2. Lexical analysis
*******************

A Python program is read by a *parser*.  Input to the parser is a
stream of *tokens*, generated by the *lexical analyzer* (also known as
the *tokenizer*). This chapter describes how the lexical analyzer
breaks a file into tokens.

Python reads program text as Unicode code points; the encoding of a
source file can be given by an encoding declaration and defaults to
UTF-8, see **PEP 3120** for details.  If the source file cannot be
decoded, a "SyntaxError" is raised.


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 "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.


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 ‘Classic Mac OS’ 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 string
literals.) 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>


2.1.3. Comments
---------------

A comment starts with a hash character ("#") that is not part of a
string literal, and ends at the end of the physical line.  A comment
signifies the end of the logical line unless the implicit line joining
rules are invoked. Comments are ignored by the syntax.


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 Standard Encodings). 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:

   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.


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 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.


2.1.7. Blank lines
------------------

A logical line that contains only spaces, tabs, formfeeds and possibly
a comment, is ignored (i.e., no "NEWLINE" token is generated). During
interactive input of statements, handling of a blank line may differ
depending on the implementation of the read-eval-print loop. In the
standard interactive interpreter, an entirely blank logical line (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 "TabError" is raised in that case.

**Cross-platform compatibility note:** because of the nature of text
editors on non-UNIX platforms, it is unwise to use a mixture of spaces
and tabs for the indentation in a single source file.  It should also
be noted that different platforms may explicitly limit the maximum
indentation level.

A formfeed character may be present at the start of the line; it will
be ignored for the indentation calculations above.  Formfeed
characters occurring elsewhere in the leading whitespace have an
undefined effect (for instance, they may reset the space count to
zero).

The indentation levels of consecutive lines are used to generate
"INDENT" and "DEDENT" tokens, using a stack, as follows.

Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again.  The numbers pushed on
the stack will always be strictly increasing from bottom to top.  At
the beginning of each logical line, the line’s indentation level is
compared to the top of the stack. If it is equal, nothing happens. If
it is larger, it is pushed on the stack, and one "INDENT" token is
generated.  If it is smaller, it *must* be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a "DEDENT" token is
generated. At the end of the file, a "DEDENT" token is generated for
each number remaining on the stack that is larger than zero.

Here is an example of a correctly (though confusingly) indented piece
of Python code:

   def perm(l):
           # Compute the list of all permutations of l
       if len(l) <= 1:
                     return [l]
       r = []
       for i in range(len(l)):
                s = l[:i] + l[i+1:]
                p = perm(s)
                for x in p:
                 r.append(l[i:i+1] + x)
       return r

The following example shows various indentation errors:

    def perm(l):                       # error: first line indented
   for i in range(len(l)):             # error: not indented
       s = l[:i] + l[i+1:]
           p = perm(l[:i] + l[i+1:])   # error: unexpected indent
           for x in p:
                   r.append(l[i:i+1] + x)
               return r                # error: inconsistent dedent

(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer — the indentation of
"return r" does not match a level popped off the stack.)


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 is needed between two
tokens only if their concatenation could otherwise be interpreted as a
different token. For example, "ab" is one token, but "a b" 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
"ENDMARKER" token.


2.2. Other tokens
=================

Besides "NEWLINE", "INDENT" and "DEDENT", the following categories of
tokens exist: *identifiers* and *keywords* ("NAME"), *literals* (such
as "NUMBER" and "STRING"), and other symbols (*operators* and
*delimiters*, "OP"). 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)
=====================================

"NAME" tokens represent *identifiers*, *keywords*, and *soft
keywords*.

Within the ASCII range (U+0001..U+007F), the valid characters for
names include the uppercase and lowercase letters ("A-Z" and "a-z"),
the underscore "_" and, except for the first character, the digits "0"
through "9".

Names must contain at least one character, but have no upper length
limit. Case is significant.

Besides "A-Z", "a-z", "_" and "0-9", names can also use “letter-like”
and “number-like” characters from outside the ASCII range, as detailed
below.

