Let's try an example that static type checkers should have no problem
with:
Python 3.9.0 (default, Oct 7 2020, 23:09:01)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from __future__ import annotations
>>>
>>> def make_a_class():
... class A:
... def get_b(self) -> B:
... return B()
... class B:
... def get_a(self) -> A:
... return A()
... return A
...
>>> A = make_a_class()
>>> a = A()
>>>
>>> import typing
>>> typing.get_type_hints(a.get_b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.9/typing.py", line 1386, in get_type_hints
value = _eval_type(value, globalns, localns)
File "/usr/lib/python3.9/typing.py", line 254, in _eval_type
return t._evaluate(globalns, localns, recursive_guard)
File "/usr/lib/python3.9/typing.py", line 493, in _evaluate
eval(self.__forward_code__, globalns, localns),
File "<string>", line 1, in <module>
NameError: name 'B' is not defined
>>>
On Tue, 2020-12-08 at 18:48 -0800, Guido van Rossum wrote:
> Yeah, static type checkers won't like it regardless.
>
> On Tue, Dec 8, 2020 at 6:39 PM Paul Bryan <pbr...@anode.ca> wrote:
> > It appears that when from future import __annotations__, a type
> > hint annotation derived from a closure loses scope.
> >
> > Simplistic example:
> >
> > Python 3.9.0 (default, Oct 7 2020, 23:09:01)
> > [GCC 10.2.0] on linux
> > Type "help", "copyright", "credits" or "license" for more
> > information.
> > >>> def make_a_class(data_type):
> > ... class Foo:
> > ... def put_data(self, data: data_type):
> > ... self.data = data
> > ... return Foo
> > ...
> > >>> import typing
> > >>> foo = make_a_class(str)()
> > >>> typing.get_type_hints(foo.put_data)
> > {'data': <class 'str'>}
> > >>>
> >
> >
> > If I add a single import to the top, it breaks:
> >
> > Python 3.9.0 (default, Oct 7 2020, 23:09:01)
> > [GCC 10.2.0] on linux
> > Type "help", "copyright", "credits" or "license" for more
> > information.
> > >>> from __future__ import annotations # added this line
> > >>> def make_a_class(data_type):
> > ... class Foo:
> > ... def put_data(self, data: data_type):
> > ... self.data = data
> > ... return Foo
> > ...
> > >>> import typing
> > >>> foo = make_a_class(str)()
> > >>> typing.get_type_hints(foo.put_data)
> > Traceback (most recent call last):
> > File "<stdin>", line 1, in <module>
> > File "/usr/lib/python3.9/typing.py", line 1386, in get_type_hints
> > value = _eval_type(value, globalns, localns)
> > File "/usr/lib/python3.9/typing.py", line 254, in _eval_type
> > return t._evaluate(globalns, localns, recursive_guard)
> > File "/usr/lib/python3.9/typing.py", line 493, in _evaluate
> > eval(self.__forward_code__, globalns, localns),
> > File "<string>", line 1, in <module>
> > NameError: name 'data_type' is not defined
> > >>>
> >
> >
> > I don't see how I can supply the closure scope as localns to
> > get_type_hints. Any suggestions? Is constructing a (dynamically-
> > type-annotated) class in a function like this an anti-pattern?
> >
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> >
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>
>
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