New submission from Valerio G :
I encountered one condition where calling get_type_hints causes infinite
recursion when dealing with forward declaration and cyclic types.
Here's an example:
from typing import Union, List, get_type_hints
ValueList = List['Value']
Value = Union[str, ValueList]
class A:
a: List[Value]
get_type_hints(A, globals(), locals())
This reaches the recursion limit as of 3.8.0b2.
It seems that the combining _GenericAlias with ForwardRef is what triggers this
condition:
ForwardRef._evaluate sets __forward_value__ on its first call on a given
instance
_GenericAlias tries to compare its args post evaluation
If one of the arguments is a previously evaluated forward reference containing
a cycle, then it will infinitely recurse in the hash function when building a
frozen set for the comparison.
The above is, of course, a very artificial example, but I can imagine this
happening a lot in code with trees or similar structures.
My initial reproduction case was using _eval_type to resolve forward references
returned by get_args (side note: it would be nice to have a public function to
do that).
--
components: Library (Lib)
messages: 349330
nosy: vg0377467
priority: normal
severity: normal
status: open
title: Infinite recursion with typing.get_type_hints
versions: Python 3.8
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<https://bugs.python.org/issue37806>
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