DataFrame.nunique is incorrect for DataFrame with no columns #21959
Description
(edit by @TomAugspurger)
Current output:
In [33]: pd.DataFrame(index=[0, 1]).nunique()
Out[33]:
Empty DataFrame
Columns: []
Index: [0, 1]Expected Output is an empty series:
Out[34]: Series([], dtype: float64)Not sure what the expected dtype of that Series should be... probably object.
original post below:
Code Sample, a copy-pastable example if possible
With Pandas 0.20.3
# create a DataFrame with 3 rows
df = pd.DataFrame({'a': ['A','B','C']})
# lookup unique values for each column, excluding 'a'
unique = df.loc[:, (df.columns != 'a')].nunique()
# this results in an empty Series, the index is also empty
unique.index.tolist()
>>> []
# and
unique[unique == 1].index.tolist()
>>> []With pandas 0.23.3
# create a DataFrame with 3 rows
df = pd.DataFrame({'a': ['A','B','C']})
# lookup unique values for each column, excluding 'a'
unique = df.loc[:, (df.columns != 'a')].nunique()
# this results in an empty Series, but the index is not empty
unique.index.tolist()
>>> [1,2,3]
also:
unique[unique == 1].index.tolist()
>>> [1,2,3]Note:
# if we have don't have an empty df, the behavior of nunique() seems fine:
df = pd.DataFrame({'a': ['A','B','C'], 'b': [1,1,1]})
unique = df.loc[:, (df.columns != 'a')].nunique()
unique[unique == 1]
>>> b 1
>>> dtype: int64
# and
unique[unique == 1].index.tolist()
>>> ['b']Problem description
The change of behavior is a bit disturbing, and seems like it is a bug:
nunique() ends up creating a Series, and it should be a Series of the df columns, but that doesn't seem to be the case here, instead it is picking up the index of the df.
This is likely related to:
I am posting this because in my use case I use the list to drop the columns, but i end up with column names that do not exist in the df