mass rename files in folders

[kerzol81]

I use python 3.6, i modified the script, but it still doesnt work

import os

def renamer(*folder):
    dirname = folder
    files = os.listdir(folder)
    os.chdir(folder)
    for i in files:
        os.rename(i, dirname + '_' + i)



for root, dirs, files in os.walk('.'):
    dirs[:] = [d for d in dirs if not d[0] == '.']

    renamer(dirs)