A Python program is read by a parser. Input to the parser is a stream of
, generated by the lexical analyzer (also known as the tokenizer). This chapter describes how the lexical analyzer produces these tokens.
The lexical analyzer determines the program text’s
(UTF-8 by default), and decodes the text into
. If the text cannot be decoded, a
is raised.
Next, the lexical analyzer uses the source characters to generate a stream of tokens. The type of a generated token generally depends on the next source character to be processed. Similarly, other special behavior of the analyzer depends on the first source character that hasn’t yet been processed. The following table gives a quick summary of these source characters, with links to sections that contain more information.
Character
Next token (or other relevant documentation)
space
tab
formfeed
CR, LF
backslash (\)
(Also significant in
)
hash (#)
quote (', ")
ASCII letter (a-z, A-Z)
non-ASCII character
Prefixed
underscore (_)
(Can also be part of
)
number (0-9)
dot (.)
question mark (?)
dollar ($)
backquote (`)
control character
Error (outside string literals and comments)
other printing character
end of file
2.1. Line structure
A Python program is divided into a number of logical lines.
2.1.1. Logical lines
The end of a logical line is represented by the token
. 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
or
line joining rules.
2.1.2. Physical lines
A physical line is a sequence of characters terminated by one the following end-of-line sequences:
the Unix form using ASCII LF (linefeed),
the Windows form using the ASCII sequence CR LF (return followed by linefeed),
the ‘
’ form using the ASCII CR (return) character.
Regardless of platform, each of these sequences is replaced by a single ASCII LF (linefeed) character. (This is done even inside
.) Each line can use any of the sequences; they do not need to be consistent within a file.
The end of input also serves as an implicit terminator for the final physical line.
Formally:
newline: <ASCII LF> | <ASCII CR> <ASCII LF> | <ASCII CR> 2.1.4. 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 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
). The encoding is used for all lexical analysis, including string literals, comments and identifiers.
All lexical analysis, including string literals, comments and identifiers, works on Unicode text decoded using the source encoding. Any Unicode code point, except the NUL control character, can appear in Python source.
source_character: <any Unicode code point, except NUL> 2.1.5. 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:
if1900<year<2100and1<=month<=12 \ and1<=day<=31and0<=hour<24 \ and0<=minute<60and0<=second<60:# Looks like a valid datereturn1A 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.
2.1.6. 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 yearImplicitly 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.
2.1.7. Blank lines
A logical line that contains only spaces, tabs, formfeeds and possibly a comment, is ignored (i.e., no
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 (that is, one containing not even whitespace or a comment) terminates a multi-line statement.
2.1.8. 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
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
and
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:
defperm(l):# Compute the list of all permutations of liflen(l)<=1:return[l]r=[]foriinrange(len(l)):s=l[:i]+l[i+1:]p=perm(s)forxinp:r.append(l[i:i+1]+x)returnrThe following example shows various indentation errors:
defperm(l):# error: first line indentedforiinrange(len(l)):# error: not indenteds=l[:i]+l[i+1:]p=perm(l[:i]+l[i+1:])# error: unexpected indentforxinp:r.append(l[i:i+1]+x)returnr# 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 returnr does not match a level popped off the stack.)
2.1.9. 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: ' ' | tab | formfeed Whitespace is needed between two tokens only if their concatenation could otherwise be interpreted as a different token. For example, ab is one token, but ab is two tokens. However, +a and +a both produce two tokens, + and a, as +a is not a valid token.
2.1.10. End marker
At the end of non-interactive input, the lexical analyzer generates an
token.
2.2. Other tokens
Besides
,
and
, the following categories of tokens exist: identifiers and keywords (
), literals (such as
and
), and other symbols (operators and delimiters,
). Whitespace characters (other than logical 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.
2.3. Names (identifiers and keywords)
tokens represent identifiers, keywords, and soft keywords.
Names are composed of the following characters:
uppercase and lowercase letters (A-Z and a-z),
the underscore (_),
digits (0 through 9), which cannot appear as the first character, and
non-ASCII characters. Valid names may only contain “letter-like” and “digit-like” characters; see
for details.
Names must contain at least one character, but have no upper length limit. Case is significant.
