"annotationlib" --- Functionality for introspecting annotations
***************************************************************

**Source code:** Lib/annotationlib.py

======================================================================

The "annotationlib" module provides tools for introspecting
*annotations* on modules, classes, and functions.

Annotations are lazily evaluated and often contain forward references
to objects that are not yet defined when the annotation is created.
This module provides a set of low-level tools that can be used to
retrieve annotations in a reliable way, even in the presence of
forward references and other edge cases.

This module supports retrieving annotations in three main formats (see
"Format"), each of which works best for different use cases:

* "VALUE" evaluates the annotations and returns their value. This is
  most straightforward to work with, but it may raise errors, for
  example if the annotations contain references to undefined names.

* "FORWARDREF" returns "ForwardRef" objects for annotations that
  cannot be resolved, allowing you to inspect the annotations without
  evaluating them. This is useful when you need to work with
  annotations that may contain unresolved forward references.

* "SOURCE" returns the annotations as a string, similar to how it
  would appear in the source file. This is useful for documentation
  generators that want to display annotations in a readable way.

The "get_annotations()" function is the main entry point for
retrieving annotations. Given a function, class, or module, it returns
an annotations dictionary in the requested format. This module also
provides functionality for working directly with the *annotate
function* that is used to evaluate annotations, such as
"get_annotate_function()" and "call_annotate_function()", as well as
the "call_evaluate_function()" function for working with *evaluate
functions*.

参考:

  **PEP 649** proposed the current model for how annotations work in
  Python.

  **PEP 749** expanded on various aspects of **PEP 649** and
  introduced the "annotationlib" module.

  Annotations Best Practices provides best practices for working with
  annotations.

  typing-extensions provides a backport of "get_annotations()" that
  works on earlier versions of Python.


Annotation semantics
====================

The way annotations are evaluated has changed over the history of
Python 3, and currently still depends on a future import. There have
been execution models for annotations:

* *Stock semantics* (default in Python 3.0 through 3.13; see **PEP
  3107** and **PEP 526**): Annotations are evaluated eagerly, as they
  are encountered in the source code.

* *Stringified annotations* (used with "from __future__ import
  annotations" in Python 3.7 and newer; see **PEP 563**): Annotations
  are stored as strings only.

* *Deferred evaluation* (default in Python 3.14 and newer; see **PEP
  649** and **PEP 749**): Annotations are evaluated lazily, only when
  they are accessed.

As an example, consider the following program:

   def func(a: Cls) -> None:
       print(a)

   class Cls: pass

   print(func.__annotations__)

This will behave as follows:

* Under stock semantics (Python 3.13 and earlier), it will throw a
  "NameError" at the line where "func" is defined, because "Cls" is an
  undefined name at that point.

* Under stringified annotations (if "from __future__ import
  annotations" is used), it will print "{'a': 'Cls', 'return':
  'None'}".

* Under deferred evaluation (Python 3.14 and later), it will print
  "{'a': <class 'Cls'>, 'return': None}".

Stock semantics were used when function annotations were first
introduced in Python 3.0 (by **PEP 3107**) because this was the
simplest, most obvious way to implement annotations. The same
execution model was used when variable annotations were introduced in
Python 3.6 (by **PEP 526**). However, stock semantics caused problems
when using annotations as type hints, such as a need to refer to names
that are not yet defined when the annotation is encountered. In
addition, there were performance problems with executing annotations
at module import time. Therefore, in Python 3.7, **PEP 563**
introduced the ability to store annotations as strings using the "from
__future__ import annotations" syntax. The plan at the time was to
eventually make this behavior the default, but a problem appeared:
stringified annotations are more difficult to process for those who
introspect annotations at runtime. An alternative proposal, **PEP
649**, introduced the third execution model, deferred evaluation, and
was implemented in Python 3.14. Stringified annotations are still used
if "from __future__ import annotations" is present, but this behavior
will eventually be removed.


