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

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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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bpo-41341: Recursive evaluation of ForwardRef in get_type_hints 
The issue raised by recursive evaluation is infinite recursion with
recursive types. In that case, only the first recursive ForwardRef is
evaluated.
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@wyfo
wyfo committed Jul 20, 2020
commit 2389c257ea8a480c9a011cac5cda7eb947ed3bca
6 changes: 6 additions & 0 deletions Lib/test/test_typing.py
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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\']'):
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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.

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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.

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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')
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20 changes: 14 additions & 6 deletions Lib/typing.py
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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