Formally, names are described by the following lexical definitions:
NAME:
* name_start: "a"..."z" | "A"..."Z" | "_" | <non-ASCII character> name_continue: name_start | "0"..."9"identifier: <
, except keywords> Note that not all names matched by this grammar are valid; see
for details.
2.3.1. Keywords
The following names 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 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 2.3.2. Soft Keywords
Added in version 3.10.
Some names are only reserved under specific contexts. These are known as soft keywords:
match, case, and _, when used in the
statement.
type, when used in the
statement.
lazy, when used before an
statement.
These syntactically act as keywords in their specific 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.
Changed in version 3.12: type is now a soft keyword.
Changed in version 3.15: lazy is now a soft keyword.
2.3.3. 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 frommoduleimport*.
_In a case pattern within a
statement, _ is a
that denotes a
.
Separately, the interactive interpreter makes the result of the last evaluation available in the variable _. (It is stored in the
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
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
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
.
2.3.4. Non-ASCII characters in names
Names that contain non-ASCII characters need additional normalization and validation beyond the rules and grammar explained
. For example, ř_1, 蛇, or साँप are valid names, but r〰2, €, or 🐍 are not.
This section explains the exact rules.
All names are converted into the
NFKC while parsing. This means that, for example, some typographic variants of characters are converted to their “basic” form. For example, fiⁿₐˡᵢᶻₐᵗᵢᵒₙ normalizes to finalization, so Python treats them as the same name:
>>> fiⁿₐˡᵢᶻₐᵗᵢᵒₙ=3>>> finalization3Note
Normalization is done at the lexical level only. Run-time functions that take names as strings generally do not normalize their arguments. For example, the variable defined above is accessible at run time in the
dictionary as globals()["finalization"] but not globals()["fiⁿₐˡᵢᶻₐᵗᵢᵒₙ"].
Similarly to how ASCII-only names must contain only letters, digits and the underscore, and cannot start with a digit, a valid name must start with a character in the “letter-like” set xid_start, and the remaining characters must be in the “letter- and digit-like” set xid_continue.
These sets are based on the XID_Start and XID_Continue sets as defined by the Unicode standard annex
. Python’s xid_start additionally includes the underscore (_). Note that Python does not necessarily conform to
.
A non-normative listing of characters in the XID_Start and XID_Continue sets as defined by Unicode is available in the
file in the Unicode Character Database. For reference, the construction rules for the xid_* sets are given below.
The set id_start is defined as the union of:
Unicode category <Lu> - uppercase letters (includes A to Z)
Unicode category <Ll> - lowercase letters (includes a to z)
Unicode category <Lt> - titlecase letters
Unicode category <Lm> - modifier letters
Unicode category <Lo> - other letters
Unicode category <Nl> - letter numbers
{"_"} - the underscore
<Other_ID_Start> - an explicit set of characters in
to support backwards compatibility
The set xid_start then closes this set under NFKC normalization, by removing all characters whose normalization is not of the form id_startid_continue*.
The set id_continue is defined as the union of:
id_start (see above)
Unicode category <Nd> - decimal numbers (includes 0 to 9)
Unicode category <Pc> - connector punctuations
Unicode category <Mn> - nonspacing marks
Unicode category <Mc> - spacing combining marks
<Other_ID_Continue> - another explicit set of characters in
to support backwards compatibility
Again, xid_continue closes this set under NFKC normalization.
Unicode categories use the version of the Unicode Character Database as included in the
module.
See also
– Supporting Non-ASCII Identifiers
– Unicode-related Security Considerations for Python
2.4. Literals
Literals are notations for constant values of some built-in types.
In terms of lexical analysis, Python has
and
literals.
Other “literals” are lexically denoted using
(None, True, False) and the special
(...).
2.5. String and Bytes literals
String literals are text enclosed in single quotes (') or double quotes ("). For example:
"spam"'eggs'The quote used to start the literal also terminates it, so a string literal can only contain the other quote (except with escape sequences, see below). For example:
'Say "Hello", please.'"Don't do that!"Except for this limitation, the choice of quote character (' or ") does not affect how the literal is parsed.
Inside a string literal, the backslash (\) character introduces an escape sequence, which has special meaning depending on the character after the backslash. For example, \" denotes the double quote character, and does not end the string:
>>> print("Say \"Hello\" to everyone!")Say "Hello" to everyone!See
below for a full list of such sequences, and more details.
