# `__future__` — Future statement definitions

Código fuente: Lib/__future__.py

Imports of the form `from __future__ import feature` are called future statements. These are special-cased by the Python compiler to allow the use of new Python features in modules containing the future statement before the release in which the feature becomes standard.

While these future statements are given additional special meaning by the Python compiler, they are still executed like any other import statement and the `__future__` exists and is handled by the import system the same way any other Python module would be. This design serves three purposes:

- Para evitar confundir las herramientas existentes que analizan las declaraciones de importación y esperan encontrar los módulos que están importando.

- To document when incompatible changes were introduced, and when they will be — or were — made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing `__future__` and examining its contents.

- To ensure that future statements run under releases prior to Python 2.1 at least yield runtime exceptions (the import of `__future__` will fail, because there was no module of that name prior to 2.1).

## Module Contents

No feature description will ever be deleted from `__future__`. Since its introduction in Python 2.1 the following features have found their way into the language using this mechanism:

característica | opcional en | obligatorio en | efecto
--- | --- | --- | ---
__future__.nested_scopes | 2.1.0b1 | 2.2 | PEP 227: Ámbitos anidados estáticamente
__future__.generators | 2.2.0a1 | 2.3 | PEP 255: Generadores simples
__future__.division | 2.2.0a2 | 3.0 | PEP 238: Cambio de operador de división
__future__.absolute_import | 2.5.0a1 | 3.0 | PEP 328: Importaciones: Multilínea y Absoluto/Relativo
__future__.with_statement | 2.5.0a1 | 2.6 | PEP 343: The “with” Statement
__future__.print_function | 2.6.0a2 | 3.0 | PEP 3105: Hacer de print una función
__future__.unicode_literals | 2.6.0a2 | 3.0 | PEP 3112: Bytes literales en Python 3000
__future__.generator_stop | 3.5.0b1 | 3.7 | PEP 479: Manejo de StopIteration dentro de generadores
__future__.annotations | 3.7.0b1 | Never [1] | PEP 563: Postponed evaluation of annotations, PEP 649: Deferred evaluation of annotations using descriptors

class __future__._Feature

  Cada declaración en `__future__.py` tiene la forma:

      FeatureName = _Feature(OptionalRelease, MandatoryRelease,
                             CompilerFlag)

  donde, normalmente, OptionalRelease es menor que MandatoryRelease y ambos son 5-tuplas de la misma forma que `sys.version_info`:

      (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
       PY_MINOR_VERSION, # the 1; an int
       PY_MICRO_VERSION, # the 0; an int
       PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
       PY_RELEASE_SERIAL # the 3; an int
      )

_Feature.getOptionalRelease()

  OptionalRelease registra la primera versión en la que se aceptó la característica.

_Feature.getMandatoryRelease()

  En el caso de un MandatoryRelease que aún no se ha producido, MandatoryRelease predice el lanzamiento en el que la característica pasará a formar parte del lenguaje.

  De otro modo, MandatoryRelease registra cuándo la característica se convirtió en parte del lenguaje; en versiones en o después de este, los módulos ya no necesitan una declaración futura para usar la característica en cuestión, pero pueden continuar usando dichas importaciones.

  MandatoryRelease may also be `None`, meaning that a planned feature got dropped or that it is not yet decided.

_Feature.compiler_flag

  CompilerFlag is the (bitfield) flag that should be passed in the fourth argument to the built-in function `compile()` to enable the feature in dynamically compiled code. This flag is stored in the `_Feature.compiler_flag` attribute on `_Feature` instances.

[1]

`from __future__ import annotations` was previously scheduled to become mandatory in Python 3.10, but the change was delayed and ultimately canceled. This feature will eventually be deprecated and removed. See PEP 649 and PEP 749.

Ver también

Declaraciones Futuras

  Cómo trata el compilador las importaciones futuras.

PEP 236 - Back to the __future__

  The original proposal for the __future__ mechanism.