__slots__ isn't effective for limiting attributes #10801

Bug Report

Thank you for Mypy! It's been great help :) Recently, I want to use __slots__ in a class definition to limit the attributes. This way I don't accidentally define attributes with similar but different names (like "bar" in one class but "baz" in another) when I copy my code to a new class. But Mypy seems to ignore it completely, even if CPython throws an AttributeError when executing the code.

Originally I just want to prevent definition of attributes outside __init__, but Mypy doesn't seem to provide an option to check this, so I thought about __slots__ as a workaround, but unfortunately this failed too. Please point out any probable workarounds for the above issues.

To Reproduce

Put the following in "main.py" and first execute with python then check with mypy.

class TestClass:
    __slots__ = ('bar',)
    def __init__(self) -> None:
        self.bar: int = 0
    def modify(self) -> None:
        self.baz = 42
    def __str__(self) -> str:
        return f"TestClass(bar={self.bar})"

t = TestClass()
t.modify()
print(t)

$ python main.py
$ mypy main.py

Expected Behavior

Actual Python output:

Traceback (most recent call last):
  File "main.py", line 11, in <module>
    t.modify()
  File "main.py", line 6, in modify
    self.baz = 42
AttributeError: 'TestClass' object has no attribute 'baz'

Expected Mypy output, to be consistent with the Python output above:

main.py:6: error: "TestClass" has no attribute "baz"
Found 1 error in 1 file (checked 1 source file)

Actual Behavior

Actual Mypy output:

Success: no issues found in 1 source file

Your Environment (flags, configs, and OS are probably irrelevant)

• Mypy version used: 0.910 (pypi)
• Mypy command-line flags: (None)
• Mypy configuration options from mypy.ini (and other config files): (None)
• Python version used: 3.7.6 (conda-forge)
• Operating system and version: macOS Catalina Version 10.15.7