The parameter inplace=False should be deprecated across the board in preparation for pandas 2, which will not support that input (we will always return a copy). That would give people time to stop using it.

Thoughts?

Methods using inplace:

Deprecation non controvertial (a copy will be made anyway, and inplace=True does not add value):

• (Series/DataFrame).drop
• (Series/DataFrame).drop_duplicates
• (Series/DataFrame).dropna
• DataFrame.set_index (with drop=False wouldn't change the data, but that doesn't seem the main use case)
• DataFrame.query
• DataFrame.eval

Not sure:

• (Series/DataFrame).sort_values
• (Series/DataFrame).sort_index

Should be able to not copy memory (under discussion on what to do):

• (Series/DataFrame).clip
• (Series/DataFrame).where
• (Series/DataFrame).fillna
• (Series/DataFrame).rename_axis
• (Series/DataFrame).reset_index
• (Series/DataFrame).replace
• (Series/DataFrame).set_axis
• (Series/DataFrame).mask
• (Series/DataFrame).interpolate
• DataFrame.rename
• Index.rename
• Index.set_names
• MultiIndex.set_levels
• MultiIndex.set_labels
• pandas.core.resample.Resampler.interpolate

Special cases:

• pandas.eval (with inplace=False the value is not returned but set to an argument target)