2.5.1. Triple-quoted strings
Strings can also be enclosed in matching groups of three single or double quotes. These are generally referred to as triple-quoted strings:
"""This is a triple-quoted string."""In triple-quoted literals, unescaped quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the literal, if they are of the same kind (' or ") used at the start:
"""This string has "quotes" inside."""Unescaped newlines are also allowed and retained:
'''This triple-quoted stringcontinues on the next line.'''2.5.2. String prefixes
String literals can have an optional prefix that influences how the content of the literal is parsed, for example:
b"data"f'{result=}'The allowed prefixes are:
b:
r:
f:
(“f-string”)
t:
(“t-string”)
u: No effect (allowed for backwards compatibility)
See the linked sections for details on each type.
Prefixes are case-insensitive (for example, ‘B’ works the same as ‘b’). The ‘r’ prefix can be combined with ‘f’, ‘t’ or ‘b’, so ‘fr’, ‘rf’, ‘tr’, ‘rt’, ‘br’, and ‘rb’ are also valid prefixes.
Added in version 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
for more information.
2.5.3. Formal grammar
String literals, except
and
, are described by the following lexical definitions.
These definitions use
(!) to indicate that an ending quote ends the literal.
STRING: [
] (
) stringprefix: <("r" | "u" | "b" | "br" | "rb"), case-insensitive> stringcontent: | "'''" ( !"'''"
)* "'''" | '"""' ( !'"""'
)* '"""' | "'" ( !"'"
)* "'" | '"' ( !'"'
)* '"'stringitem:
|
stringchar: <any
, except backslash and newline> longstringitem:
| newline stringescapeseq: "\" <any
> Note that as in all lexical definitions, whitespace is significant. In particular, the prefix (if any) must be immediately followed by the starting quote.
2.5.4. 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
\<newline>
\\
\'
\"
\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
\xhh
\N{name}
\uxxxx
\Uxxxxxxxx
2.5.4.1. Ignored end of line
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
, or parentheses and
.
2.5.4.2. Escaped characters
To include a backslash in a non-
Python string literal, it must be doubled. The \\ escape sequence denotes a single backslash character:
>>> print('C:\\Program Files')C:\Program FilesSimilarly, the \' and \" sequences denote the single and double quote character, respectively:
>>> print('\' and \"')' and "2.5.4.3. Octal character
The sequence \ooo denotes a character with the octal (base 8) value ooo:
>>> '\120''P'Up to three octal digits (0 through 7) are accepted.
In a bytes literal, character means a byte with the given value. In a string literal, it means a Unicode character with the given value.
Changed in version 3.11: Octal escapes with value larger than 0o377 (255) produce a
.
Changed in version 3.12: Octal escapes with value larger than 0o377 (255) produce a
. In a future Python version they will raise a
.
2.5.4.4. Hexadecimal character
The sequence \xhh denotes a character with the hex (base 16) value hh:
>>> '\x50''P'Unlike in Standard C, exactly two hex digits are required.
In a bytes literal, character means a byte with the given value. In a string literal, it means a Unicode character with the given value.
2.5.4.5. Named Unicode character
The sequence \N{name} denotes a Unicode character with the given name:
>>> '\N{LATIN CAPITAL LETTER P}''P'>>> '\N{SNAKE}''🐍'This sequence cannot appear in
.
Changed in version 3.3: Support for
has been added.
2.5.4.6. Hexadecimal Unicode characters
These sequences \uxxxx and \Uxxxxxxxx denote the Unicode character with the given hex (base 16) value. Exactly four digits are required for \u; exactly eight digits are required for \U. The latter can encode any Unicode character.
>>> '\u1234''ሴ'>>> '\U0001f40d''🐍'These sequences cannot appear in
.
2.5.4.7. Unrecognized escape sequences
Unlike in Standard C, all unrecognized escape sequences are left in the string unchanged, that is, the backslash is left in the result:
>>> print('\q')\q>>> list('\q')['\\', 'q']Note that for bytes literals, the escape sequences only recognized in string literals (\N..., \u..., \U...) fall into the category of unrecognized escapes.
Changed in version 3.6: Unrecognized escape sequences produce a
.
Changed in version 3.12: Unrecognized escape sequences produce a
. In a future Python version they will raise a
.