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

Formally, the first character of a normalized identifier must belong
to the set "id_start", which is 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 PropList.txt
  to support backwards compatibility

The remaining characters must belong to the set "id_continue", which
is the union of:

* all characters in "id_start"

* 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
  PropList.txt to support backwards compatibility

Unicode categories use the version of the Unicode Character Database
as included in the "unicodedata" module.

These sets are based on the Unicode standard annex UAX-31. See also
**PEP 3131** for further details.

Even more formally, names are described by the following lexical
definitions:

   NAME:         xid_start xid_continue*
   id_start:     <Lu> | <Ll> | <Lt> | <Lm> | <Lo> | <Nl> | "_" | <Other_ID_Start>
   id_continue:  id_start | <Nd> | <Pc> | <Mn> | <Mc> | <Other_ID_Continue>
   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*)">
   identifier:   <NAME, except keywords>

A non-normative listing of all valid identifier characters as defined
by Unicode is available in the DerivedCoreProperties.txt file in the
Unicode Character Database.


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 "match" statement.

* "type", when used in the "type" 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.


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 "from module import *".

"_"
   In a "case" pattern within a "match" statement, "_" is a soft
   keyword that denotes a wildcard.

   Separately, the interactive interpreter makes the result of the
   last evaluation available in the variable "_". (It is stored in the
   "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
     "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 Special method names section and elsewhere. More
   will likely be defined in future versions of Python.  *Any* use of
   "__*__" names, in any context, that does not follow explicitly
   documented use, is subject to breakage without warning.

"__*"
   Class-private names.  Names in this category, when used within the
   context of a class definition, are re-written to use a mangled form
   to help avoid name clashes between “private” attributes of base and
   derived classes. See section Identifiers (Names).


2.4. Literals
=============

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

In terms of lexical analysis, Python has string, bytes and numeric
literals.

Other “literals” are lexically denoted using keywords ("None", "True",
"False") and the special ellipsis token ("...").


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 escape sequences 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 string
   continues 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": Bytes literal

* "r": Raw string

* "f": Formatted string literal (“f-string”)

* "t": Template string literal (“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 **PEP 414** for more information.


2.5.3. Formal grammar
---------------------

String literals, except “f-strings” and “t-strings”, are described by
the following lexical definitions.

These definitions use negative lookaheads ("!") to indicate that an
ending quote ends the literal.

   STRING:          [stringprefix] (stringcontent)
   stringprefix:    <("r" | "u" | "b" | "br" | "rb"), case-insensitive>
   stringcontent:
      | "'" ( !"'" stringitem)* "'"
      | '"' ( !'"' stringitem)* '"'
      | "'''" ( !"'''" longstringitem)* "'''"
      | '"""' ( !'"""' longstringitem)* '"""'
   stringitem:      stringchar | stringescapeseq
   stringchar:      <any source_character, except backslash and newline>
   longstringitem:  stringitem | newline
   stringescapeseq: "\" <any source_character>

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>                                       | Ignored end of line                                |
+----------------------------------------------------+----------------------------------------------------+
| "\\"                                               | Backslash                                          |
+----------------------------------------------------+----------------------------------------------------+
| "\'"                                               | Single quote                                       |
+----------------------------------------------------+----------------------------------------------------+
| "\""                                               | Double quote                                       |
+----------------------------------------------------+----------------------------------------------------+
| "\a"                                               | ASCII Bell (BEL)                                   |
+----------------------------------------------------+----------------------------------------------------+
| "\b"                                               | ASCII Backspace (BS)                               |
+----------------------------------------------------+----------------------------------------------------+
| "\f"                                               | ASCII Formfeed (FF)                                |
+----------------------------------------------------+----------------------------------------------------+
| "\n"                                               | ASCII Linefeed (LF)                                |
+----------------------------------------------------+----------------------------------------------------+
| "\r"                                               | ASCII Carriage Return (CR)                         |
+----------------------------------------------------+----------------------------------------------------+
| "\t"                                               | ASCII Horizontal Tab (TAB)                         |
+----------------------------------------------------+----------------------------------------------------+
| "\v"                                               | ASCII Vertical Tab (VT)                            |
+----------------------------------------------------+----------------------------------------------------+
| "\*ooo*"                                           | Octal character                                    |
+----------------------------------------------------+----------------------------------------------------+
| "\x*hh*"                                           | Hexadecimal character                              |
+----------------------------------------------------+----------------------------------------------------+
| "\N{*name*}"                                       | Named Unicode character                            |
+----------------------------------------------------+----------------------------------------------------+
| "\u*xxxx*"                                         | Hexadecimal Unicode character                      |
+----------------------------------------------------+----------------------------------------------------+
| "\U*xxxxxxxx*"                                     | Hexadecimal Unicode character                      |
+----------------------------------------------------+----------------------------------------------------+