Classes
=======

class annotationlib.Format

   An "IntEnum" describing the formats in which annotations can be
   returned. Members of the enum, or their equivalent integer values,
   can be passed to "get_annotations()" and other functions in this
   module, as well as to "__annotate__" functions.

   VALUE = 1

      Values are the result of evaluating the annotation expressions.

   FORWARDREF = 2

      Values are real annotation values (as per "Format.VALUE" format)
      for defined values, and "ForwardRef" proxies for undefined
      values. Real objects may contain references to, "ForwardRef"
      proxy objects.

   SOURCE = 3

      Values are the text string of the annotation as it appears in
      the source code, up to modifications including, but not
      restricted to, whitespace normalizations and constant values
      optimizations.

      The exact values of these strings may change in future versions
      of Python.

   Added in version 3.14.

class annotationlib.ForwardRef

   A proxy object for forward references in annotations.

   Instances of this class are returned when the "FORWARDREF" format
   is used and annotations contain a name that cannot be resolved.
   This can happen when a forward reference is used in an annotation,
   such as when a class is referenced before it is defined.

   __forward_arg__

      A string containing the code that was evaluated to produce the
      "ForwardRef". The string may not be exactly equivalent to the
      original source.

   evaluate(*, globals=None, locals=None, type_params=None, owner=None)

      Evaluate the forward reference, returning its value.

      This may throw an exception, such as "NameError", if the forward
      reference refers to names that do not exist. The arguments to
      this method can be used to provide bindings for names that would
      otherwise be undefined.

      "ForwardRef" instances returned by "get_annotations()" retain
      references to information about the scope they originated from,
      so calling this method with no further arguments may be
      sufficient to evaluate such objects. "ForwardRef" instances
      created by other means may not have any information about their
      scope, so passing arguments to this method may be necessary to
      evaluate them successfully.

      *globals* and *locals* are passed to "eval()", representing the
      global and local namespaces in which the name is evaluated.
      *type_params*, if given, must be a tuple of type parameters that
      are in scope while the forward reference is being evaluated.
      *owner* is the object that owns the annotation from which the
      forward reference derives, usually a function, class, or module.

      重要:

        Once a "ForwardRef" instance has been evaluated, it caches the
        evaluated value, and future calls to "evaluate()" will return
        the cached value, regardless of the parameters passed in.

   Added in version 3.14.


Functions
=========

annotationlib.call_annotate_function(annotate, format, *, owner=None)

   Call the *annotate function* *annotate* with the given *format*, a
   member of the "Format" enum, and return the annotations dictionary
   produced by the function.

   This helper function is required because annotate functions
   generated by the compiler for functions, classes, and modules only
   support the "VALUE" format when called directly. To support other
   formats, this function calls the annotate function in a special
   environment that allows it to produce annotations in the other
   formats. This is a useful building block when implementing
   functionality that needs to partially evaluate annotations while a
   class is being constructed.

   *owner* is the object that owns the annotation function, usually a
   function, class, or module. If provided, it is used in the
   "FORWARDREF" format to produce a "ForwardRef" object that carries
   more information.

   参考:

     **PEP 649** contains an explanation of the implementation
     technique used by this function.

   Added in version 3.14.

annotationlib.call_evaluate_function(evaluate, format, *, owner=None)

   Call the *evaluate function* *evaluate* with the given *format*, a
   member of the "Format" enum, and return the value produced by the
   function. This is similar to "call_annotate_function()", but the
   latter always returns a dictionary mapping strings to annotations,
   while this function returns a single value.

   This is intended for use with the evaluate functions generated for
   lazily evaluated elements related to type aliases and type
   parameters:

   * "typing.TypeAliasType.evaluate_value()", the value of type
     aliases

   * "typing.TypeVar.evaluate_bound()", the bound of type variables

   * "typing.TypeVar.evaluate_constraints()", the constraints of type
     variables

   * "typing.TypeVar.evaluate_default()", the default value of type
     variables

   * "typing.ParamSpec.evaluate_default()", the default value of
     parameter specifications

   * "typing.TypeVarTuple.evaluate_default()", the default value of
     type variable tuples

   *owner* is the object that owns the evaluate function, such as the
   type alias or type variable object.

