pandas dataframe remove constant column

I believe this option will be faster than the other answers here as it will traverse the data frame only once for the comparison and short-circuit if a non-unique value is found.

>>> df

   0  1  2
0  1  9  0
1  2  7  0
2  3  7  0

>>> df.loc[:, (df != df.iloc[0]).any()] 

   0  1
0  1  9
1  2  7
2  3  7

Ignoring NaNs like usual, a column is constant if nunique() == 1. So:

>>> df
   A  B  row
0  9  0    1
1  7  0    2
2  5  0    3
3  2  0    4
>>> df = df.loc[:,df.apply(pd.Series.nunique) != 1]
>>> df
   A  row
0  9    1
1  7    2
2  5    3
3  2    4