renamedir.py

#!/usr/bin/env python

import os
import sys

if len(sys.argv) < 2:
    print("must specify a folder to run against for rename")
    sys.exit()

#lists so they maintain order
src = []
dest = []

for root, dirs, files in os.walk(sys.argv[1]):
    #print("root is %s" % root)
    for directory in dirs:
        #print("directory is %s" % directory)
        #remove _ from folder name
        folder = directory.replace('_', '')
        #add 'g' to the beginning of the folder name
        folder = 'g' + folder
        #print("folder name would be: %s" % folder)
        src.append(os.path.join(root, directory))
        dest.append(os.path.join(root, folder))
        
#rename the folders. 
#Had to build a list first otherwise it will not properly continue into sub directories after rename
#we start with the most sub folder so that when we rename it, the other paths aren't broken
for i in reversed(range(0, len(src))):
    print("renaming %s to %s" %(src[i], dest[i]))
    os.rename(src[i], dest[i])