Closed
Description
Bug Report
In a function, mypy fails to detect an incompatible assignment in a function, if that is after a __get_item__ access inside a branch. Weirdly, mypy correctly deduces the types, as shown with reveal_type but does not complain about the incorrect assignment anyways.
To Reproduce
I would have liked to give a playground link, but the MWE involves a pandas.DataFrame which seems not to work on the playground.
import pandas as pd
# fails as expected
some_condition: bool = "123".isnumeric()
list_of_dicts = [{"some": 1, "columns": 2, "will": 3, "go": 4, "here": 5}]
if some_condition > 0:
output_df = pd.DataFrame(list_of_dicts)
output_df = output_df[["some", "columns", "here"]]
else:
output_df = None # fails here
def fails_as_expected_1(some_condition: bool) -> pd.DataFrame:
list_of_dicts = [{"some": 1, "columns": 2, "will": 3, "go": 4, "here": 5}]
output_df = pd.DataFrame(list_of_dicts)
output_df = output_df[["some", "columns", "here"]]
output_df = None # fails here
other_df: pd.DataFrame = output_df
return other_df
def fails_as_expected_2(some_condition: bool) -> pd.DataFrame:
list_of_dicts = [{"some": 1, "columns": 2, "will": 3, "go": 4, "here": 5}]
if some_condition > 0:
output_df = pd.DataFrame(list_of_dicts)
else:
output_df = None
other_df: pd.DataFrame = output_df # fails here
return other_df
def does_not_fail_but_should(some_condition: bool) -> pd.DataFrame:
list_of_dicts = [{"some": 1, "columns": 2, "will": 3, "go": 4, "here": 5}]
if some_condition > 0:
output_df = pd.DataFrame(list_of_dicts)
reveal_type(output_df)
output_df = output_df[["some", "columns", "here"]]
reveal_type(output_df)
else:
output_df = None
other_df: pd.DataFrame = output_df
return other_dfExpected Behavior
Each block and function should contain a mypy error regarding an incompatible assignment.
Actual Behavior
For the last function, no such error is produced:
mwe2.py:10: error: Incompatible types in assignment (expression has type "None", variable has type "DataFrame") [assignment]
mwe2.py:17: error: Incompatible types in assignment (expression has type "None", variable has type "DataFrame") [assignment]
mwe2.py:28: error: Incompatible types in assignment (expression has type "DataFrame | None", variable has type "DataFrame") [assignment]
mwe2.py:36: note: Revealed type is "pandas.core.frame.DataFrame"
mwe2.py:38: note: Revealed type is "pandas.core.frame.DataFrame"
Found 3 errors in 1 file (checked 1 source file)
Your Environment
- Mypy version used: mypy 1.14.1 (compiled: yes)
- Mypy command-line flags: only the filename
- Mypy configuration options from
pyproject.toml:
[tool.mypy]
warn_unreachable = true
enable_error_code = [
"possibly-undefined"
]
strict = true- Python version used: Python 3.12.8 from Debian Testing