change_tab_to_space.py

#!/usr/bin/env python

import os
import re
import sys

tabs = re.compile(r'^(\t+)')
spaces = '    '

def doit(filepath):
    path = os.path.join(os.getcwd(), filepath)
    lines = []
    with open(path, 'r') as f:
        for line in f.readlines():
            m = tabs.search(line)
            if m:
                line = line.replace(m.group(1), spaces*len(m.group(1)))
            lines.append(line)
    with open(path, 'w') as f:
        f.write(''.join(lines))


if __name__ == '__main__':
    for line in sys.stdin:
        doit(line.strip())

example.sh

find . -name "*.extention" | /path/to/change_tab_to_space.py