A simple patch script in Python

patch.py

#By Robin Wellner (gvx)
#I hereby waive copyright and related or neighboring rights to this work
#See the Creative Commons Zero Waiver at <http://creativecommons.org/publicdomain/zero/1.0/>

import sys
diff_file = sys.stdin.read().split('\n')

if len(sys.argv) > 1:
	filename = sys.argv[1]
else:
	filename = diff_file[0][4:].split('\t', 2)[0]

try:
	f = open(filename)
except:
	sys.stderr.write("Fatal: File could not be opened.\n")
	sys.exit(1)

old_file = f.read().split('\n')
f.close()
diff_file.pop(0);diff_file.pop(0)

for line in diff_file:
	try:
		first, rest = line[0], line[1:]
	except IndexError:
		first = None
	if first == '@':
		new_index = int(rest.split('+', 2)[1].split(',', 2)[0]) - 1
	elif first == '+': #new line
		old_file.insert(new_index, rest)
		new_index += 1
	elif first == '-': #deleted line
		old_file.pop(new_index)
	else:
		#if ' ' or '\', simply ignore
		new_index += 1

sys.stdout.write('\n'.join(old_file))