python pandas compare columns and add list of columns that differ

I would like to compare multiple columns in a data frame and add a new column that tells me which columns are different for each row.

for example, for this dataframe i want to compare a1 to a2 and b1 to b2:

   a1  b1  a2  b2
0   1   2   1   2
1   1   2   1   3
2   1   2   3   4

the output should be something like:

   a1  b1  a2  b2  diff
0   1   2   1   2  
1   1   2   1   3  'b1-b2'
2   1   2   3   4  'a1-a2,b1-b2'

this is what i have so far:

import numpy as np
import pandas as pd
data = [{'a1': 1, 'b1': 2, 'a2':1, 'b2':2},
        {'a1':1, 'b1': 2, 'a2': 1, 'b2':3},
        {'a1':1, 'b1': 2, 'a2':3 , 'b2':4}]
df = pd.DataFrame(data)

compare = [('a1','a2'),('b1','b2')]
comp_result = np.array([(df[x[0]] != df[x[1]]) for x in compare])

comp_result is a list of lists of True/False values for each of the comparisons but i am not sure how to use that to create the "diff" column.

>Solution :

Fast one-liner without loops:

col_groups = [c.columns for _, c in df.groupby(df.columns.str[0], axis=1)]

df['diff'] = pd.Series(np.sum([(df[l] != df[r]).map({True: f'{l}-{r}',False:''}) + ',' for l, r in col_groups], axis=0)).str.strip(',')

Output:

>>> df
   a1  b1  a2  b2         diff
0   1   2   1   2             
1   1   2   1   3        b1-b2
2   1   2   3   4  a1-a2,b1-b2