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 triple-quoted strings, or
parentheses and string literal concatenation.


2.5.4.2. Escaped characters
~~~~~~~~~~~~~~~~~~~~~~~~~~~

To include a backslash in a non-raw Python string literal, it must be
doubled. The "\\" escape sequence denotes a single backslash
character:

   >>> print('C:\\Program Files')
   C:\Program Files

Similarly, 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 "DeprecationWarning".

Changed in version 3.12: Octal escapes with value larger than "0o377"
(255) produce a "SyntaxWarning". In a future Python version they will
raise a "SyntaxError".


2.5.4.4. Hexadecimal character
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The sequence "\x*hh*" 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 bytes literals.

Changed in version 3.3: Support for name aliases has been added.


2.5.4.6. Hexadecimal Unicode characters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

These sequences "\u*xxxx*" and "\U*xxxxxxxx*" 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 bytes literals.


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
"DeprecationWarning".

Changed in version 3.12: Unrecognized escape sequences produce a
"SyntaxWarning". In a future Python version they will raise a
"SyntaxError".


2.5.5. Bytes literals
---------------------

*Bytes literals* are always prefixed with "'b'" or "'B'"; they produce
an instance of the "bytes" type instead of the "str" type. They may
only contain ASCII characters; bytes with a numeric value of 128 or
greater must be expressed with escape sequences (typically Hexadecimal
character or Octal character):

   >>> 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, escape
sequences 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.

A *formatted string literal* or *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:

   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 "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.

   >>> f"abc{a # This is a comment }"
   ... + 3}"
   'abc5'

Changed in version 3.7: Prior to Python 3.7, an "await" expression and
comprehensions containing an "async for" clause were illegal in the
expressions in formatted string literals due to a problem with the
implementation.

Changed in version 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 "repr()" of
the expression to be provided, unless there is a format specified.
When a format is specified it defaults to the "str()" of the
expression unless a conversion "'!r'" is declared.

Added in version 3.8: The equal sign "'='".

If a conversion is specified, the result of evaluating the expression
is converted before formatting.  Conversion "'!s'" calls "str()" on
the result, "'!r'" calls "repr()", and "'!a'" calls "ascii()".

The result is then formatted using the "format()" protocol.  The
format specifier is passed to the "__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 format
specifiers, but may not include more deeply nested replacement fields.
The format specifier mini-language is the same as that used by the
"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'

Changed in version 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

Changed in version 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 "str.format()", 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 and
evaluation rules as formatted string literals, with the following
differences:

* Rather than evaluating to a "str" object, t-strings evaluate to a
  "Template" object from the "string.templatelib" module.

* The "format()" protocol is not used. Instead, the format specifier
  and conversions (if any) are passed to a new "Interpolation" object
  that is created for each evaluated expression. It is up to code that
  processes the resulting "Template" object to decide how to handle
  format specifiers and conversions.

* Format specifiers containing nested replacement fields are evaluated
  eagerly, prior to being passed to the "Interpolation" object. For
  instance, an interpolation of the form "{amount:.{precision}f}" will
  evaluate the expression "{precision}" before setting the
  "format_spec" attribute of the resulting "Interpolation" object; if
  "precision" is (for example) "2", the resulting format specifier
  will be "'.2f'".

* When the equal sign "'='" is provided in an interpolation
  expression, the resulting "Template" object will have the expression