2.5.5. Bytes literals
Bytes literals are always prefixed with ‘b’ or ‘B’; they produce an instance of the
type instead of the
type. They may only contain ASCII characters; bytes with a numeric value of 128 or greater must be expressed with escape sequences (typically
or
):
>>> b'\x89PNG\r\n\x1a\n'b'\x89PNG\r\n\x1a\n'>>> list(b'\x89PNG\r\n\x1a\n')[137, 80, 78, 71, 13, 10, 26, 10]Similarly, a zero byte must be expressed using an escape sequence (typically \0 or \x00).
2.5.6. Raw string literals
Both string and bytes literals may optionally be prefixed with a letter ‘r’ or ‘R’; such constructs are called raw string literals and raw bytes literals respectively and treat backslashes as literal characters. As a result, in raw string literals,
are not treated specially:
>>> r'\d{4}-\d{2}-\d{2}''\\d{4}-\\d{2}-\\d{2}'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.
2.5.7. f-strings
Added in version 3.6.
Changed in version 3.7: The
and
can be used in expressions within f-strings.
Changed in version 3.8: Added the debug specifier (=)
Changed in version 3.12: Many restrictions on expressions within f-strings have been removed. Notably, nested strings, comments, and backslashes are now permitted.
A formatted string literal or f-string is a string literal that is prefixed with ‘f’ or ‘F’. Unlike other string literals, f-strings do not have a constant value. They may contain replacement fields delimited by curly braces {}. Replacement fields contain expressions which are evaluated at run time. For example:
>>> who='nobody'>>> nationality='Spanish'>>> f'{who.title()} expects the {nationality} Inquisition!''Nobody expects the Spanish Inquisition!'Any doubled curly braces ({{ or }}) outside replacement fields are replaced with the corresponding single curly brace:
>>> print(f'{{...}}'){...}Other characters outside replacement fields are treated like in ordinary string literals. This means that escape sequences are decoded (except when a literal is also marked as a raw string), and newlines are possible in triple-quoted f-strings:
>>> name='Galahad'>>> favorite_color='blue'>>> print(f'{name}:\t{favorite_color}')Galahad: blue>>> print(rf"C:\Users\{name}")C:\Users\Galahad>>> print(f'''Three shall be the number of the counting... and the number of the counting shall be three.''')Three shall be the number of the countingand the number of the counting shall be three.Expressions in formatted string literals are treated like regular Python expressions. Each expression is evaluated in the context where the formatted string literal appears, in order from left to right. An empty expression is not allowed, and both
and assignment expressions := must be surrounded by explicit parentheses:
>>> f'{(half:=1/2)}, {half*42}''0.5, 21.0'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'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:abcIt is possible to nest f-strings:
>>> name='world'>>> f'Repeated:{f' hello {name}'*3}''Repeated: hello world hello world hello world'Portable Python programs should not use more than 5 levels of nesting.
CPython implementation detail: CPython does not limit nesting of f-strings.
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). This means that replacement fields with comments must be closed in a different line:
>>> a = 2 >>> f"abc{a # This comment }" continues until the end of the line ... + 3}" 'abc5' After the expression, replacement fields may optionally contain:
a debug specifier – an equal sign (=), optionally surrounded by whitespace on one or both sides;
a conversion specifier – !s, !r or !a; and/or
a format specifier prefixed with a colon (:).
See the
Standard Library section on f-strings
for details on how these fields are evaluated.
As that section explains, format specifiers are passed as the second argument to the
function to format a replacement field value. For example, they can be used to specify a field width and padding characters using the
Format Specification Mini-Language
:
>>> number=14.3>>> f'{number:20.7f}'' 14.3000000'Top-level format specifiers may include nested replacement fields:
>>> field_size=20>>> precision=7>>> f'{number:{field_size}.{precision}f}'' 14.3000000'These nested fields may include their own conversion fields and
:
>>> number=3>>> f'{number:{field_size}}'' 3'>>> f'{number:{field_size:05}}''00000000000000000003'However, these nested fields may not include more deeply nested replacement fields.
Formatted string literals cannot be used as
, even if they do not include expressions:
>>> deffoo():... f"Not a docstring"...>>> print(foo.__doc__)NoneSee also
– Literal String Interpolation
– Syntactic formalization of f-strings
, which uses a related format string mechanism.
2.5.8. t-strings
Added in version 3.14.
