:mod:!annotationlib --- Functionality for introspecting annotations

.. module:: annotationlib :synopsis: Functionality for introspecting annotations

.. versionadded:: 3.14

Source code: :source:Lib/annotationlib.py

.. testsetup:: default

import annotationlib from annotationlib import *

The :mod:!annotationlib module provides tools for introspecting :term:annotations <annotation> on modules, classes, and functions.

Annotations are :ref:lazily evaluated <lazy-evaluation> 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 :class:Format), each of which works best for different use cases:

• :attr:~Format.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.
• :attr:~Format.FORWARDREF returns :class: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.
• :attr:~Format.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 :func: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 :term:annotate function that is used to evaluate annotations, such as :func:get_annotate_from_class_namespace and :func:call_annotate_function, as well as the :func:call_evaluate_function function for working with :term:evaluate functions <evaluate function>.

.. caution::

Most functionality in this module can execute arbitrary code; see :ref:the security section <annotationlib-security> for more information.

.. seealso::

:pep:649 proposed the current model for how annotations work in Python.

:pep:749 expanded on various aspects of :pep:649 and introduced the :mod:!annotationlib module.

:ref:annotations-howto provides best practices for working with annotations.

:pypi:typing-extensions provides a backport of :func: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 :ref:future import <future>. 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 :exc: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:: Format

An :class:~enum.IntEnum describing the formats in which annotations can be returned. Members of the enum, or their equivalent integer values, can be passed to :func:get_annotations and other functions in this module, as well as to :attr:~object.__annotate__ functions.

.. attribute:: VALUE :value: 1

  Values are the result of evaluating the annotation expressions.

.. attribute:: VALUE_WITH_FAKE_GLOBALS :value: 2

  Special value used to signal that an annotate function is being
  evaluated in a special environment with fake globals. When passed this
  value, annotate functions should either return the same value as for
  the :attr:`Format.VALUE` format, or raise :exc:`NotImplementedError`
  to signal that they do not support execution in this environment.
  This format is only used internally and should not be passed to
  the functions in this module.

.. attribute:: FORWARDREF :value: 3

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

.. attribute:: STRING :value: 4

  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.

.. versionadded:: 3.14

.. class:: ForwardRef

A proxy object for forward references in annotations.

Instances of this class are returned when the :attr:~Format.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.

.. attribute:: forward_arg

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

.. method:: evaluate(*, owner=None, globals=None, locals=None, type_params=None, format=Format.VALUE)

  Evaluate the forward reference, returning its value.

  If the *format* argument is :attr:`~Format.VALUE` (the default),
  this method may throw an exception, such as :exc:`NameError`, if the forward
  reference refers to a name that cannot be resolved. The arguments to this
  method can be used to provide bindings for names that would otherwise
  be undefined. If the *format* argument is :attr:`~Format.FORWARDREF`,
  the method will never throw an exception, but may return a :class:`~ForwardRef`
  instance. For example, if the forward reference object contains the code
  ``list[undefined]``, where ``undefined`` is a name that is not defined,
  evaluating it with the :attr:`~Format.FORWARDREF` format will return
  ``list[ForwardRef('undefined')]``. If the *format* argument is
  :attr:`~Format.STRING`, the method will return :attr:`~ForwardRef.__forward_arg__`.

  The *owner* parameter provides the preferred mechanism for passing scope
  information to this method. The owner of a :class:`~ForwardRef` is the
  object that contains the annotation from which the :class:`~ForwardRef`
  derives, such as a module object, type object, or function object.

  The *globals*, *locals*, and *type_params* parameters provide a more precise
  mechanism for influencing the names that are available when the :class:`~ForwardRef`
  is evaluated. *globals* and *locals* are passed to :func:`eval`, representing
  the global and local namespaces in which the name is evaluated.
  The *type_params* parameter is relevant for objects created using the native
  syntax for :ref:`generic classes <generic-classes>` and :ref:`functions <generic-functions>`.
  It is a tuple of :ref:`type parameters <type-params>` that are in scope
  while the forward reference is being evaluated. For example, if evaluating a
  :class:`~ForwardRef` retrieved from an annotation found in the class namespace
  of a generic class ``C``, *type_params* should be set to ``C.__type_params__``.

