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bpo-38605: Make 'from __future__ import annotations' the default (GH-20434)
The hard part was making all the tests pass; there are some subtle issues here, because apparently the future import wasn't tested very thoroughly in previous Python versions. For example, `inspect.signature()` returned type objects normally (except for forward references), but strings with the future import. We changed it to try and return type objects by calling `typing.get_type_hints()`, but fall back on returning strings if that function fails (which it may do if there are future references in the annotations that require passing in a specific namespace to resolve).
1 parent bef7d29 commit 044a104

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‎Doc/reference/compound_stmts.rst

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@@ -610,13 +610,9 @@ following the parameter name. Any parameter may have an annotation, even those
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``*identifier`` or ``**identifier``. Functions may have "return" annotation of
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the form "``-> expression``" after the parameter list. These annotations can be
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any valid Python expression. The presence of annotations does not change the
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semantics of a function. The annotation values are available as values of
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a dictionary keyed by the parameters' names in the :attr:`__annotations__`
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attribute of the function object. If the ``annotations`` import from
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:mod:`__future__` is used, annotations are preserved as strings at runtime which
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enables postponed evaluation. Otherwise, they are evaluated when the function
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definition is executed. In this case annotations may be evaluated in
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a different order than they appear in the source code.
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semantics of a function. The annotation values are available as string values
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in a dictionary keyed by the parameters' names in the :attr:`__annotations__`
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attribute of the function object.
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.. index:: pair: lambda; expression
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‎Doc/whatsnew/3.10.rst

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@@ -70,6 +70,23 @@ Summary -- Release highlights
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New Features
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============
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.. _whatsnew310-pep563:
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PEP 563: Postponed Evaluation of Annotations Becomes Default
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------------------------------------------------------------
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In Python 3.7, postponed evaluation of annotations was added,
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to be enabled with a ``from __future__ import annotations``
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directive. In 3.10 this became the default behavior, even
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without that future directive. With this being default, all
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annotations stored in :attr:`__annotations__` will be strings.
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If needed, annotations can be resolved at runtime using
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:func:`typing.get_type_hints`. See :pep:`563` for a full
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description. Also, the :func:`inspect.signature` will try to
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resolve types from now on, and when it fails it will fall back to
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showing the string annotations. (Contributed by Batuhan Taskaya
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in :issue:`38605`.)
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* The :class:`int` type has a new method :meth:`int.bit_count`, returning the
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number of ones in the binary expansion of a given integer, also known
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as the population count. (Contributed by Niklas Fiekas in :issue:`29882`.)

‎Lib/dataclasses.py

Lines changed: 10 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -399,8 +399,10 @@ def _create_fn(name, args, body, *, globals=None, locals=None,
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ns = {}
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exec(txt, globals, ns)
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return ns['__create_fn__'](**locals)
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func = ns['__create_fn__'](**locals)
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for arg, annotation in func.__annotations__.copy().items():
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func.__annotations__[arg] = locals[annotation]
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return func
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def _field_assign(frozen, name, value, self_name):
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# If we're a frozen class, then assign to our fields in __init__
@@ -651,6 +653,11 @@ def _is_type(annotation, cls, a_module, a_type, is_type_predicate):
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# a eval() penalty for every single field of every dataclass
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# that's defined. It was judged not worth it.
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# Strip away the extra quotes as a result of double-stringifying when the
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# 'annotations' feature became default.
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if annotation.startswith(("'", '"')) and annotation.endswith(("'", '"')):
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annotation = annotation[1:-1]
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match = _MODULE_IDENTIFIER_RE.match(annotation)
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if match:
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ns = None
@@ -991,7 +998,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
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if not getattr(cls, '__doc__'):
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# Create a class doc-string.
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cls.__doc__ = (cls.__name__ +
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str(inspect.signature(cls)).replace(' -> None', ''))
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str(inspect.signature(cls)).replace(' -> NoneType', ''))
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abc.update_abstractmethods(cls)
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‎Lib/inspect.py

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@@ -45,6 +45,7 @@
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import tokenize
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import token
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import types
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import typing
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import warnings
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import functools
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import builtins
@@ -1877,7 +1878,10 @@ def _signature_is_functionlike(obj):
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code = getattr(obj, '__code__', None)
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defaults = getattr(obj, '__defaults__', _void) # Important to use _void ...
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kwdefaults = getattr(obj, '__kwdefaults__', _void) # ... and not None here
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annotations = getattr(obj, '__annotations__', None)
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try:
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annotations = _get_type_hints(obj)
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except AttributeError:
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annotations = None
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return (isinstance(code, types.CodeType) and
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isinstance(name, str) and
@@ -2118,6 +2122,16 @@ def p(name_node, default_node, default=empty):
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return cls(parameters, return_annotation=cls.empty)
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def _get_type_hints(func):
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try:
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return typing.get_type_hints(func)
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except Exception:
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# First, try to use the get_type_hints to resolve
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# annotations. But for keeping the behavior intact
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# if there was a problem with that (like the namespace
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# can't resolve some annotation) continue to use
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# string annotations
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return func.__annotations__
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def _signature_from_builtin(cls, func, skip_bound_arg=True):
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"""Private helper function to get signature for
@@ -2161,7 +2175,8 @@ def _signature_from_function(cls, func, skip_bound_arg=True):
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positional = arg_names[:pos_count]
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keyword_only_count = func_code.co_kwonlyargcount
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keyword_only = arg_names[pos_count:pos_count + keyword_only_count]
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annotations = func.__annotations__
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annotations = _get_type_hints(func)
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defaults = func.__defaults__
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kwdefaults = func.__kwdefaults__
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‎Lib/test/dataclass_module_1.py

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@@ -1,9 +1,3 @@
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#from __future__ import annotations
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USING_STRINGS = False
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# dataclass_module_1.py and dataclass_module_1_str.py are identical
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# except only the latter uses string annotations.
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import dataclasses
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import typing
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‎Lib/test/dataclass_module_1_str.py

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This file was deleted.

‎Lib/test/dataclass_module_2.py

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@@ -1,9 +1,3 @@
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#from __future__ import annotations
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USING_STRINGS = False
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# dataclass_module_2.py and dataclass_module_2_str.py are identical
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# except only the latter uses string annotations.
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from dataclasses import dataclass, InitVar
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from typing import ClassVar
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‎Lib/test/dataclass_module_2_str.py

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This file was deleted.

‎Lib/test/dataclass_textanno.py

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@@ -1,5 +1,3 @@
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from __future__ import annotations
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import dataclasses
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