Using mutually recursive types causes AttributeError

[Varriount]

I'm currently testing some code that deals with a recursive tree structure. Since the code already has type definitions, I was hoping to reuse them in Hypothesis by way of from_type. While I was successful in finding a way forward, some of the failed attempts seemed to uncover errors within Hypothesis.

from typing import ForwardRef, Mapping, Sequence, Union

import hypothesis.strategies as st

if True:
    # Doesn't work
    A = Mapping[str, "B"]
    B = Union[Sequence[str], A]

    st.register_type_strategy(
        ForwardRef("B"),
        lambda x: st.deferred(lambda: b_strategy),
    )

    b_strategy = st.from_type(B)
    print(b_strategy)
    print(repr(b_strategy.example()))

else:
    # Works
    C = Union[Sequence[str], "D"]
    D = Mapping[str, C]

    st.register_type_strategy(
        ForwardRef("D"),
        lambda x: st.deferred(lambda: d_strategy),
    )

    d_strategy = st.from_type(D)
    print(d_strategy)
    print(repr(d_strategy.example()))

The version that doesn't work spits out a rather long traceback:

Traceback (most recent call last):
  File "<string>", 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 1023, in wrapped_test
    processed_args = process_arguments_to_given(
  File "...\hypothesis\core.py", line 441, 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\strategies.py", line 595, in do_validate
    for e in self.element_strategies:
  File "...\hypothesis\strategies\_internal\strategies.py", line 570, in element_strategies
    if not arg.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\lazy.py", line 86, in calc_is_empty
    return recur(self.wrapped_strategy)
  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 55, in wrapped_strategy
    del self.__definition
AttributeError: _DeferredStrategy__definition

[Zac-HD]

Thanks for reporting this! To be honest it's probably cleaner to register explicit st.recursive() strategies for each type, but I can see why you'd want to use st.from_type() - and I consider this a bug to be fixed either way 🙂

---

I also need to appreciate this symphony of lazy evaluation for a minute:

A = Dict[str, "B"]          # "B" is a forward reference, to avoid NameError.  Standard.
B = Union[List[str], A]     # Mypy doesn't support direct recursion, but mutual is fine! 🤔

# We're not actually using the "register a function" feature of inspecting the
# type instance... just using the function closure to *defer harder* because
# st.deferred() itself isn't enough inside the optionally-lazy st.from_type() 😕
st.register_type_strategy(ForwardRef("B"), lambda _: st.deferred(lambda: bs))

							# st.from_type(B) would fail here
bs = st.from_type(B)        # works
							# st.from_type(B) works here because bs is now in scope 🤯