How to find column names in pandas dataframe that contain all unique values except NaN?

Question

I want to find columns that contain all non-duplicates from a pandas data frame except NaN.

   x   y   z
a  1   2   A
b  2   2   B
c  NaN 3   D
d  4   NaN NaN
e  NaN NaN NaN 

The columns "x" and "z" have non-duplicate values except NaN, so I want to pick them out and create a new data frame.

Answer 1

Let us use nunique

m=df.nunique()==df.notnull().sum()
subdf=df.loc[:,m]
     x    z
a  1.0    A
b  2.0    B
c  NaN    D
d  4.0  NaN
e  NaN  NaN

m.index[m].tolist()
['x', 'z']