BUG: inplace behavior is inconsist for fillna · Issue #47188 · pandas-dev/pandas (original) (raw)

Pandas version checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of pandas.
• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd import numpy as np pdf = pd.DataFrame({"x": [np.nan, 2, 3, 4, np.nan, 6], "y": [np.nan, 2, 3, 4, np.nan, 6]}) pser = pdf.x

Didn't make a copy write to existing (it's unexpected behavior)

pdf.fillna(0, inplace=True)

pser 0 0.0 1 2.0 2 3.0 3 4.0 4 0.0 5 6.0

import pandas as pd import numpy as np pdf = pd.DataFrame({"x": [np.nan, 2, 3, 4, np.nan, 6], "y": [np.nan, 2, 3, 4, np.nan, 6]}) pser = pdf.x

Make a copy and doesn't write to existing (it's expected behavior)

pdf.fillna({"x": -1, "y": -2}, inplace=True) pser 0 NaN 1 2.0 2 3.0 3 4.0 4 NaN 5 6.0 Name: x, dtype: float64

Issue Description

Looks like behavior is a little bit differet in current pandas.

Expected Behavior

Always make a copy and doesn't write to existing array for Do in-place operation in all functions with inplace (such like eval/update/fillna).

Installed Versions

1.4+