Pandas: Modify a particular level of Multiindex

Question

I have a dataframe with Multiindex and would like to modify one particular level of the Multiindex. For instance, the first level might be strings and I may want to remove t

Answer 1

As mentioned in the comments, indexes are immutable and must be remade when modifying, but you do not have to use reset_index for that, you can create a new multi-index directly:

df.index = pd.MultiIndex.from_tuples([(x[0], x[1].replace(' ', ''), x[2]) for x in df.index])

This example is for a 3-level index, where you want to modify the middle level. You need to change the size of the tuple for different level sizes.

Update

John's improvement is great performance-wise, but as mentioned in the comments it causes an error. So here's the corrected implementation with small improvements:

df.index.set_levels(
    df.index.levels[0].str.replace(' ',''), 
    level=0,
    inplace=True,  # If False, you will need to use `df.index = ...`
)

If you'd like to use level names instead of numbers, you'll need another small variation:

df.index.set_levels(
    df.index.levels[df.index.names.index('level_name')].str.replace(' ',''), 
    level='level_name',
    inplace=True,
)