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private (name mangled) attributes within inherited classes can not have different types #19598
Description
Bug Report
mypy does not recognise that a dunder class attribute does not clash with the inherited one
To Reproduce
Consider the following example:
class BaseSpecialValue:
def __init__(self, special_value:int):
self.__special_value = special_value
@property
def base_special_value(self) -> int:
return self.__special_value
class SpecialValue(BaseSpecialValue):
def __init__(self, base_special_value:int, special_value:str):
super().__init__(special_value=base_special_value)
self.__special_value = special_value
@property
def special_value(self) -> str:
return self.__special_value
if __name__ == '__main__':
foo = SpecialValue(special_value='one', base_special_value=1)
print(f'special_value={foo.special_value}, base_special_value={foo.base_special_value}')In this case both SpecialValue and BaseSpecialValue define __special_value, when run it is clear that python treats them separately as the result is:
special_value=one, base_special_value=1
Expected Behavior
No mypy error reported
Actual Behavior
However, when mypy is run the following result is returned by mypy:
mypy --config-file=default_mypy.ini mypy_dunder_test.py
mypy_dunder_test.py:15: error: Incompatible types in assignment (expression has type "str", variable has type "int") [assignment]
mypy_dunder_test.py:19: error: Incompatible return value type (got "int", expected "str") [return-value]
Found 2 errors in 1 file (checked 1 source file)Your Environment
- Mypy version used: 1.17.1
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): See below - Python version used: 3.12.0
default configuration used
# Global options:
[mypy]
warn_return_any = True
warn_unused_configs = True
Activity
sterliakov commentedon Aug 6, 2025
Very similar to #8267 (please note that "dunder attributes" usually refer to
__module__and similar attributes that start and end with two underscores, this is just mangled/private name). Mypy indeed does not fully support name mangling, and we really should.[-]dunder attributes within inherited classes can not have different types[/-][+]private (name mangled) attributes within inherited classes can not have different types[/+]krcb197 commentedon Aug 6, 2025
@sterliakov
Thank you for the feedback. I agree that the terminology "dunder" was not quite right. I have updated the description and title to match the terminology used within the Python documentation Private Variables.
krcb197 commentedon Aug 6, 2025
I believe this issue was previously fixed but may have regressed, see #6505
There is even a unit test for the case, see testIgnorePrivateAttributesTypeCheck