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Hibou57 opened this issue Sep 3, 2020 · 6 comments
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Wrong inference of the target of __new__ #9405

Hibou57 opened this issue Sep 3, 2020 · 6 comments
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@Hibou57
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Hibou57 commented Sep 3, 2020

🐛 Bug Report

The target of an invocation of __new__ is an explicit class, but mypy erroneously believes it is another class.

To Reproduce

Create a file test.py with this content:

from typing import Any
class C(str):
    def __new__(cls, value: str) -> Any:
        return str.__new__(cls, value)

Then run mypy test.py

Side note # 1: this is a stripped down version to reproduce the issue, the original case has more real interest.
Side note # 2: __new__ should return C but the name C is not defined during class creation, so __new__ is declared to return Any, instead.

Expected Behavior

There should be no error, as one can check this way:

>>> import test
>>> s = test.C("abc")
# No exception raised
>>> type(s)
<class 'test.C'>

Actual Behavior

mypy complains:

test.py:4: error: Too many arguments for "__new__" of "object"
Found 1 error in 1 file (checked 1 source file)

It erroneously believes the target of __new__ is object while it is str

Your Environment

  • Mypy version used: mypy 0.782
  • Mypy command-line flags: none (just the file name)
  • Mypy configuration options from mypy.ini (and other config files): none (no config file)
  • Python version used: Python 3.8.2
@Hibou57 Hibou57 added the bug mypy got something wrong label Sep 3, 2020
@Hibou57 Hibou57 changed the title Wrong inference of the type of the target of __new__ Wrong inference of the target of __new__ Sep 3, 2020
@gvanrossum
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I think this could actually be fixed in typeshed -- the problem seems to be that no __new__ method is declared for class str.

But another interpretation would be that we ought to automatically derive a signature for __new__ if one is given for __init__. That would not be a simple feature to add, and I suspect there's already some other issue open for it. (Maybe #5647 is related?)

@gvanrossum gvanrossum added feature and removed bug mypy got something wrong labels Sep 3, 2020
@Hibou57
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Hibou57 commented Sep 3, 2020

I’m not good at Python insides, anyway, my understanding is that the two issues are not that much related. The other issue as I understand it, is about __new__ vs __init__ to infer the type returned by a class constructor (I agree __new__ is the reference for that).

I did not knew about typeshed, so just had a look: https://github.com/python/typeshed/blob/master/stdlib/2and3/builtins.pyi#L443

Indeed, no __new__ is defined. But I was believing the arguments to __new__ and __init__, except for cls and self, are typically the same. If I’m not too wrong, when no __new__ is defined, what about relying on __init__ to infer its expected arguments? In that case, __new__(cls, o: object) could be inferred and would match the invocation.

@Hibou57
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Hibou57 commented Sep 6, 2020

That said, I wonder if the error is finally in typeshed: I believe that’s a __new__ which should be declared for str, not an __init__ (and if so, probably the same for others like int, float, bool). There seems to be no __init__ for str.

@Hibou57 Hibou57 mentioned this issue Sep 6, 2020
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@gvanrossum
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gvanrossum commented Sep 6, 2020 via email

@JelleZijlstra
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We tried this before in python/typeshed#1352 and terrible things happened (see python/typeshed#1464). Perhaps the enum issue I found back then got solved some other way since then, though.

@AlexWaygood
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Cannot be reproduced on mypy 0.800+

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@JelleZijlstra @Hibou57 @gvanrossum @AlexWaygood