  text along with a "'='" character placed in its "strings" attribute.
  The "interpolations" attribute will also contain an "Interpolation"
  instance for the expression. By default, the "conversion" attribute
  will be set to "'r'" (that is, "repr()"), unless there is a
  conversion explicitly specified (in which case it overrides the
  default) or a format specifier is provided (in which case, the
  "conversion" defaults to "None").


2.6. Numeric literals
=====================

"NUMBER" tokens represent numeric literals, of which there are three
types: integers, floating-point numbers, and imaginary numbers.

   NUMBER: integer | floatnumber | imagnumber

The numeric value of a numeric literal is the same as if it were
passed as a string to the "int", "float" or "complex" 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:

   7
   3
   2147483647

There is no limit for the length of integer literals apart from what
can be stored in available memory:

   7922816251426433759354395033679228162514264337593543950336

Underscores 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_000
   100000000000
   1_00_00_00_00_000

Underscores 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:

   0b100110111
   0b_1110_0101
   0o177
   0o377
   0xdeadbeef
   0xDead_Beef

An 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 | octinteger | hexinteger | zerointeger
   decinteger:   nonzerodigit (["_"] digit)*
   bininteger:   "0" ("b" | "B") (["_"] bindigit)+
   octinteger:   "0" ("o" | "O") (["_"] octdigit)+
   hexinteger:   "0" ("x" | "X") (["_"] hexdigit)+
   zerointeger:  "0"+ (["_"] "0")*
   nonzerodigit: "1"..."9"
   digit:        "0"..."9"
   bindigit:     "0" | "1"
   octdigit:     "0"..."7"
   hexdigit:     digit | "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, ".":

   2.71828
   4.0

Unlike in integer literals, leading zeros are allowed in the numeric
parts. For example, "077.010" is legal, and denotes the same number as
"77.10".

As in integer literals, single underscores may occur between digits to
help readability:

   96_485.332_123
   3.14_15_93

Either 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 "." [digitpart] [exponent]
      | "." digitpart [exponent]
      | digitpart exponent
   digitpart: digit (["_"] digit)*
   exponent:  ("e" | "E") ["+" | "-"] digitpart

Changed in version 3.6: Underscores are now allowed for grouping
purposes in literals.


2.6.3. Imaginary literals
-------------------------

Python has complex number objects, but no complex literals. Instead,
*imaginary literals* denote complex numbers with a zero real part.

For example, in math, the complex number 3+4.2*i* is written as the
real number 3 added to the imaginary number 4.2*i*. Python uses a
similar syntax, except the imaginary unit is written as "j" rather
than *i*:

   3+4.2j

This is an expression composed of the integer literal "3", the
operator ‘"+"’, and the imaginary literal "4.2j". Since these are
three separate tokens, whitespace is allowed between them:

   3 + 4.2j

No 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:

   4.2j
   3.14j
   10.j
   .001j
   1e100j
   3.14e-10j
   3.14_15_93j

Unlike 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:

   10j
   0j
   1000000000000000000000000j   # equivalent to 1e+24j

The "j" suffix is case-insensitive. That means you can use "J"
instead:

   3.14J   # equivalent to 3.14j

Formally, imaginary literals are described by the following lexical
definition:

   imagnumber: (floatnumber | digitpart) ("j" | "J")


2.7. Operators
==============

The following tokens are operators:

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


2.8. Delimiters
===============

The following tokens serve as delimiters in the grammar:

   (       )       [       ]       {       }
   ,       :       !       .       ;       @       =

The period can also occur in floating-point and imaginary literals.

A sequence of three periods has a special meaning as an "Ellipsis"
literal:

   ...

The following *augmented assignment operators* serve lexically as
delimiters, but also perform an operation:

   ->      +=      -=      *=      /=      //=     %=
   @=      &=      |=      ^=      >>=     <<=     **=

The following printing ASCII characters have special meaning as part
of other tokens or are otherwise significant to the lexical analyzer:

   '       "       #       \

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

   $       ?       `
