Fails on typing.Type and ForwardRef

[sobolevn]

When we use Type[MyType] - everything is good. But, Type['MyType'] fails.

Code:

>>> from hypothesis import strategies as st
>>> from typing import Type
>>> st.from_type(Type[int])
just(<class 'int'>)
>>> st.from_type(Type['int'])
from_type(typing.Type[ForwardRef('int')])
>>> st.from_type(Type['int']).example()

Output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "hypothesis/strategies/_internal/strategies.py", line 314, in example
    example_generating_inner_function()
  File "hypothesis/strategies/_internal/strategies.py", line 302, in example_generating_inner_function
    @settings(
  File "hypothesis/core.py", line 1001, in wrapped_test
    processed_args = process_arguments_to_given(
  File "hypothesis/core.py", line 420, in process_arguments_to_given
    search_strategy.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/collections.py", line 39, in do_validate
    s.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/strategies.py", line 638, in do_validate
    self.mapped_strategy.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/lazy.py", line 118, in do_validate
    w.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/strategies.py", line 638, in do_validate
    self.mapped_strategy.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/collections.py", line 39, in do_validate
    s.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 377, in validate
    self.do_validate()
  File "hypothesis/strategies/_internal/lazy.py", line 118, in do_validate
    w.validate()
  File "hypothesis/strategies/_internal/strategies.py", line 378, in validate
    self.is_empty
  File "hypothesis/strategies/_internal/strategies.py", line 125, in accept
    recur(self)
  File "hypothesis/strategies/_internal/strategies.py", line 121, in recur
    mapping[strat] = getattr(strat, calculation)(recur)
  File "hypothesis/strategies/_internal/deferred.py", line 80, in calc_is_empty
    return recur(self.wrapped_strategy)
  File "hypothesis/strategies/_internal/deferred.py", line 43, in wrapped_strategy
    result = self.__definition()
  File "hypothesis/strategies/_internal/core.py", line 1389, in <lambda>
    lambda thing: deferred(lambda: _from_type(thing)),
  File "hypothesis/strategies/_internal/core.py", line 1477, in _from_type
    return types.from_typing_type(thing)
  File "hypothesis/strategies/_internal/types.py", line 282, in from_typing_type
    strategies = [
  File "hypothesis/strategies/_internal/types.py", line 283, in <listcomp>
    v if isinstance(v, st.SearchStrategy) else v(thing)
  File "hypothesis/strategies/_internal/types.py", line 507, in resolve_Type
    raise ResolutionFailed(
hypothesis.errors.ResolutionFailed: 
	thing=typing.Type[ForwardRef('int')] cannot be resolved.
	Upgrading to python>=3.6 may fix this problem via improvements to the typing module.

[Zac-HD]

Looks like _try_import_forward_ref() from #2550 could be useful here too! Are there other places ForwardRef can occur that we should be looking at?

[sobolevn]

Pretty much any generic. Like List, Callable, etc
I agree that they should all use the same logic. 👍

[Zac-HD]

Hmm. Sounds like we should be calling _try_import_forward_ref() pretty early in st.from_type(), then it would be shared across all users and we could raise ResolutionFailed instead of returning a ForwardRef if we can't resolve it.

[sobolevn]

We can make it "safe" for this general use-case. Like "Let's try to resolve this, failed? Ok, no worries."