Python; Encode to MD5 (hashlib) shows error: "NoneType"

I wrote a code that will generate random password for 5 times, and I would like to encode that passwords to MD5, but when I try to encode it, it will show an error that 'NoneType' object has no attribute 'encode' and I dont know how to change the code to avoid this error. Sorry I'm beginner in python... My Code is below. Thanks for help

import random, string
import hashlib

length = 6
chars = string.ascii_letters + string.digits

def ff():
    rnd = random.SystemRandom()
    a = (''.join(rnd.choice(chars) for i in range(length)))
    c = a
    return(c)

def ff2():
    for i in range(5):
        print(ff(),' ')


srt = ff2()
result = hashlib.md5(str.encode())

print("The hexadecimal equivalent of hash is : ", end ="")
print(result.hexdigest())