import hashlib
import json
from time import time
from uuid import uuid4


class Blockhain(object):
    def proof_of_work(self,last_proof):
        proof=0
        while self.valid_proof(last_proof,proof) is False:
            proof+=1
        return proof

@staticmethod
def valid_proof(last_proof,proof):
     guess=f'{last_proof}{proof}'.encode()
     guess_hash=hashlib.sha256(guess).hexdigest()
     return guess_hash[:4]=="0000"
