Type check signature of __new__ #1019

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1. JukkaL

   added

   feature

   on Nov 29, 2015
2. JukkaL commented on Nov 29, 2015

   

   JukkaL

   on Nov 29, 2015

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   This is follow-up to #982.
3. gvanrossum commented on Nov 29, 2015

   

   gvanrossum

   on Nov 29, 2015

   Member

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   Except for the special case where the implementation for one or the other is inherited from object. In that case the signature of the one that's overridden prevails.
4. JukkaL

   added

   priority-1-normal

   on May 18, 2018
5. msullivan commented on Sep 19, 2018

   

   msullivan

   on Sep 19, 2018

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   A test case

   class A:
       def __init__(self, x: int) -> None:
           pass
   
   class B(A):
       def __new__(cls) -> 'B':
           return object.__new__(cls)
   
   B()
   

   even though it crashes at runtime.

   Per #5642 (review):
   The rule CPython uses is this (for a class C with superclass B, where B is not object):

   • If both C.__new__ and C.__init__ refer to their versions in object, the signature is def () -> C.
   • If exactly one of C.__new__ and C.__object__ refers to its object version, the other determines the signature.
   • If both override their corresponding object versions, they must match -- even if they don't come from the same class (say, if one comes from C and the other from B).