cattrs: Breaks when PEP 563 is active

• cattrs version: All versions, as far as I can tell
• Python version: Python 3.7
• Operating System: Arch Linux

Description

cattr.structure breaks when PEP 563 is activated. This can be done in Python 3.7 with the new from __future__ import annotations.

In short, this defers the evaluation of type annotations. So consider this class:

@attr.s(auto_attribs=True)
class Foo:
    bar: str

Traditionally, __annotations__ looks like this:

>>> Foo.__annotations__
... {'bar': str}

but under PEP 563 (and eventually the default behaviour), it’ll look like this:

>>> Foo.__annotations__
... {'bar': 'str'}

This seems to confuse cattr and causes problems. Round-tripping the above class, we now see exceptions like this:

In [16]: cattr.structure(cattr.unstructure(Foo('Hello World')), Foo)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-16-1bbcc538d3df> in <module>()
----> 1 cattr.structure(cattr.unstructure(Foo('Hello World')), Foo)

~/.local/share/virtualenvs/aastrology-5jK3bc4O/lib/python3.7/site-packages/cattr/converters.py in structure(self, obj, cl)
    176         """Convert unstructured Python data structures to structured data."""
    177         # type: (Any, Type) -> Any
--> 178         return self._structure_func.dispatch(cl)(obj, cl)
    179 
    180     # Classes to Python primitives.

~/.local/share/virtualenvs/aastrology-5jK3bc4O/lib/python3.7/site-packages/cattr/converters.py in structure_attrs_fromdict(self, obj, cl)
    294             except KeyError:
    295                 continue
--> 296             conv_obj[name] = dispatch(type_)(val, type_)
    297 
    298         return cl(**conv_obj)

~/.local/share/virtualenvs/aastrology-5jK3bc4O/lib/python3.7/site-packages/cattr/converters.py in _structure_default(self, obj, cl)
    238             "it.".format(cl)
    239         )
--> 240         raise ValueError(msg)
    241 
    242     def _structure_call(self, obj, cl):

ValueError: Unsupported type: str. Register a structure hook for it.

This fix is probably to apply this logic: https://www.python.org/dev/peps/pep-0563/#resolving-type-hints-at-runtime

Here:

https://github.com/Tinche/cattrs/blob/6e90fd28af8e23c44798b5cbd98e8d5a06eb87ba/src/cattr/converters.py#L287-L296

Which I’m currently digging into.