   *format* can be used to control the format in which the value is
   returned:

      >>> type Alias = undefined
      >>> call_evaluate_function(Alias.evaluate_value, Format.VALUE)
      Traceback (most recent call last):
      ...
      NameError: name 'undefined' is not defined
      >>> call_evaluate_function(Alias.evaluate_value, Format.FORWARDREF)
      ForwardRef('undefined')
      >>> call_evaluate_function(Alias.evaluate_value, Format.SOURCE)
      'undefined'

   Added in version 3.14.

annotationlib.get_annotate_function(obj)

   Retrieve the *annotate function* for *obj*. Return "None" if *obj*
   does not have an annotate function.

   This is usually equivalent to accessing the "__annotate__"
   attribute of *obj*, but direct access to the attribute may return
   the wrong object in certain situations involving metaclasses. This
   function should be used instead of accessing the attribute
   directly.

   Added in version 3.14.

annotationlib.get_annotations(obj, *, globals=None, locals=None, eval_str=False, format=Format.VALUE)

   Compute the annotations dict for an object.

   *obj* may be a callable, class, module, or other object with
   "__annotate__" and "__annotations__" attributes. Passing in an
   object of any other type raises "TypeError".

   The *format* parameter controls the format in which annotations are
   returned, and must be a member of the "Format" enum or its integer
   equivalent.

   Returns a dict. "get_annotations()" returns a new dict every time
   it's called; calling it twice on the same object will return two
   different but equivalent dicts.

   This function handles several details for you:

   * If *eval_str* is true, values of type "str" will be un-stringized
     using "eval()". This is intended for use with stringized
     annotations ("from __future__ import annotations"). It is an
     error to set *eval_str* to true with formats other than
     "Format.VALUE".

   * If *obj* doesn't have an annotations dict, returns an empty dict.
     (Functions and methods always have an annotations dict; classes,
     modules, and other types of callables may not.)

   * Ignores inherited annotations on classes, as well as annotations
     on metaclasses. If a class doesn't have its own annotations dict,
     returns an empty dict.

   * All accesses to object members and dict values are done using
     "getattr()" and "dict.get()" for safety.

   *eval_str* controls whether or not values of type "str" are
   replaced with the result of calling "eval()" on those values:

   * If eval_str is true, "eval()" is called on values of type "str".
     (Note that "get_annotations()" doesn't catch exceptions; if
     "eval()" raises an exception, it will unwind the stack past the
     "get_annotations()" call.)

   * If *eval_str* is false (the default), values of type "str" are
     unchanged.

   *globals* and *locals* are passed in to "eval()"; see the
   documentation for "eval()" for more information. If *globals* or
   *locals* is "None", this function may replace that value with a
   context-specific default, contingent on "type(obj)":

   * If *obj* is a module, *globals* defaults to "obj.__dict__".

   * If *obj* is a class, *globals* defaults to
     "sys.modules[obj.__module__].__dict__" and *locals* defaults to
     the *obj* class namespace.

   * If *obj* is a callable, *globals* defaults to "obj.__globals__",
     although if *obj* is a wrapped function (using
     "functools.update_wrapper()") or a "functools.partial" object, it
     is unwrapped until a non-wrapped function is found.

   Calling "get_annotations()" is best practice for accessing the
   annotations dict of any object. See Annotations Best Practices for
   more information on annotations best practices.

      >>> def f(a: int, b: str) -> float:
      ...     pass
      >>> get_annotations(f)
      {'a': <class 'int'>, 'b': <class 'str'>, 'return': <class 'float'>}

   Added in version 3.14.
