Proof of work implementation for Python3

pow.py

from hashlib import sha512

def count_noughts(s):
    n=0
    for c in s:
        if c == '0':n+=1
        else:
            return n
    return 0

def work(prefix,noughts,search_depth=10):
    if search_depth < 1:
        return None
    for _ in range(256):
        h=sha512((prefix+chr(_)).encode()).hexdigest()
        if noughts==count_noughts(h):return chr(_)
    for d in range(search_depth-1):
        for _ in range(256):
            r=work(prefix+chr(_),noughts,d)
            if r is not None:return chr(_)+r

s=work('',3)
print('{}:{}'.format(sha512(s.encode()).hexdigest(),s))