A template string literal or t-string is a string literal that is prefixed with ‘t’ or ‘T’. These strings follow the same syntax rules as
. For differences in evaluation rules, see the
Standard Library section on t-strings
2.5.9. Formal grammar for f-strings
F-strings are handled partly by the
, which produces the tokens
,
and
, and partly by the parser, which handles expressions in the replacement field. The exact way the work is split is a CPython implementation detail.
Correspondingly, the f-string grammar is a mix of
lexical and syntactic definitions
.
Whitespace is significant in these situations:
There may be no whitespace in
(between the prefix and quote).
Whitespace in
is part of the literal string contents.
In fstring_replacement_field, if f_debug_specifier is present, all whitespace after the opening brace until the f_debug_specifier, as well as whitespace immediately following f_debug_specifier, is retained as part of the expression.
CPython implementation detail: The expression is not handled in the tokenization phase; it is retrieved from the source code using locations of the { token and the token after =.
The FSTRING_MIDDLE definition uses
(!) to indicate special characters (backslash, newline, {, }) and sequences (f_quote).
fstring:
*
FSTRING_START:
("'" | '"' | "'''" | '"""') FSTRING_END:
fstringprefix: <("f" | "fr" | "rf"), case-insensitive> f_debug_specifier: '='f_quote: <the quote character(s) used in FSTRING_START> fstring_middle: |
|
FSTRING_MIDDLE: | (!"\" !
!'{' !'}' !
)
|
| "{{" | "}}" | <newline, in triple-quoted f-strings only> fstring_replacement_field: | '{'
[
] [
] [
] '}'fstring_conversion: | "!" ("s" | "r" | "a") fstring_full_format_spec: | ':'
* fstring_format_spec: |
|
f_expression: | ','.(
| "*"
)+ [","] |
Note
In the above grammar snippet, the f_quote and FSTRING_MIDDLE rules are context-sensitive – they depend on the contents of FSTRING_START of the nearest enclosing fstring.
Constructing a more traditional formal grammar from this template is left as an exercise for the reader.
The grammar for t-strings is identical to the one for f-strings, with t instead of f at the beginning of rule and token names and in the prefix.
tstring: TSTRING_START tstring_middle* TSTRING_END <rest of the t-string grammar is omitted; see above> 2.6. Numeric literals
tokens represent numeric literals, of which there are three types: integers, floating-point numbers, and imaginary numbers.
NUMBER:
|
|
The numeric value of a numeric literal is the same as if it were passed as a string to the
,
or
class constructor, respectively. Note that not all valid inputs for those constructors are also valid literals.
Numeric literals do not include a sign; a phrase like -1 is actually an expression composed of the unary operator ‘-’ and the literal 1.
2.6.1. Integer literals
Integer literals denote whole numbers. For example:
732147483647There is no limit for the length of integer literals apart from what can be stored in available memory:
7922816251426433759354395033679228162514264337593543950336Underscores can be used to group digits for enhanced readability, and are ignored for determining the numeric value of the literal. For example, the following literals are equivalent:
100_000_000_0001000000000001_00_00_00_00_000Underscores can only occur between digits. For example, _123, 321_, and 123__321 are not valid literals.
Integers can be specified in binary (base 2), octal (base 8), or hexadecimal (base 16) using the prefixes 0b, 0o and 0x, respectively. Hexadecimal digits 10 through 15 are represented by letters A-F, case-insensitive. For example:
0b1001101110b_1110_01010o1770o3770xdeadbeef0xDead_BeefAn underscore can follow the base specifier. For example, 0x_1f is a valid literal, but 0_x1f and 0x__1f are not.
Leading zeros in a non-zero decimal number are not allowed. For example, 0123 is not a valid literal. This is for disambiguation with C-style octal literals, which Python used before version 3.0.
Formally, integer literals are described by the following lexical definitions:
integer:
|
|
|
|
decinteger:
(["_"]
)* bininteger: "0" ("b" | "B") (["_"]
)+ octinteger: "0" ("o" | "O") (["_"]
)+ hexinteger: "0" ("x" | "X") (["_"]
)+ zerointeger: "0"+ (["_"] "0")* nonzerodigit: "1"..."9"digit: "0"..."9"bindigit: "0" | "1"octdigit: "0"..."7"hexdigit:
| "a"..."f" | "A"..."F"Changed in version 3.6: Underscores are now allowed for grouping purposes in literals.
