# 2. 字句解析

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.

文字 | Next token (or other relevant documentation)
--- | ---
空白 タブ 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. 行構造

Python プログラムは多数の 論理行 (logical lines) に分割されます。

### 2.1.1. 論理行 (logical line)

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. 物理行 (physical line)

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. コメント (Comments)

コメントは文字列リテラル内に入っていないハッシュ文字 (`#`) から始まり、同じ物理行の末端で終わります。 非明示的な行継続規則が適用されていない限り、コメントは論理行を終端させます。 コメントは構文上無視されます。

### 2.1.4. エンコード宣言 (encoding declaration)

Python スクリプト中の一行目か二行目にあるコメントが正規表現 `coding[=:]\s*([-\w.]+)` にマッチする場合、コメントはエンコード宣言として処理されます; この表現の最初のグループがソースコードファイルのエンコードを指定します。エンコード宣言は自身の行になければなりません。二行目にある場合、一行目もコメントのみの行でなければなりません。エンコード宣言式として推奨する形式は

    # -*- coding: <encoding-name> -*-

これは GNU Emacs で認識できます。または

    # vim:fileencoding=<encoding-name>

これは、Bram Moolenar による VIM が認識できる形式です。

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.

エンコーディングが宣言される場合、そのエンコーディング名は Python によって認識できなければなりません (標準エンコーディング を参照してください)。宣言されたエンコーディングは、例えば文字列リテラル、コメント、識別子などの、全ての字句解析に使われます。

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. 明示的な行継続

二つまたはそれ以上の物理行を論理行としてつなげるためには、バックスラッシュ文字 (`\`) を使って以下のようにします: 物理行が文字列リテラルやコメント中の文字でないバックスラッシュで終わっている場合、後続する行とつなげて一つの論理行を構成し、バックスラッシュおよびバックスラッシュの後ろにある行末文字を削除します。例えば:

    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

バックスラッシュで終わる行にはコメントを入れることはできません。また、バックスラッシュを使ってコメントを継続することはできません。バックスラッシュが文字列リテラル中にある場合を除き、バックスラッシュの後ろにトークンを継続することはできません (すなわち、物理行内の文字列リテラル以外のトークンをバックスラッシュを使って分断することはできません)。上記以外の場所では、文字列リテラル外にあるバックスラッシュはどこにあっても不正となります。

### 2.1.6. 非明示的な行継続

丸括弧 (parentheses)、角括弧 (square bracket) 、および波括弧 (curly brace) 内の式は、バックスラッシュを使わずに一行以上の物理行に分割することができます。例えば:

    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

非明示的に継続された行にはコメントを含めることができます。継続行のインデントは重要ではありません。空の継続行を書くことができます。非明示的な継続行中には、NEWLINE トークンは存在しません。非明示的な行の継続は、三重クオートされた文字列 (下記参照) でも発生します; この場合には、コメントを含めることができません。

### 2.1.7. 空行

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. インデント

論理行の行頭にある、先頭の空白 (スペースおよびタブ) の連なりは、その行のインデントレベルを計算するために使われます。インデントレベルは、実行文のグループ化方法を決定するために用いられます。

タブは (左から右の方向に) 1 つにつき 8 つのスペースで置き換えられ、置き換え後の文字数は 8 の倍数になります (Unix で使われている規則と同じになるよう意図されています)。そして、最初の非空白文字までのスペースの総数が、その行のインデントを決定します。インデントは、バックスラッシュで複数の物理行に分割できません; 最初のバックスラッシュまでの空白がインデントを決定します。

ソースファイルがタブとスペースを混在させ、その意味づけがタブのスペース換算数に依存するようなら、インデントは不合理なものとして却下されます。その場合は `TabError` が送出されます。

プラットフォーム間の互換性に関する注意: 非 UNIX プラットフォームにおけるテキストエディタの性質上、一つのソースファイル内でタブとインデントを混在させて使うのは賢明ではありません。また、プラットフォームによっては、最大インデントレベルを明示的に制限しているかもしれません。

フォームフィード文字が行の先頭にあっても構いません; フォームフィード文字は上のインデントレベル計算時には無視されます。フォームフィード文字が先頭の空白中の他の場所にある場合、その影響は未定義です (例えば、スペースの数を 0 にリセットするかもしれません)。

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.

