Pandas: combining header rows of a multiIndex DataFrame

1. Update using Python 3.6+ use f-string formatting with list comprehension:

df.columns = [f'{i}{j}' for i, j in df.columns]

2. Use map and join:

df.columns = df.columns.map(''.join)

3. If your columns has numeric datatypes, use map and format:

df.columns = df.columns.map('{0[0]}{0[1]}'.format) 

Output:

   barone  bartwo  bazone  baztwo  fooone  footwo  foothree
A       1       2       3       4       5       6         7
B       8       9      10      11      12      13        14