Support builds(Optional[MyDataclass]) or document alternative

[ysangkok]

>>> import dataclasses
>>> @dataclasses.dataclass
... class A:
...     b: int

Since builds(A) works, I was thinking I can just use builds(Optional[A]), since if A is a type, Optional[A] is also a type.

But it doesn't work:

>>> hypothesis.strategies.builds(Optional[A]).example()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/janus/.pyenv/versions/3.9.1/lib/python3.9/site-packages/hypothesis/strategies/_internal/strategies.py", line 314, in example
    example_generating_inner_function()
  File "/home/janus/.pyenv/versions/3.9.1/lib/python3.9/site-packages/hypothesis/strategies/_internal/strategies.py", line 302, in example_generating_inner_function
    @settings(
  File "/home/janus/.pyenv/versions/3.9.1/lib/python3.9/site-packages/hypothesis/core.py", line 1164, in wrapped_test
    raise the_error_hypothesis_found
  File "/home/janus/.pyenv/versions/3.9.1/lib/python3.9/typing.py", line 664, in __call__
    result = self.__origin__(*args, **kwargs)
  File "/home/janus/.pyenv/versions/3.9.1/lib/python3.9/typing.py", line 329, in __call__
    raise TypeError(f"Cannot instantiate {self!r}")
TypeError: Cannot instantiate typing.Union

It would be nice if it worked, or this use case was documented. I searched in vain for a strategy combinator optional(s: Strategy[A]) -> Strategy[Optional[A]] or something like that, so that I could do optional(builds(A)) and sometimes get a None, sometimes get an A.

[ysangkok]

aaah, you can use from_type!

>>> st.from_type(A)
builds(A)
>>> st.from_type(Optional[A])
one_of(none(), builds(A))

It would be nice if the builds documentation noted that from_type is strictly more capable for dataclasses. What do you think?

[Zac-HD]

It would be nice if the builds documentation noted that from_type is strictly more capable for dataclasses. What do you think?

It's not though - the real distinction is that st.builds() calls the thing, while st.from_type() gives you instances (possibly by using builds()). On the other hand, you can st.builds(A, b=st.integers(min_value=0) and this is a level of flexibility that st.from_type() doesn't offer (unless you use st.builds() and st.register_type_strategy() first).

I think we could improve the error message when you attempt to build a typing type though, to explain the difference and suggest from_type() instead.

[ysangkok]

Awesome, thank you so much!