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ViewsPyCharm type checker expected type 'None', got 'str' instead when using pandas dataframe.to_csv
Using PyCharm 2021.2 (Community Edition) and Pandas 1.3.1, with the following code:
import pandas as pd
d = {'col1': [1, 2], 'col2': [3, 4]}
df = pd.DataFrame(data=d)
df.to_csv("testfile.csv")
PyCharm will produce a Warning, highlight "testfile.csv", and display "Expected type 'None', got 'str' instead" when hovering over the Warning.
testfile.csv is created and no issues appear to be happening.
Is there a way to fix this? Does anyone know why this Warning appears?
Thanks
2 answers
Answer question0
You can suppress the PyCharm warning as follows:
# noinspection PyTypeChecker
df.to_csv("testfile.csv")
You can also add the noinspection comment at the start of a function or class to disable warnings within, but careful not to hide other real warnings. I prefer per-statement suppression only as needed.
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This appears to be a bug in Pycharm that prevents it from properly interpreting |; the new operator for typing.Union.
If I ctrl+p while inside of the argument list of to_csv, I get this
Note that it thinks the argument is of type None (indicated by : None).
The problem is, this is the actual signature of the function:
def to_csv(
self,
path_or_buf: FilePathOrBuffer[AnyStr] | None = None,
. . .
The type of the parameter is actually FilePathOrBuffer[AnyStr] | None, which prior to Python 3.10 would be typing.Union[FilePathOrBuffer[AnyStr], None] or simply typing.Optional[FilePathOrBuffer[AnyStr]].
Pycharm apparently can't yet read fancy type hints, despite saying it supports Python 3.10.