Problem Description​

The same codebase produces ​different type-checking results​ under ​Python 3.9.21​ (detects an error) vs ​Python 3.9.23​ (no error), using the ​same Mypy version (1.6.0 and 1.17.1, tested both)​​ and ​identical Mypy configuration.

This inconsistency breaks team collaboration, as developers using different minor Python 3.9.x versions (e.g., 3.9.21 vs 3.9.23) get conflicting type-check results for the same code.

Reproduction Steps​

​ Environment 1 (Error Detected)​:

Python Version: 3.9.21
Mypy Command: mypy .
Mypy Config (pyproject.toml):

[tool.mypy]
ignore_missing_imports = true 
disallow_untyped_defs = true
explicit_package_bases = true
mypy_path = './'             
exclude = [
    '^script/.*',
    '.*setup.py$',
]

​Environment 2 (No Error)​:

Python Version: 3.9.23
Mypy Command: mypy .
Same Mypy Config and Version (1.6.0 / 1.17.1).
Result: ​No errors reported​ for the same file/code.

Here is the Mypy check results screenshot for two different Python versions