python - 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 the 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 use nunique to calculate the number of unique values in each column of the dataframe, and drop the columns which only have a single unique value:

In [285]:
nunique = df.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, take abs values and sums them:

In [298]:
cols = df.select_dtypes([np.number]).columns
diff = df[cols].diff().abs().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