以下の例に正しく (しかし当惑させるように) インデントされた Python コードの一部を示します:

    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

以下の例は、様々なインデントエラーになります:

     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

(実際は、最初の 3 つのエラーはパーザによって検出されます; 最後のエラーのみが字句解析器で見つかります --- `return r` のインデントは、スタックから逐次除去されていくどのインデントレベル値とも一致しません)

### 2.1.9. トークン間の空白

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 `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. その他のトークン

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.

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 Non-ASCII characters in names 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 name_continue*
    name_start:    "a"..."z" | "A"..."Z" | "_" | <non-ASCII character>
    name_continue: name_start | "0"..."9"
    identifier:    <NAME, except keywords>

Note that not all names matched by this grammar are valid; see Non-ASCII characters in names for details.

### 2.3.1. キーワード (keyword)

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. ソフトキーワード

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.

- `lazy`, when used before an `import` 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.

バージョン 3.12 で変更: `type` is now a soft keyword.

バージョン 3.15 で変更: `lazy` is now a soft keyword.

### 2.3.3. 予約済みの識別子種 (reserved classes of identifiers)

ある種の (キーワードを除く) 識別子には、特殊な意味があります。これらの識別子種は、先頭や末尾にあるアンダースコア文字のパターンで区別されます:

`_*`

  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.

  注釈

  名前 `_` は、しばしば国際化 (internationalization) と共に用いられます; この慣習についての詳しい情報は、 `gettext` を参照してください。

  It is also commonly used for unused variables.

`__*__`

  システムで定義された (system-defined) 名前です。非公式には"dunder"な名前と呼ばれます(訳注: double underscoresの略)。これらの名前はインタープリタと (標準ライブラリを含む) 実装上で定義されています。 現行のシステムでの名前は 特殊メソッド名 などで話題に挙げられています。 Python の将来のバージョンではより多くの名前が定義されることになります。このドキュメントで明記されている用法に従わない、 あらゆる `__*__` の名前は、いかなるコンテキストにおける利用でも、警告無く損害を引き起こすことがあります。

`__*`

  クラスプライベート (class-private) な名前です。このカテゴリに属する名前は、クラス定義のコンテキスト上で用いられた場合、基底クラスと派生クラスの "プライベートな" 属性間で名前衝突が起こるのを防ぐために書き直されます。 識別子 (identifier、または名前 (name)) を参照してください。

### 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 above. 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 normalization form 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
    >>> finalization
    3

注釈

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 `globals()` 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 UAX-31. Python's `xid_start` additionally includes the underscore (`_`). Note that Python does not necessarily conform to UAX-31.

A non-normative listing of characters in the XID_Start and XID_Continue sets as defined by Unicode is available in the DerivedCoreProperties.txt 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 PropList.txt 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_start id_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 PropList.txt 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 `unicodedata` module.

参考

- PEP 3131 -- Supporting Non-ASCII Identifiers

- PEP 672 -- Unicode-related Security Considerations for Python

## 2.4. リテラル

リテラル (literal) とは、いくつかの組み込み型の定数を表記したものです。

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. 文字列およびバイト列リテラル

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: raw バイト列リテラルの `'rb'` プレフィックスが `'br'` の同義語として追加されました。

Python 2.x と 3.x 両対応のコードベースのメンテナンスを単純化するために、レガシー unicode リテラル (`u'value'`) のサポートが再び導入されました。詳細は PEP 414 を参照してください。

### 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:

エスケープシーケンス | 意味
--- | ---
`\`<newline> | Ignored end of line
`\\` | Backslash
`\'` | Single quote
`\"` | Double quote
`\a` | ASCII 端末ベル (BEL)
`\b` | ASCII バックスペース (BS)
`\f` | ASCII フォームフィード (FF)
`\n` | ASCII 行送り (LF)
`\r` | ASCII 復帰 (CR)
`\t` | ASCII 水平タブ (TAB)
`\v` | ASCII 垂直タブ (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.

