Pekka Klärck <pekka.kla...@gmail.com> added the comment:
While studying the types in the typing module more, I noticed they have a
special `__origin__` attribute which seems to contain the "real" type they
represent. I was able to make my type conversion code to work by adding these
lines:
if hasattr(type_, '__origin__'):
type_ = type_.__origin__
All our tests pass with this simple fix, but I'm slightly worried using it
because `__origin__` doesn't seem to be documented. This means I'm not sure is
my usage OK and, more importantly, makes me worried that another change in
typing changes the behavior or removes the attribute altogether. Hopefully
someone with more insight on this can comment my worries. Perhaps the attribute
should also be documented as discussed earlier:
https://github.com/python/typing/issues/335
I'd also be a little bit happier with the above fix if I could write it like
if isinstance(type_, typing.SomeBaseType):
type_ = type_.__origin__
but apparently types in the typing module don't have any public base class. I
guess odds that some unrelated class would have `__origin__` defined is small
enough that using `hasattr(type_, '__origin__')` is safe.
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<https://bugs.python.org/issue34568>
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