Fix type object signature when both __new__ and __init__ present #5642
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Currently mypy will prefer __init__ to __new__ for determining the signature of a type object if there exists any __init__ other than object's. Instead, prefer the closest definition in the MRO, so that subclass __new__ can override parent __init__. Fixes #1435.
gvanrossum
reviewed
Sep 19, 2018
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Note that only the first example fails in CPython -- the others are OK.
The rule CPython uses is this (for a class C with superclass B, where B is not object):
- If both
C.__new__andC.__init__refer to their versions inobject, the signature isdef () -> C. - If exactly one of
C.__new__andC.__object__refers to itsobjectversion, the other determines the signature. - If both override their corresponding
objectversions, they must match -- even if they don't come from the same class (say, if one comes fromCand the other fromB).
| def __new__(cls) -> 'B': | ||
| ... | ||
|
|
||
| B() |
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But at runtime in CPython this is an error:
$ Traceback (most recent call last):
File "_.py", line 9, in <module>
b = B()
TypeError: __init__() missing 1 required positional argument: 'x'
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Oh, hm:
The original bug example had the __new__ call return an int, which works because __init__ isn't called if it isn't a subclass.
gvanrossum
approved these changes
Sep 19, 2018
This was referenced Sep 19, 2018
hauntsaninja
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Nov 3, 2024
Fixes python#5647 See also python#5642
hauntsaninja
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Nov 3, 2024
Fixes python#5647 See also python#5642
hauntsaninja
added a commit
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this pull request
Nov 3, 2024
Fixes python#5647 See also python#5642
hauntsaninja
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Dec 27, 2024
Fixes python#5647 See also python#5642
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Currently mypy will prefer
__init__to__new__for determiningthe signature of a type object if there exists any
__init__other than object's.
Instead, prefer the closest definition in the MRO, so that
subclass
__new__can override parent__init__.Fixes #1435.