子集具有索引的pandas数据框，该索引包含重复项

Question

For the data frame:

df = pd.DataFrame({
    'key': [1,2,3,4,5, np.nan, np.nan],
    'value': ['one','two','three', 'four', 'five', 'six', 'seven']
}).set_index('key')

That looks like this:

        value
key     
1.0     one
2.0     two
3.0     three
4.0     four
5.0     five
NaN     six
NaN     seven

I would like to subset it to:

    value
key     
1   one
1   one
6   NaN

This produces a warning:

df.loc[[1,1,6],]

Passing list-likes to .loc or [] with any missing label will raise
KeyError in the future, you can use .reindex() as an alternative.

This produces an error:

df.reindex([1, 1, 6])

ValueError: cannot reindex from a duplicate axis

How to do it while referencing a missing index and without using apply?

Answer 1

The thing is you have duplicated values NaN s as indexes. You should disconsider those when reindexing because they are duplicates and there is ambiguity on which value use in the new index.

df.loc[df.index.dropna()].reindex([1, 1, 6])

    value
key 
1   one
1   one
6   NaN

For a generalized solution, use duplicated

df.loc[~df.index.duplicated(keep=False)].reindex([1, 1, 6])

If you want to keep duplicated indexes and use reindex , you'll fail. This has actually been asked before a couple of times