  :class:`~ForwardRef` instances returned by :func:`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. :class:`~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.

  If no *owner*, *globals*, *locals*, or *type_params* are provided and the
  :class:`~ForwardRef` does not contain information about its origin,
  empty globals and locals dictionaries are used.

.. versionadded:: 3.14

Functions

.. function:: 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 :func:type_repr. This is meant as a helper for user-provided annotate functions that support the :attr:~Format.STRING format but do not have access to the code creating the annotations.

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

.. doctest::

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

.. versionadded:: 3.14

.. function:: call_annotate_function(annotate, format, *, owner=None)

Call the :term:annotate function annotate with the given format, a member of the :class: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 :attr:~Format.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 :attr:~Format.FORWARDREF format to produce a :class:ForwardRef object that carries more information.

.. seealso::

  :PEP:`PEP 649 <649#the-stringizer-and-the-fake-globals-environment>`
  contains an explanation of the implementation technique used by this
  function.

.. versionadded:: 3.14

.. function:: call_evaluate_function(evaluate, format, *, owner=None)

Call the :term:evaluate function evaluate with the given format, a member of the :class:Format enum, and return the value produced by the function. This is similar to :func: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:

• :meth:typing.TypeAliasType.evaluate_value, the value of type aliases
• :meth:typing.TypeVar.evaluate_bound, the bound of type variables
• :meth:typing.TypeVar.evaluate_constraints, the constraints of type variables
• :meth:typing.TypeVar.evaluate_default, the default value of type variables
• :meth:typing.ParamSpec.evaluate_default, the default value of parameter specifications
• :meth: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:

.. doctest::

  >>> 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'

.. versionadded:: 3.14

.. function:: get_annotate_from_class_namespace(namespace)

Retrieve the :term:annotate function from a class namespace dictionary namespace. Return :const:!None if the namespace does not contain an annotate function. This is primarily useful before the class has been fully created (e.g., in a metaclass); after the class exists, the annotate function can be retrieved with cls.__annotate__. See :ref:below <annotationlib-metaclass> for an example using this function in a metaclass.

.. versionadded:: 3.14

.. function:: 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 :attr:~object.__annotate__ or :attr:~object.__annotations__ attributes. Passing any other object raises :exc:TypeError.

The format parameter controls the format in which annotations are returned, and must be a member of the :class:Format enum or its integer equivalent. The different formats work as follows:

• VALUE: :attr:!object.__annotations__ is tried first; if that does not exist, the :attr:!object.__annotate__ function is called if it exists.

• FORWARDREF: If :attr:!object.__annotations__ exists and can be evaluated successfully, it is used; otherwise, the :attr:!object.__annotate__ function is called. If it does not exist either, :attr:!object.__annotations__ is tried again and any error from accessing it is re-raised.

  • When calling :attr:!object.__annotate__ it is first called with :attr:~Format.FORWARDREF. If this is not implemented, it will then check if :attr:~Format.VALUE_WITH_FAKE_GLOBALS is supported and use that in the fake globals environment. If neither of these formats are supported, it will fall back to using :attr:~Format.VALUE. If :attr:~Format.VALUE fails, the error from this call will be raised.

• STRING: If :attr:!object.__annotate__ exists, it is called first; otherwise, :attr:!object.__annotations__ is used and stringified using :func:annotations_to_string.

  • When calling :attr:!object.__annotate__ it is first called with :attr:~Format.STRING. If this is not implemented, it will then check if :attr:~Format.VALUE_WITH_FAKE_GLOBALS is supported and use that in the fake globals environment. If neither of these formats are supported, it will fall back to using :attr:~Format.VALUE with the result converted using :func:annotations_to_string. If :attr:~Format.VALUE fails, the error from this call will be raised.