バージョン 3.11 で変更: Octal escapes with value larger than `0o377` (255) produce a `DeprecationWarning`.

バージョン 3.12 で変更: 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'

標準 C とは違い、ちょうど 2 桁の 16 進数しか受理されません。

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.

バージョン 3.3 で変更: 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.

バージョン 3.6 で変更: Unrecognized escape sequences produce a `DeprecationWarning`.

バージョン 3.12 で変更: 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}'

raw リテラルでも、引用符はバックスラッシュでエスケープできますが、バックスラッシュ自体も文字列に残ります; 例えば、`r"\""` は有効な文字列リテラルで、バックスラッシュと二重引用符からなる文字列を表します; `r"\"` は無効な文字列リテラルです (raw リテラルを奇数個連なったバックスラッシュで終わらせることはできません)。具体的には、(バックスラッシュが直後のクオート文字をエスケープしてしまうので) raw文字列を単一のバックスラッシュで終わらせることはできません さらに、バックスラッシュの直後に改行がきても、行継続を意味する のではなく、リテラルの一部であるそれら二つの文字として解釈されます。

### 2.5.7. f-strings

Added in version 3.6.

バージョン 3.7 で変更: The `await` and `async for` can be used in expressions within f-strings.

バージョン 3.8 で変更: Added the debug specifier (`=`)

バージョン 3.12 で変更: 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 counting
    and 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 `lambda` 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:
    a
    b
    c

It 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 実装の詳細: 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 `format()` 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 format specifiers:

    >>> 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 docstrings, even if they do not include expressions:

    >>> def foo():
    ...     f"Not a docstring"
    ...
    >>> print(foo.__doc__)
    None

参考

- PEP 498 -- Literal String Interpolation

- PEP 701 -- Syntactic formalization of f-strings

- `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 rules as formatted string literals. 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 lexical analyzer, which produces the tokens `FSTRING_START`, `FSTRING_MIDDLE` and `FSTRING_END`, 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 `FSTRING_START` (between the prefix and quote).

- Whitespace in `FSTRING_MIDDLE` 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 実装の詳細: 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 negative lookaheads (`!`) to indicate special characters (backslash, newline, `{`, `}`) and sequences (`f_quote`).

    fstring:    FSTRING_START fstring_middle* FSTRING_END

    FSTRING_START:      fstringprefix ("'" | '"' | "'''" | '"""')
    FSTRING_END:        f_quote
    fstringprefix:      <("f" | "fr" | "rf"), case-insensitive>
    f_debug_specifier:  '='
    f_quote:            <the quote character(s) used in FSTRING_START>

    fstring_middle:
       | fstring_replacement_field
       | FSTRING_MIDDLE
    FSTRING_MIDDLE:
       | (!"\" !newline !'{' !'}' !f_quote) source_character
       | stringescapeseq
       | "{{"
       | "}}"
       | <newline, in triple-quoted f-strings only>
    fstring_replacement_field:
       | '{' f_expression [f_debug_specifier] [fstring_conversion]
             [fstring_full_format_spec] '}'
    fstring_conversion:
       | "!" ("s" | "r" | "a")
    fstring_full_format_spec:
       | ':' fstring_format_spec*
    fstring_format_spec:
       | FSTRING_MIDDLE
       | fstring_replacement_field
    f_expression:
       | ','.(conditional_expression | "*" or_expr)+ [","]
       | yield_expression

注釈

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. 数値リテラル

`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. 整数リテラル

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"

バージョン 3.6 で変更: グループ化を目的としたリテラル中のアンダースコアが許されるようになりました。

### 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

バージョン 3.6 で変更: グループ化を目的としたリテラル中のアンダースコアが許されるようになりました。

### 2.6.3. 虚数 (imaginary) リテラル

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:              "."  | "@"

注釈

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.