Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
Step 5: Complete Class
import hashlib
import json
from textwrap import dedent
from time import time
from uuid import uuid4
class Blockchain(object):
def __init__(self):
self.current_transactions = []
self.chain = []
# Create the genesis block
genesis_block = self.new_block(previous_hash=1, proof=100)
def new_block(self, proof, previous_hash=None):
Create a new Block in the Blockchain
:param proof: <int> The proof given by the Proof of Work algorithm
:param previous_hash: (Optional) <str> Hash of previous Block
:return: <dict> New Block
block = {
'index': len(self.chain) + 1,
'timestamp': time(),
'transactions': self.current_transactions,
'proof': proof,
'previous_hash': previous_hash or self.chain[-1]['hash'],
# Calculate the hash of this new Block
block['hash'] = self.hash(block)
# Reset the current list of transactions
self.current_transactions = []
return block
def new_transaction(self, sender, recipient, amount):
Creates a new transaction to go into the next mined Block
:param sender: <str> Address of the Sender
:param recipient: <str> Address of the Recipient
:param amount: <int> Amount
:return: <int> The index of the Block that will hold this transaction
'sender': sender,
'recipient': recipient,
'amount': amount,
return self.last_block['index'] + 1
def last_block(self):
return self.chain[-1]
def hash(block):
Creates a SHA-256 hash of a Block
:param block: <dict> Block
:return: <str>
block_string = json.dumps(block).encode()
return hashlib.sha256(block_string).hexdigest()
def proof_of_work(last_proof):
Simple Proof of Work Algorithm:
- Find a number p' such that hash(pp') contains leading 4 zeroes, where p is the previous p'
- p is the previous proof, and p' is the new proof
:param last_proof: <int>
:return: <int>
proof = 0
while True:
guess = f'{last_proof}{proof}'.encode()
guess_hash = hashlib.sha256(guess).hexdigest()
if guess_hash[:4] == "0000":
proof += 1
New Proof found!
New Proof: {proof}
Last Proof: {last_proof}
Hash: {guess_hash}
return proof
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment