# 2. Análisis léxico

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)
--- | ---
espacio tabulador 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. Estructura de línea

Un programa Python se divide en un número de líneas lógicas.

### 2.1.1. Líneas lógicas

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. Líneas físicas

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. Comentarios

Un comentario comienza con un carácter de almohadilla (`#`) que no es parte de un literal de cadena, y termina al final de la línea física. Un comentario implica el final de la línea lógica, a menos que se invoque la regla implícita de unión de líneas. Los comentarios son ignorados por la sintaxis.

### 2.1.4. Declaración de Codificación

Si un comentario en la primera o segunda línea del script de Python coincide con la expresión regular `coding[=:]\s*([-\w.]+)`, este comentario se procesa como una declaración de codificación; el primer grupo de esta expresión denomina la codificación del archivo de código fuente. La declaración de codificación debe aparecer en una línea propia. Si se trata de la segunda línea, la primera línea debe ser también una línea solamente de comentario. Las formas recomendadas de una expresión de codificación son

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

que también es reconocido por GNU Emacs y

    # vim:fileencoding=<encoding-name>

que es reconocido por el VIM de Bram Moolenaar.

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.

Si se declara una codificación, Python debe reconocer el nombre de la codificación (ver Codificaciones estándar). La codificación se utiliza para todos los análisis léxicos, incluidos las cadenas literales, los comentarios y los identificadores.

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. Unión explícita de líneas

Dos o más líneas físicas pueden unirse en líneas lógicas utilizando caracteres de barra invertida (`\`), de la siguiente manera: cuando una línea física termina en una barra invertida que no es parte de literal de cadena o de un comentario, se une con la siguiente formando una sola línea lógica, borrando la barra invertida y el siguiente carácter de fin de línea. Por ejemplo:

    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

Una línea que termina en una barra invertida no puede llevar un comentario. Una barra invertida no continúa un comentario. Una barra invertida no continúa un token excepto para los literales de la cadena (es decir, los tokens que no sean literales de la cadena no pueden ser divididos a través de líneas físicas usando una barra invertida). La barra invertida es ilegal en cualquier parte de una línea fuera del literal de la cadena.

### 2.1.6. Unión implícita de líneas

Las expresiones entre paréntesis, entre corchetes o entre rizos pueden dividirse en más de una línea física sin usar barras invertidas. Por ejemplo:

    month_names = ['Januari', 'Februari', 'Maart',      # Son los
                   'April',   'Mei',      'Juni',       # nombres holandeses
                   'Juli',    'Augustus', 'September',  # para los meses
                   'Oktober', 'November', 'December']   # del año

Las líneas continuas implícitas pueden llevar comentarios. La sangría de las líneas de continuación no es importante. Se permiten líneas de continuación en blanco. No hay ningún token NEWLINE (nueva línea) entre las líneas de continuación implícitas. Las líneas de continuación implícitas también pueden aparecer dentro de cadenas de triple comilla ( ver más adelante); en ese caso no pueden llevar comentarios.

### 2.1.7. Líneas en blanco

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. Sangría

El espacio en blanco ( espacios y tabulaciones) al principio de una línea lógica se utiliza para calcular el nivel de sangría de la línea, que a su vez se utiliza para determinar la agrupación de las declaraciones.

Los tabuladores se sustituyen (de izquierda a derecha) por uno a ocho espacios, de manera que el número total de caracteres hasta el reemplazo inclusive es un múltiplo de ocho (se pretende que sea la misma regla que la utilizada por Unix). El número total de espacios que preceden al primer carácter no en blanco determina entonces la sangría de la línea. La sangría no puede dividirse en múltiples líneas físicas utilizando barras invertidas; el espacio en blanco hasta la primera barra invertida determina la sangría.

La indentación se rechaza como inconsistente si un archivo fuente mezcla tabulaciones y espacios de manera que el significado depende del valor de una tabulación en los espacios; un `TabError` se produce en ese caso.

Nota de compatibilidad entre plataformas: debido a la naturaleza de los editores de texto en plataformas que no sean UNIX, no es aconsejable utilizar una mezcla de espacios y tabuladores para la sangría en un solo archivo de origen. También debe tenerse en cuenta que las diferentes plataformas pueden limitar explícitamente el nivel máximo de sangría.

Un carácter formfeed puede estar presente al comienzo de la línea; será ignorado para los cálculos de sangría anteriores. Los caracteres formfeed que aparecen en otras partes del espacio en blanco inicial tienen un efecto indefinido (por ejemplo, pueden poner a cero el recuento de espacio).

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.

Aquí hay un ejemplo de un código de Python con una correcta (aunque no tan clara) sangría:

    def perm(l):
            # Calcular la lista de todas las permutaciones de 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

El siguiente ejemplo muestra varios errores de sangría:

     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

(En realidad, los tres primeros errores son detectados por el analizador; sólo el último error es encontrado por el analizador léxico — la sangría de `return r` no coincide con un nivel sacado de la pila.)

### 2.1.9. Espacios en blanco entre tokens

Except at the beginning of a logical line or in string literals, the whitespace characters space, tab and formfeed can be used interchangeably to separate tokens:

    whitespace:  ' ' | tab | formfeed

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

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. Palabras clave

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. Palabras clave suaves

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.

Como palabras clave suaves, su uso en la gramática es posible sin dejar de preservar la compatibilidad con el código existente que usa esos nombres como nombres de identificadores.

Distinto en la versión 3.12: `type` ahora es un palabra clave suave.

### 2.3.3. Clases reservadas de identificadores

Ciertas clases de identificadores (además de las palabras clave) tienen significados especiales. Estas clases se identifican por los patrones de los caracteres de guión bajo que van delante y detrás:

`_*`

  No importado por `from module import *`.

`_`

  En un patrón `case` dentro de una declaración `match`, `_` es una palabra clave suave que denota un comodín wildcard.

  Por separado, el intérprete interactivo pone a disposición el resultado de la última evaluación en la variable `_`. (Se almacena en el módulo `builtins`, junto con funciones incorporadas como `print`).

  En otros lugares, `_` es un identificador regular. A menudo se usa para nombrar elementos «especiales», pero no es especial para Python en sí.

  Nota

  El nombre `_` se usa a menudo en conjunción con la internacionalización; consultar la documentación del módulo `gettext`` para más información sobre esta convención.

  También se usa comúnmente para variables no utilizadas.

`__*__`

  Nombres definidos por el sistema, conocidos informalmente como nombres «dunder». Estos nombres son definidos por el intérprete y su aplicación (incluida la biblioteca estándar). Los nombres actuales del sistema se discuten en la sección Nombres especiales de método y en otros lugares. Es probable que se definan más en futuras versiones de Python. Cualquier uso de nombres `__*__`, en cualquier contexto, que no siga un uso explícitamente documentado, está sujeto a que se rompa sin previo aviso.

`__*`

  Nombres de clase privada. Los nombres de esta categoría, cuando se utilizan en el contexto de una definición de clase, se reescriben para utilizar una forma desfigurada que ayude a evitar conflictos de nombres entre los atributos «privados» de las clases base y derivadas. Ver la sección Identificadores (Nombres).

### 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

Nota

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 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.

Ver también

- PEP 3131 – Supporting Non-ASCII Identifiers

- PEP 672 – Unicode-related Security Considerations for Python

## 2.4. Literales

Los literales son notaciones para los valores constantes de algunos tipos incorporados.

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. Literales de cadenas y bytes

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: El prefijo `'rb'` de literales de bytes raw se ha añadido como sinónimo de `'br'`.

Se reintrodujo el soporte para el legado unicode literal (`u'value'`) para simplificar el mantenimiento de las bases de código dual Python 2.x y 3.x. Ver PEP 414 para más información.

### 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. Secuencias de escape

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:

Secuencia de escape | Significado
--- | ---
`\`<newline> | Ignored end of line
`\\` | Backslash
`\'` | Single quote
`\"` | Double quote
`\a` | ASCII Bell (BEL)
`\b` | ASCII Retroceso (BS)
`\f` | ASCII Formfeed (FF)
`\n` | ASCII Linefeed (LF)
`\r` | ASCII Retorno de carro (CR)
`\t` | ASCII Sangría horizontal (TAB)
`\v` | ASCII Sangría vertical (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

Se puede agregar una barra invertida al final de una línea para ignorar la nueva línea:

    >>> 'Esta cadena no incluirá \
    ... caracteres de barra invertida o nueva linea.'
    'Esta cadena no incluirá caracteres de barra invertida o nueva linea.'

Se puede lograr el mismo resultado usando triple-quoted strings, o paréntesis y 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.

Distinto en la versión 3.11: Octal escapes with value larger than `0o377` (255) produce a `DeprecationWarning`.

Distinto en la versión 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'

A diferencia de C estándar, se requieren exactamente dos dígitos hexadecimales.

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.

Distinto en la versión 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.

Distinto en la versión 3.6: Secuencias de escape no conocidas producen un `DeprecationWarning`.

Distinto en la versión 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}'

Incluso en un literal raw, las comillas se pueden escapar con una barra inversa, pero la barra inversa permanece en el resultado; por ejemplo, `r"\""` es un literal de cadena válido que consiste en dos caracteres: una barra inversa y una comilla doble; `r"\"` no es un literal de cadena válido (incluso una cadena en bruto no puede terminar en un número impar de barras inversas). Específicamente, un literal raw no puede terminar en una sola barra inversa (ya que la barra inversa se escaparía del siguiente carácter de comillas). Nótese también que una sola barra inversa seguida de una nueva línea se interpreta como esos dos caracteres como parte del literal, no como una continuación de línea.

### 2.5.7. f-strings

Added in version 3.6.

Distinto en la versión 3.7: The `await` and `async for` can be used in expressions within f-strings.

Distinto en la versión 3.8: Added the debug specifier (`=`)

Distinto en la versión 3.12: Many restrictions on expressions within f-strings have been removed. Notably, nested strings, comments, and backslashes are now permitted.

A formatted string literal or f-string is a string literal that is prefixed with “`f`” or “`F`”. Unlike other string literals, f-strings do not have a constant value. They may contain replacement fields delimited by curly braces `{}`. Replacement fields contain expressions which are evaluated at run time. For example:

    >>> who = 'nobody'
    >>> nationality = 'Spanish'
    >>> f'{who.title()} expects the {nationality} Inquisition!'
    'Nobody expects the Spanish Inquisition!'

Any doubled curly braces (`{{` or `}}`) outside replacement fields are replaced with the corresponding single curly brace:

    >>> print(f'{{...}}')
    {...}

Other characters outside replacement fields are treated like in ordinary string literals. This means that escape sequences are decoded (except when a literal is also marked as a raw string), and newlines are possible in triple-quoted f-strings:

    >>> name = 'Galahad'
    >>> favorite_color = 'blue'
    >>> print(f'{name}:\t{favorite_color}')
    Galahad:       blue
    >>> print(rf"C:\Users\{name}")
    C:\Users\Galahad
    >>> print(f'''Three shall be the number of the counting
    ... and the number of the counting shall be three.''')
    Three shall be the number of the 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'

Se puede reutilizar el tipo de comilla del f-string exterior dentro de un campo de reemplazo:

    >>> a = dict(x=2)
    >>> f"abc {a["x"]} def"
    'abc 2 def'

También se permiten barras invertidas en campos de reemplazo y se evalúan de la misma manera que cualquier otro contexto:

    >>> a = ["a", "b", "c"]
    >>> print(f"Lista a contiene:\n{"\n".join(a)}")
    Lista a contiene:
    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.

Detalles de implementación de CPython: 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

Ver también

- 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.

  Detalles de implementación de CPython: 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

Nota

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. Literales numéricos

`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. Literales enteros

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"

Distinto en la versión 3.6: Los guiones bajos están ahora permitidos para agrupar en literales.

### 2.6.2. Literales de punto flotante

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

Distinto en la versión 3.6: Los guiones bajos están ahora permitidos para agrupar en literales.

### 2.6.3. Literales imaginarios

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:              "."  | "@"

Nota

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.