Returns a dict. :func:!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 :class:!str will be un-stringized using :func: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 :attr: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 :class:!str are replaced with the result of calling :func:eval on those values:

• If eval_str is true, :func:eval is called on values of type :class:!str. (Note that :func:!get_annotations doesn't catch exceptions; if :func:eval raises an exception, it will unwind the stack past the :func:!get_annotations call.)
• If eval_str is false (the default), values of type :class:!str are unchanged.

globals and locals are passed in to :func:eval; see the documentation for :func:eval for more information. If globals or locals is :const:!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 :attr:obj.__globals__ <function.__globals__>, although if obj is a wrapped function (using :func:functools.update_wrapper) or a :class:functools.partial object, it is unwrapped until a non-wrapped function is found.

Calling :func:!get_annotations is best practice for accessing the annotations dict of any object. See :ref:annotations-howto for more information on annotations best practices.

.. doctest::

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

.. versionadded:: 3.14

.. function:: type_repr(value)

Convert an arbitrary Python value to a format suitable for use by the :attr:~Format.STRING format. This calls :func: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 :attr:~Format.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.

.. versionadded:: 3.14

Recipes

.. _annotationlib-metaclass:

Using annotations in a metaclass ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A :ref:metaclass <metaclasses> may want to inspect or even modify the annotations in a class body during class creation. Doing so requires retrieving annotations from the class namespace dictionary. For classes created with from __future__ import annotations, the annotations will be in the __annotations__ key of the dictionary. For other classes with annotations, :func:get_annotate_from_class_namespace can be used to get the annotate function, and :func:call_annotate_function can be used to call it and retrieve the annotations. Using the :attr:~Format.FORWARDREF format will usually be best, because this allows the annotations to refer to names that cannot yet be resolved when the class is created.

To modify the annotations, it is best to create a wrapper annotate function that calls the original annotate function, makes any necessary adjustments, and returns the result.

Below is an example of a metaclass that filters out all :class:typing.ClassVar annotations from the class and puts them in a separate attribute:

.. code-block:: python

import annotationlib import typing

class ClassVarSeparator(type): def new(mcls, name, bases, ns): if "annotations" in ns: # from future import annotations annotations = ns["annotations"] classvar_keys = { key for key, value in annotations.items() # Use string comparison for simplicity; a more robust solution # could use annotationlib.ForwardRef.evaluate if value.startswith("ClassVar") } classvars = {key: annotations[key] for key in classvar_keys} ns["annotations"] = { key: value for key, value in annotations.items() if key not in classvar_keys } wrapped_annotate = None elif annotate := annotationlib.get_annotate_from_class_namespace(ns): annotations = annotationlib.call_annotate_function( annotate, format=annotationlib.Format.FORWARDREF ) classvar_keys = { key for key, value in annotations.items() if typing.get_origin(value) is typing.ClassVar } classvars = {key: annotations[key] for key in classvar_keys}

        def wrapped_annotate(format):
           annos = annotationlib.call_annotate_function(annotate, format, owner=typ)
           return {key: value for key, value in annos.items() if key not in classvar_keys}

     else:  # no annotations
        classvars = {}
        wrapped_annotate = None
     typ = super().__new__(mcls, name, bases, ns)

     if wrapped_annotate is not None:
        # Wrap the original __annotate__ with a wrapper that removes ClassVars
        typ.__annotate__ = wrapped_annotate
     typ.classvars = classvars  # Store the ClassVars in a separate attribute
     return typ

Limitations of the STRING format

The :attr:~Format.STRING format is meant to approximate the source code of the annotation, but the implementation strategy used means that it is not always possible to recover the exact source code.

First, the stringifier of course cannot recover any information that is not present in the compiled code, including comments, whitespace, parenthesization, and operations that get simplified by the compiler.

Second, the stringifier can intercept almost all operations that involve names looked up in some scope, but it cannot intercept operations that operate fully on constants. As a corollary, this also means it is not safe to request the STRING format on untrusted code: Python is powerful enough that it is possible to achieve arbitrary code execution even with no access to any globals or builtins. For example:

.. code-block:: pycon

def f(x: (1).class.base.subclasses()[-1].init.builtins["print"]("Hello world")): pass ... annotationlib.get_annotations(f, format=annotationlib.Format.STRING) Hello world {'x': 'None'}

.. note:: This particular example works as of the time of writing, but it relies on implementation details and is not guaranteed to work in the future.

