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.

  • STRING 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.

STRING = 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.annotations_to_string(annotations)

Convert an annotations dict containing runtime values to a dict containing only strings. If the values are not already strings, they are converted using value_to_string(). This is meant as a helper for user-provided annotate functions that support the STRING format but do not have access to the code creating the annotations.

For example, this is used to implement the STRING for typing.TypedDict classes created through the functional syntax:

>>> from typing import TypedDict
>>> Movie = TypedDict("movie", {"name": str, "year": int})
>>> get_annotations(Movie, format=Format.STRING)
{'name': 'str', 'year': 'int'}

Added in version 3.14.

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:

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.STRING)
'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.

annotationlib.value_to_string(value)

Convert an arbitrary Python value to a format suitable for use by the STRING format. This calls repr() for most objects, but has special handling for some objects, such as type objects.

This is meant as a helper for user-provided annotate functions that support the STRING format but do not have access to the code creating the annotations. It can also be used to provide a user-friendly string representation for other objects that contain values that are commonly encountered in annotations.

Added in version 3.14.