BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty

[morotti]

BUG: DataFrame.resample is changing the index type to MultiIndex when the dataframe is empty

very easy bug to reproduce. the bug has been opened for 1 year.

# repro
import pandas as pd

df = pd.DataFrame(columns = ["a", "b"], index = pd.DatetimeIndex([]))
sampled = df.resample("8h").apply(pd.Series.mean)

print(df.index)
print(sampled.index)

output on pandas 1

DatetimeIndex([], dtype='datetime64[ns]', freq=None)
DatetimeIndex([], dtype='datetime64[ns]', freq='8H')

output on pandas 2 and main branch

DatetimeIndex([], dtype='datetime64[ns]', freq=None)
MultiIndex([], )

• closes BUG: When upsampling an empty dataframe, the index type changes from DatetimeIndex to MultiIndex #55572 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Thanks @morotti