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Slonegg opened this issue Jun 24, 2020 · 3 comments
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Issue using Protocol as a base class with Python 3.5.2 #734

Slonegg opened this issue Jun 24, 2020 · 3 comments

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@Slonegg
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Slonegg commented Jun 24, 2020 •
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Python: 3.5.2
OS: macOS Mojave

Seems that _generic_new implementation from typing_extensions module is outdated or incorrect. When typing_extensions relies on its own implementation of _generic_new I get the following error.

Script to reproduce:

import abc
from typing_extensions import Protocol, runtime_checkable


@runtime_checkable
class Proto(Protocol):
    @abc.abstractmethod
    def foo(self):
        pass


class SomeClass(Proto):
    def __init__(self, some_arg):
        pass

    def foo(self):
        return 5


x = SomeClass(3)

Traceback:

Traceback (most recent call last):
  File "/Users/dikobraz/repro.py", line 20, in <module>
    x = SomeClass(3)
  File "/Users/dikobraz/env-py-3.5-tf-2/lib/python3.5/site-packages/typing_extensions.py", line 1357, in __new__
    return _generic_new(cls.__next_in_mro__, cls, *args, **kwds)
  File "/Users/dikobraz/env-py-3.5-tf-2/lib/python3.5/site-packages/typing_extensions.py", line 87, in _generic_new
    return base_cls.__new__(cls, *args, **kwargs)
TypeError: object() takes no parameters

In debugger:
Screen Shot 2020-06-23 at 6 47 37 PM
@gvanrossum
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Could you try this with a newer version? 3.5 is nearing its end of life, and the latest version is 3.5.9. The latest version for which I can find downloadable binaries is 3.5.4: https://www.python.org/downloads/release/python-354/

If your problem is specifically with 3.5.2, I'm not sure that we can do much for you unless you can come up with a fix (I don't even have 3.5 installed on my machine any more).

@Slonegg
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Slonegg commented Jun 24, 2020 •
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Thank you @gvanrossum for the quick reply. I've verified that it works with 3.5.9 because it comes with newer typing module and typing_extensions module is just using _generic_new from typing.

Maybe we can just use _generic_new from more recent version of typing module before it was removed completely in this PR (python/cpython#4906):

def _generic_new(base_cls, cls, *args, **kwargs):
    if cls.__origin__ is None:
        return base_cls.__new__(cls)
    else:
        origin = cls._gorg
        obj = base_cls.__new__(origin)
        obj.__init__(*args, **kwargs)
        return obj

However, since Python 3.5 nearing its end of life maybe it's not worth it. We just have 3.5.2 baked in our Docker images. What is your advise? Submit a PR for typing_extensions and see if it works or drop it? Worth noting though that Python 3.5.2 is default for Ubuntu 16.04 which is still widely used for deployment.

@gvanrossum
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I strongly advise you to upgrade your Python version.

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@Slonegg @gvanrossum