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bpo-41341: Recursive evaluation of ForwardRef in get_type_hints by wyfo · Pull Request #21553 · python/cpython · GitHub

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bpo-41341: Recursive evaluation of ForwardRef in get_type_hints #21553

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gvanrossum merged 2 commits into from
Jul 22, 2020
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bpo-41341: Recursive evaluation of ForwardRef in get_type_hints #21553

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bpo-41341: Recursive evaluation of ForwardRef in get_type_hints
wyfo Jul 20, 2020
0a29c8b
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 20, 2020
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6 changes: 6 additions & 0 deletions Lib/test/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2456,6 +2456,12 @@ def foo(a: tuple[ForwardRef('T')]):
self.assertEqual(get_type_hints(foo, globals(), locals()),
{'a': tuple[T]})

def test_double_forward(self):
def foo(a: 'List[\'int\']'):

gvanrossum Jul 21, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I was about to approve this PR when I realized that we should make this work with list -- I tried and it doesn't currently.

wyfo Jul 22, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

You're right concerning the fact that it doesn't work with list, but I've found that it's not related to this PR; actually, I think there is a significant defect in current PEP 585 implementation. In fact:

from dataclasses import dataclass, field
from typing import get_type_hints, List, ForwardRef

@dataclass
class Node:
    children: list["Node"] = field(default_factory=list)
    children2: List["Node"] = field(default_factory=list)

assert get_type_hints(Node) == {"children": list["Node"], "children2": List[Node]}
assert List["Node"].__args__ == (ForwardRef("Node"),)
assert list["Node"].__args__ == ("Node",)  # No ForwardRef here, so no evaluation by get_type_hints

I will open an issue on bugs.python.org to discuss about this, because I think this PR is not the best place to do it.

gvanrossum Jul 22, 2020

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Heh, you're right, thanks for the analysis. We probably will not ever support this: importing ForwardRef from the built-in generic alias code would be problematic, and once from __future__ import annotations is always on there's no need to quote the argument anyway. Sorry for the hiccup. I will now approve your PR.
pass
self.assertEqual(get_type_hints(foo, globals(), locals()),
{'a': List[int]})

def test_forward_recursion_actually(self):
def namespace1():
a = typing.ForwardRef('A')
Expand Down
20 changes: 14 additions & 6 deletions Lib/typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -244,14 +244,16 @@ def inner(*args, **kwds):
return inner


def _eval_type(t, globalns, localns):
def _eval_type(t, globalns, localns, recursive_guard=frozenset()):
"""Evaluate all forward reverences in the given type t.
For use of globalns and localns see the docstring for get_type_hints().
recursive_guard is used to prevent prevent infinite recursion
with recursive ForwardRef.
"""
if isinstance(t, ForwardRef):
return t._evaluate(globalns, localns)
return t._evaluate(globalns, localns, recursive_guard)
if isinstance(t, (_GenericAlias, GenericAlias)):
ev_args = tuple(_eval_type(a, globalns, localns) for a in t.__args__)
ev_args = tuple(_eval_type(a, globalns, localns, recursive_guard) for a in t.__args__)
if ev_args == t.__args__:
return t
if isinstance(t, GenericAlias):
Expand Down Expand Up @@ -477,18 +479,24 @@ def __init__(self, arg, is_argument=True):
self.__forward_value__ = None
self.__forward_is_argument__ = is_argument

def _evaluate(self, globalns, localns):
def _evaluate(self, globalns, localns, recursive_guard):
if self.__forward_arg__ in recursive_guard:
return self
if not self.__forward_evaluated__ or localns is not globalns:
if globalns is None and localns is None:
globalns = localns = {}
elif globalns is None:
globalns = localns
elif localns is None:
localns = globalns
self.__forward_value__ = _type_check(
type_ =_type_check(
eval(self.__forward_code__, globalns, localns),
"Forward references must evaluate to types.",
is_argument=self.__forward_is_argument__)
is_argument=self.__forward_is_argument__,
)
self.__forward_value__ = _eval_type(
type_, globalns, localns, recursive_guard | {self.__forward_arg__}
)
self.__forward_evaluated__ = True
return self.__forward_value__

Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2020-07-20-19-13-17.bpo-41341.wqrj8C.rst
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@@ -0,0 +1 @@
Recursive evaluation of `typing.ForwardRef` in `get_type_hints`.
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