``'='``.
If a conversion is specified, the result of evaluating the expression
is converted before formatting. Conversion ``'!s'`` calls :func:`str` on
the result, ``'!r'`` calls :func:`repr`, and ``'!a'`` calls :func:`ascii`.
The result is then formatted using the :func:`format` protocol. The
format specifier is passed to the :meth:`~object.__format__` method of the
expression or conversion result. An empty string is passed when the
format specifier is omitted. The formatted result is then included in
the final value of the whole string.
Top-level format specifiers may include nested replacement fields. These nested
fields may include their own conversion fields and :ref:`format specifiers
<formatspec>`, but may not include more deeply nested replacement fields. The
:ref:`format specifier mini-language <formatspec>` is the same as that used by
the :meth:`str.format` method.
Formatted string literals may be concatenated, but replacement fields
cannot be split across literals.
Some examples of formatted string literals::
>>> name = "Fred"
>>> f"He said his name is {name!r}."
"He said his name is 'Fred'."
>>> f"He said his name is {repr(name)}." # repr() is equivalent to !r
"He said his name is 'Fred'."
>>> width = 10
>>> precision = 4
>>> value = decimal.Decimal("12.34567")
>>> f"result: {value:{width}.{precision}}" # nested fields
'result: 12.35'
>>> today = datetime(year=2017, month=1, day=27)
>>> f"{today:%B %d, %Y}" # using date format specifier
'January 27, 2017'
>>> f"{today=:%B %d, %Y}" # using date format specifier and debugging
'today=January 27, 2017'
>>> number = 1024
>>> f"{number:#0x}" # using integer format specifier
'0x400'
>>> foo = "bar"
>>> f"{ foo = }" # preserves whitespace
" foo = 'bar'"
>>> line = "The mill's closed"
>>> f"{line = }"
'line = "The mill\'s closed"'
>>> f"{line = :20}"
"line = The mill's closed "
>>> f"{line = !r:20}"
'line = "The mill\'s closed" '
Reusing the outer f-string quoting type inside a replacement field is
permitted::
>>> a = dict(x=2)
>>> f"abc {a["x"]} def"
'abc 2 def'
.. versionchanged:: 3.12
Prior to Python 3.12, reuse of the same quoting type of the outer f-string
inside a replacement field was not possible.
Backslashes are also allowed in replacement fields and are evaluated the same
way as in any other context::
>>> a = ["a", "b", "c"]
>>> print(f"List a contains:\n{"\n".join(a)}")
List a contains:
a
b
c
.. versionchanged:: 3.12
Prior to Python 3.12, backslashes were not permitted inside an f-string
replacement field.
Formatted string literals cannot be used as docstrings, even if they do not
include expressions.
::
>>> def foo():
... f"Not a docstring"
...
>>> foo.__doc__ is None
True
See also :pep:`498` for the proposal that added formatted string literals,
and :meth:`str.format`, which uses a related format string mechanism.
.. _numbers:
Numeric literals
----------------
.. index:: number, numeric literal, integer literal
floating-point literal, hexadecimal literal
octal literal, binary literal, decimal literal, imaginary literal, complex literal
There are three types of numeric literals: integers, floating-point numbers, and
imaginary numbers. There are no complex literals (complex numbers can be formed
by adding a real number and an imaginary number).
Note that numeric literals do not include a sign; a phrase like ``-1`` is
actually an expression composed of the unary operator '``-``' and the literal
``1``.
.. index::
single: 0b; integer literal
single: 0o; integer literal
single: 0x; integer literal
single: _ (underscore); in numeric literal
.. _integers:
Integer literals
----------------
Integer literals are described by the following lexical definitions:
.. productionlist:: python-grammar
integer: `decinteger` | `bininteger` | `octinteger` | `hexinteger`
decinteger: `nonzerodigit` (["_"] `digit`)* | "0"+ (["_"] "0")*
bininteger: "0" ("b" | "B") (["_"] `bindigit`)+
octinteger: "0" ("o" | "O") (["_"] `octdigit`)+
hexinteger: "0" ("x" | "X") (["_"] `hexdigit`)+
nonzerodigit: "1"..."9"
digit: "0"..."9"
bindigit: "0" | "1"
octdigit: "0"..."7"
hexdigit: `digit` | "a"..."f" | "A"..."F"
There is no limit for the length of integer literals apart from what can be
stored in available memory.
Underscores are ignored for determining the numeric value of the literal. They
can be used to group digits for enhanced readability. One underscore can occur
between digits, and after base specifiers like ``0x``.
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 0xdeadbeef
100_000_000_000 0b_1110_0101
.. versionchanged:: 3.6
Underscores are now allowed for grouping purposes in literals.
.. index::
single: . (dot); in numeric literal
single: e; in numeric literal
single: _ (underscore); in numeric literal
.. _floating:
Floating-point literals
-----------------------
Floating-point literals are described by the following lexical definitions:
.. productionlist:: python-grammar
floatnumber: `pointfloat` | `exponentfloat`
pointfloat: [`digitpart`] `fraction` | `digitpart` "."
exponentfloat: (`digitpart` | `pointfloat`) `exponent`
digitpart: `digit` (["_"] `digit`)*
fraction: "." `digitpart`
exponent: ("e" | "E") ["+" | "-"] `digitpart`
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. As in
integer literals, underscores are supported for digit grouping.
Some examples of floating-point literals::
3.14 10. .001 1e100 3.14e-10 0e0 3.14_15_93
.. versionchanged:: 3.6
Underscores are now allowed for grouping purposes in literals.
.. index::
single: j; in numeric literal
.. _imaginary:
Imaginary literals
------------------
Imaginary literals are described by the following lexical definitions:
.. productionlist:: python-grammar
imagnumber: (`floatnumber` | `digitpart`) ("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 3.14_15_93j
.. _operators:
Operators
=========
.. index:: single: operators
The following tokens are operators:
.. code-block:: none
+ - * ** / // % @
<< >> & | ^ ~ :=
< > <= >= == !=
.. _delimiters:
Delimiters
==========
.. index:: single: delimiters
The following tokens serve as delimiters in the grammar:
.. code-block:: none
( ) [ ] { }
, : ! . ; @ =
-> += -= *= /= //= %=
@= &= |= ^= >>= <<= **=
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:
.. code-block:: none
' " # \
The following printing ASCII characters are not used in Python. Their
occurrence outside string literals and comments is an unconditional error:
.. code-block:: none
$ ? `
.. rubric:: Footnotes
.. [#] https://www.unicode.org/Public/16.0.0/ucd/NameAliases.txt
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