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Incorrect handling of Self when operating on the different class instance than self #14635
Description
Bug Report
mypy complains on a method returning Self when it returns another instance of its own class.
To Reproduce
from typing import Self
class Foo:
def spawn(self) -> Self:
return Foo()Expected Behavior
The same output as with
class Foo:
def spawn(self) -> 'Foo':
return Foo()Actual Behavior
1.py:4: error: Incompatible return value type (got "Foo", expected "Self") [return-value]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used:
mypy 1.0.0 (compiled: no) - Mypy command-line flags: -
- Mypy configuration options from
mypy.ini(and other config files): - - Python version used:
Python 3.11.1
Activity
JelleZijlstra commentedon Feb 7, 2023
This error is correct.
Selfmeans that the method will return the same type even in subclasses. Your method does not do that.johnthagen commentedon Feb 8, 2023
Thanks for the explanation @JelleZijlstra.
While I'm bummed because I was hoping I could fix the forward declaration problem nearly everywhere by applying
Selfeven to normal methods, your explanation about the type theory reasoning makes sense. I guess we'll keep usingfrom __future__ import annotationsuntil PEP 649 lands.hauntsaninja commentedon Feb 8, 2023
Maybe it could be allowed in final classes