Pandas selecting by label sometimes return Series, sometimes returns DataFrame

python pandas dataframe slice series
jobevers picture jobevers · Dec 4, 2013 · Viewed 37.9k times · Source

In Pandas, when I select a label that only has one entry in the index I get back a Series, but when I select an entry that has more then one entry I get back a data frame.

Why is that? Is there a way to ensure I always get back a data frame?

In [1]: import pandas as pd

In [2]: df = pd.DataFrame(data=range(5), index=[1, 2, 3, 3, 3])

In [3]: type(df.loc[3])
Out[3]: pandas.core.frame.DataFrame

In [4]: type(df.loc[1])
Out[4]: pandas.core.series.Series

Answer

Dan Allan picture Dan Allan · Dec 4, 2013

Granted that the behavior is inconsistent, but I think it's easy to imagine cases where this is convenient. Anyway, to get a DataFrame every time, just pass a list to loc. There are other ways, but in my opinion this is the cleanest.

In [2]: type(df.loc[[3]])
Out[2]: pandas.core.frame.DataFrame

In [3]: type(df.loc[[1]])
Out[3]: pandas.core.frame.DataFrame

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