typing/Generic __orig_class__ availability during __init__ in 3.6 vs 3.7

[baltus-atomicrules]

I observe that when executing __init__ in a Generic class, the value of __orig_class__ is available in version 3.6 but not in 3.7

(I am comparing 3.6.7 with 3.7.0)

I've attached a simple example that defines a Generic class and then accesses __orig_class__ in the associated __init__ method.

Simple.py.txt

N = TypeVar('N')
class ZZ(Generic[N]):
    def __init__(self):
        print("__orig_class__ is: ", self.__orig_class__)

In 3.6, this code runs fine:

In [42]: import Simple

In [51]: a = Simple.ZZ[int]

In [56]: b = a()
__orig_class__ is:  Simple.ZZ[int]

In 3.7 however, the value of __orig_class__ is not available:

In [4]: import Simple

In [5]: a = Simple.ZZ[int]

In [6]: b = a()

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-52a6c422c17e> in <module>
----> 1 b = a()
/tool/pandora64/.package/python-3.7.0/lib/python3.7/typing.py in __call__(self, *args, **kwargs)
    668             raise TypeError(f"Type {self._name} cannot be instantiated; "
    669                             f"use {self._name.lower()}() instead")
--> 670         result = self.__origin__(*args, **kwargs)
    671         try:
    672             result.__orig_class__ = self
/proj/emu_scratch4/users/dbaltus/python/experiment/Simple.py in __init__(self)
      4 class ZZ(Generic[N]):
      5     def __init__(self):
----> 6         print("__orig_class__ is: ", self.__orig_class__)
AttributeError: 'ZZ' object has no attribute '__orig_class__'

> /proj/emu_scratch4/users/dbaltus/python/experiment/Simple.py(6)__init__()
      2
      3 N = TypeVar('N')
      4 class ZZ(Generic[N]):
      5     def __init__(self):
----> 6         print("__orig_class__ is: ", self.__orig_class__)
ipdb> q

This occurs because in 3.7, the __call__ method of _GenericAlias does not set the value of __orig_class__ until after __init__ is called. (It is done differently in 3.6).

    def __call__(self, *args, **kwargs):
        if not self._inst:
            raise TypeError(f"Type {self._name} cannot be instantiated; "
                            f"use {self._name.lower()}() instead")
        result = self.__origin__(*args, **kwargs)
        try:
            result.__orig_class__ = self
        except AttributeError:
            pass
        return result

[ilevkivskyi]

Yes, this is unfortunate, but as you know, this is undocumented internal attribute that you are using totally at your own risk. So unless you have a simple idea how to fix this, I don't think there is anything we can do here.

[Stewori]

@baltus-atomicrules
For the record, you can use pytypes.get_orig_class to solve this.
Note: It's only available in master as of this writing. (I hope to make an new release soon.)

[mitar]

this is undocumented internal attribute that you are using totally at your own risk.

Sure, but that makes also typing_inspect fail inside constructor: ilevkivskyi/typing_inspect#35

[JelleZijlstra]

We can't change what typing looks like in 3.6 or 3.7, so I don't think there's anything actionable in this issue.

[mitar]

I mean, the same still holds true in 3.8, 3.9, 3.10. So the issue in all of those that inside __init__ you cannot access __orig_class__. The issue was only reported when 3.7 was out, but it is still valid.

[Gobot1234]

This has been annoying me as of late, couldn't we just have it not be available in __new__ but available in __init__?

[fzyzcjy]

Hi, is there any updates? Thanks