Among the different kinds of expressions that exist in Python, as represented by the :mod:ast module, some expressions are supported, meaning that the STRING format can generally recover the original source code; others are unsupported, meaning that they may result in incorrect output or an error.

The following are supported (sometimes with caveats):

• :class:ast.BinOp

• :class:ast.UnaryOp

  • :class:ast.Invert (~), :class:ast.UAdd (+), and :class:ast.USub (-) are supported
  • :class:ast.Not (not) is not supported

• :class:ast.Dict (except when using ** unpacking)

• :class:ast.Set

• :class:ast.Compare

  • :class:ast.Eq and :class:ast.NotEq are supported
  • :class:ast.Lt, :class:ast.LtE, :class:ast.Gt, and :class:ast.GtE are supported, but the operand may be flipped
  • :class:ast.Is, :class:ast.IsNot, :class:ast.In, and :class:ast.NotIn are not supported

• :class:ast.Call (except when using ** unpacking)

• :class:ast.Constant (though not the exact representation of the constant; for example, escape sequences in strings are lost; hexadecimal numbers are converted to decimal)

• :class:ast.Attribute (assuming the value is not a constant)

• :class:ast.Subscript (assuming the value is not a constant)

• :class:ast.Starred (* unpacking)

• :class:ast.Name

• :class:ast.List

• :class:ast.Tuple

• :class:ast.Slice

The following are unsupported, but throw an informative error when encountered by the stringifier:

• :class:ast.FormattedValue (f-strings; error is not detected if conversion specifiers like !r are used)
• :class:ast.JoinedStr (f-strings)

The following are unsupported and result in incorrect output:

• :class:ast.BoolOp (and and or)
• :class:ast.IfExp
• :class:ast.Lambda
• :class:ast.ListComp
• :class:ast.SetComp
• :class:ast.DictComp
• :class:ast.GeneratorExp

The following are disallowed in annotation scopes and therefore not relevant:

• :class:ast.NamedExpr (:=)
• :class:ast.Await
• :class:ast.Yield
• :class:ast.YieldFrom

Limitations of the FORWARDREF format

The :attr:~Format.FORWARDREF format aims to produce real values as much as possible, with anything that cannot be resolved replaced with :class:ForwardRef objects. It is affected by broadly the same Limitations as the :attr:~Format.STRING format: annotations that perform operations on literals or that use unsupported expression types may raise exceptions when evaluated using the :attr:~Format.FORWARDREF format.

Below are a few examples of the behavior with unsupported expressions:

.. code-block:: pycon

from annotationlib import get_annotations, Format def zerodiv(x: 1 / 0): ... get_annotations(zerodiv, format=Format.STRING) Traceback (most recent call last): ... ZeroDivisionError: division by zero get_annotations(zerodiv, format=Format.FORWARDREF) Traceback (most recent call last): ... ZeroDivisionError: division by zero def ifexp(x: 1 if y else 0): ... get_annotations(ifexp, format=Format.STRING) {'x': '1'}

.. _annotationlib-security:

Security implications of introspecting annotations

Much of the functionality in this module involves executing code related to annotations, which can then do arbitrary things. For example, :func:get_annotations may call an arbitrary :term:annotate function, and :meth:ForwardRef.evaluate may call :func:eval on an arbitrary string. Code contained in an annotation might make arbitrary system calls, enter an infinite loop, or perform any other operation. This is also true for any access of the :attr:~object.__annotations__ attribute, and for various functions in the :mod:typing module that work with annotations, such as :func:typing.get_type_hints.

Any security issue arising from this also applies immediately after importing code that may contain untrusted annotations: importing code can always cause arbitrary operations to be performed. However, it is unsafe to accept strings or other input from an untrusted source and pass them to any of the APIs for introspecting annotations, for example by editing an __annotations__ dictionary or directly creating a :class:ForwardRef object.