# 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 produces these tokens.

The lexical analyzer determines the program text’s encoding (UTF-8 by default), and decodes the text into source characters. If the text cannot be decoded, a `SyntaxError` 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 | Whitespace
CR, LF | New line Indentation
backslash (`\`) | Explicit line joining (Also significant in string escape sequences)
hash (`#`) | Comment
quote (`'`, `"`) | String literal
ASCII letter (`a`-`z`, `A`-`Z`) non-ASCII character | Name Prefixed string or bytes literal
underscore (`_`) | Name (Can also be part of numeric literals)
number (`0`-`9`) | Numeric literal
dot (`.`) | Numeric literal Operator
question mark (`?`) dollar (`$`) backquote (`​`​`) control character | Error (outside string literals and comments)
other printing character | Operator or delimiter
end of file | End marker

## 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:
       | "'''" ( !"'''" longstringitem)* "'''"
       | '"""' ( !'"""' longstringitem)* '"""'
       | "'" ( !"'" stringitem)* "'"
       | '"' ( !'"' stringitem)* '"'
    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
`\xhh` | Hexadecimal character
`\N{name}` | Named Unicode character
`\uxxxx` | Hexadecimal Unicode character
`\Uxxxxxxxx` | 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 `\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 bytes literals.

Changed in version 3.3: Support for name aliases 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 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, template string literals evaluate to a `string.templatelib.Template` object.

- 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 inner expression `{precision}` to determine the value of the `format_spec` attribute. If `precision` were to be `2`, the resulting format specifier would be `'.2f'`.

- When the equals sign `'='` is provided in an interpolation expression, the text of the expression is appended to the literal string that precedes the relevant interpolation. This includes the equals sign and any surrounding whitespace. The `Interpolation` instance for the expression will be created as normal, except that `conversion` will be set to ‘`r`’ (`repr()`) by default. If an explicit conversion or format specifier are provided, this will override the default behaviour.

## 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. 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_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+4.2i 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+4.2j

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 and delimiters

The following grammar defines operator and delimiter tokens, that is, the generic `OP` 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 expressions, 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 `getattr()` operator and the function call operator(s).

Some of Python’s operators, like `and`, `or`, and `not in`, use keyword tokens rather than “symbols” (operator tokens).

A sequence of three consecutive periods (`...`) has a special meaning as an `Ellipsis` literal.