Change number of spaces used for indentation in a file, only change spaces at start of file. Made for python, will work with any file.

change_indent_length.py

#!/bin/env python3
#===============================================================================
#
#         FILE: change_indent_length.py
#
#        USAGE: cange_indent_length.py [-h] file_name old_tab_length new_tab_length
#
#  DESCRIPTION: Change indent length, only change those at start of line
#
#       AUTHOR: Michael D Dacre, [email protected]
# ORGANIZATION: Stanford University
#      LICENSE: CC BY-NC-SA - Creative Commons Attribution-NonCommercial-ShareAlike
#      VERSION: 0.3
#      CREATED: 11-12-12 09:42:14
#===============================================================================

import sys
import argparse
import re

# Get command line arguments
parser = argparse.ArgumentParser(formatter_class=argparse.RawDescriptionHelpFormatter,
                                 description="""\
Change indent length, only effect those spaces at start of line.
Will write new file out as original file name with '_tabbed' appended""")
parser.add_argument('file_name', type=str, help="Filename of file to parse")
parser.add_argument('old_tab_length', type=int, help="The current length of tabs in the file")
parser.add_argument('new_tab_length', type=int, help="The new length for tabs in the output file")
args = parser.parse_args()

file_name       = args.file_name
old_tab_length  = args.old_tab_length
new_tab_length  = args.new_tab_length

# Create regex
split  = re.compile( '^([ ]*)([^ ].*)$' )
spaces = re.compile( ''.join( [ '([ ]{',
                                str(old_tab_length),
                                '})'
                                ] )
                    )

f = open(file_name, 'r')
o = open('_'.join([file_name, 'tabbed']), 'w')

for line in f:
    split_line  = split.search(line).groups()
    tabs        = spaces.findall(split_line[0])

    # Error checking
    if len(split_line) != 2:
        raise Exception("split_line regex failed! split_line does not contain two elements")

    new_tabs    = []
    # Loop through tabs
    for tab in tabs:
        t       = []
        # Use new_tab_length to create new_tabs
        count   = new_tab_length
        while(count):
            t.append(' ')
            count = count - 1
        new_tabs.append(''.join(t))

    print(''.join([''.join(new_tabs), split_line[1]]), file=o)

f.close()
o.close()