How to drop columns which have same values in all rows via pandas or spark dataframe?

Question

Suppose I've data similar to following:

  index id   name  value  value2  value3  data1  val5
    0  345  name1    1      99      23     3      66
    1   12  name2    1      99      23     2      66
    5    2  name6    1      99      23     7      66

How can we drop all those columns like (value, value2, value3) where all rows have same values, in one command or couple of commands using python ?

Consider we have many columns similar to value,value2,value3...value200.

Output:

   index id      name   data1
        0  345  name1    3
        1   12  name2    2
        5    2  name6    7

Answer 1

What we can do is apply nunique to calc the number of unique values in the df and drop the columns which only have a single unique value:

In [285]:
nunique = df.apply(pd.Series.nunique)
cols_to_drop = nunique[nunique == 1].index
df.drop(cols_to_drop, axis=1)

Out[285]:
   index   id   name  data1
0      0  345  name1      3
1      1   12  name2      2
2      5    2  name6      7

Another way is to just diff the numeric columns and sums them:

In [298]:
cols = df.select_dtypes([np.number]).columns
diff = df[cols].diff().sum()
df.drop(diff[diff== 0].index, axis=1)
​
Out[298]:
   index   id   name  data1
0      0  345  name1      3
1      1   12  name2      2
2      5    2  name6      7

Another approach is to use the property that the standard deviation will be zero for a column with the same value:

In [300]:
cols = df.select_dtypes([np.number]).columns
std = df[cols].std()
cols_to_drop = std[std==0].index
df.drop(cols_to_drop, axis=1)

Out[300]:
   index   id   name  data1
0      0  345  name1      3
1      1   12  name2      2
2      5    2  name6      7

Actually the above can be done in a one-liner:

In [306]:
df.drop(df.std()[(df.std() == 0)].index, axis=1)

Out[306]:
   index   id   name  data1
0      0  345  name1      3
1      1   12  name2      2
2      5    2  name6      7

Answer 2

Another solution is set_index from column which are not compared and then compare first row selected by iloc by eq with all DataFrame and last use boolean indexing:

df1 = df.set_index(['index','id','name',])
print (~df1.eq(df1.iloc[0]).all())
value     False
value2    False
value3    False
data1      True
val5      False
dtype: bool

print (df1.ix[:, (~df1.eq(df1.iloc[0]).all())].reset_index())
   index   id   name  data1
0      0  345  name1      3
1      1   12  name2      2
2      5    2  name6      7