mypy throws an error when two pipes are combined

[PetitLepton]

Hi, I am taken aback by the behavior of mypy. With the following minimal example

import pandas


def test() -> pandas.DataFrame:
    return pandas.DataFrame().pipe(lambda df: df).pipe(lambda df: df)

mypy barks at me with

error: <nothing> has no attribute "pipe"  [attr-defined]

but a single pipe is OK.

Do I have to cast manually the first pandas.DataFrame().pipe(lambda df: df)?

I am using mypy 1.51, pandas 2.0.3 and pandas-stubs 2.0.3.230814.

Thanks for your help!

[twoertwein]

The issue is that mypy (and pyright) cannot infer the type of your lambda function but the definition of pipe depends on the return value of this lambda function.

You could cast, but it would be better to provide type annotations for your lambda function:

from typing import Callable

import pandas as pd

reveal_type(lambda df: df)  # "def (df: Any) -> Any"
reveal_type(pd.DataFrame().pipe(lambda df: df))  # Any

# explicitly declare the type
fun: Callable[[pd.DataFrame], pd.DataFrame] = lambda df: df
reveal_type(pd.DataFrame().pipe(fun).pipe(fun))  # DataFrame

# or use a def
def fun2(df: pd.DataFrame) -> pd.DataFrame:
    return df

reveal_type(pd.DataFrame().pipe(fun2).pipe(fun2))  # DataFrame

[PetitLepton]

All right. With a fresh installation, it works as expected. I will dig into that. Thanks a lot!

[PetitLepton]

@twoertwein and @Dr-Irv I eventually found the reason. Fresh install works. It turns out that a mypy plugin I was using was imposing a lower version of pandas-stubs. Without it, it works as expected. Thanks for your help!

[twoertwein]

Out of curiosity, which mypy plugin uses pandas-stubs?

[PetitLepton]

pandera, you can take a look here.

[PetitLepton]

Note that we are using an older version of the pandera package which could explain the issues.

[Dr-Irv]

@PetitLepton OK to close the discussion?

[PetitLepton]

Sure. I will check on my side and reopen if necessary. 😉 Thanks!