2.6.2. Floating-point literals
Floating-point (float) literals, such as 3.14 or 1.5, denote
approximations of real numbers
.
They consist of integer and fraction parts, each composed of decimal digits. The parts are separated by a decimal point, .:
2.718284.0Unlike in integer literals, leading zeros are allowed. For example, 077.010 is legal, and denotes the same number as 77.01.
As in integer literals, single underscores may occur between digits to help readability:
96_485.332_1233.14_15_93Either of these parts, but not both, can be empty. For example:
10.# (equivalent to 10.0).001# (equivalent to 0.001)Optionally, the integer and fraction may be followed by an exponent: the letter e or E, followed by an optional sign, + or -, and a number in the same format as the integer and fraction parts. The e or E represents “times ten raised to the power of”:
1.0e3# (represents 1.0×10³, or 1000.0)1.166e-5# (represents 1.166×10⁻⁵, or 0.00001166)6.02214076e+23# (represents 6.02214076×10²³, or 602214076000000000000000.)In floats with only integer and exponent parts, the decimal point may be omitted:
1e3# (equivalent to 1.e3 and 1.0e3)0e0# (equivalent to 0.)Formally, floating-point literals are described by the following lexical definitions:
floatnumber: |
"." [
] [
] | "."
[
] |
digitpart:
(["_"]
)* exponent: ("e" | "E") ["+" | "-"]
Changed in version 3.6: Underscores are now allowed for grouping purposes in literals.
2.6.3. Imaginary literals
Python has
objects, but no complex literals. Instead, imaginary literals denote complex numbers with a zero real part.
For example, in math, the complex number 3+4.2i is written as the real number 3 added to the imaginary number 4.2i. Python uses a similar syntax, except the imaginary unit is written as j rather than i:
3+4.2jThis is an expression composed of the
3, the
‘+’, and the
4.2j. Since these are three separate tokens, whitespace is allowed between them:
3+4.2jNo whitespace is allowed within each token. In particular, the j suffix, may not be separated from the number before it.
The number before the j has the same syntax as a floating-point literal. Thus, the following are valid imaginary literals:
4.2j3.14j10.j.001j1e100j3.14e-10j3.14_15_93jUnlike in a floating-point literal the decimal point can be omitted if the imaginary number only has an integer part. The number is still evaluated as a floating-point number, not an integer:
10j0j1000000000000000000000000j# equivalent to 1e+24jThe j suffix is case-insensitive. That means you can use J instead:
3.14J# equivalent to 3.14jFormally, imaginary literals are described by the following lexical definition:
imagnumber: (
|
) ("j" | "J") 2.7. Operators and delimiters
The following grammar defines operator and delimiter tokens, that is, the generic
token type. A
list of these tokens and their names
is also available in the token module documentation.
OP: | assignment_operator | bitwise_operator | comparison_operator | enclosing_delimiter | other_delimiter | arithmetic_operator | "..." | other_op assignment_operator: "+=" | "-=" | "*=" | "**=" | "/=" | "//=" | "%=" | "&=" | "|=" | "^=" | "<<=" | ">>=" | "@=" | ":="bitwise_operator: "&" | "|" | "^" | "~" | "<<" | ">>"comparison_operator: "<=" | ">=" | "<" | ">" | "==" | "!="enclosing_delimiter: "(" | ")" | "[" | "]" | "{" | "}"other_delimiter: "," | ":" | "!" | ";" | "=" | "->"arithmetic_operator: "+" | "-" | "**" | "*" | "//" | "/" | "%"other_op: "." | "@"Note
Generally, operators are used to combine
, while delimiters serve other purposes. However, there is no clear, formal distinction between the two categories.
Some tokens can serve as either operators or delimiters, depending on usage. For example, * is both the multiplication operator and a delimiter used for sequence unpacking, and @ is both the matrix multiplication and a delimiter that introduces decorators.
For some tokens, the distinction is unclear. For example, some people consider ., (, and ) to be delimiters, while others see the
operator and the function call operator(s).
Some of Python’s operators, like and, or, and notin, use
tokens rather than “symbols” (operator tokens).
A sequence of three consecutive periods (...) has a special meaning as an
literal.