blockchain.py

import hashlib
import json
from time import time
from textwrap import dedent
from uuid import uuid4

from flask import Flask, jsonify, request

class Blockchain(object):
    def __init__(self):
        self.chain = []
        self.current_transactions = []

        #create genesis block
        self.new_block(proof=100, previous_hash=1)

    def new_block(self, proof, previous_hash=None):
        # create new block and add it to chain
        block = {
            'index': len(self.chain) + 1,
            'timestamp': time(), # type: ignore
            'transactions': self.current_transactions,
            'proof': proof,
            'previous_hash': previous_hash or self.hash(self.chain[-1]),
        }
    
        # reset current transactions
        self.current_transactions = []
        self.chain.append(block)
        return block

    def new_transaction(self, sender, recipient, amount):
        # add new transaction
        self.current_transactions.append({
            'sender': sender,
            'recipient': recipient,
            'amount': amount
        })

        return self.last_block['index'] + 1

    @staticmethod
    def hash(block):
        block_string = json.dumps(block, sort_keys=True).encode()
        return hashlib.sha256(block_string).hexdigest()

    @property
    def last_block(self):
        return self.chain[-1]
    
    def proof_of_work(self, last_proof):    # find a proof such that when hashed with the previous block's solution, a hash with 4 leading zeros is produced
        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" # return true if 4 leading zeros
    
app = Flask(__name__)

node_identifier = str(uuid4()).replace('-', '')

blockchain = Blockchain()

@app.route('/mine', methods=['GET'])
def mine():
    # run PoW algorithm to get next proof
    last_block = blockchain.last_block
    last_proof = last_block['proof']
    proof = blockchain.proof_of_work(last_proof)

    # reward for finding proof, sender is 0 to show the node has mined a new coin
    blockchain.new_transaction(
        sender="0",
        recipient=node_identifier,
        amount=1,
    )

    # forge new block by adding it to chain
    previous_hash = blockchain.hash(last_block)
    block = blockchain.new_block(proof, previous_hash)

    response = {
        'message': "New block forged",
        'index': block['index'],
        'transactions': block['transactions'],
        'proof': block['proof'],
        'previous_hash': block['previous_hash'],
    }

    return jsonify(response), 200

@app.route('/transactions/new', methods=['POST'])
def new_transaction():
    values = request.get_json()

    required = ['sender', 'recipient', 'amount']
    if not all(k in values for k in required):
        return "Missing values", 400
    
    # create new transaction
    index = blockchain.new_transaction(values['sender'], values['recipient'], values['amount'])

    response = {'message': f'Transaction will be added to Block {index}'}
    return jsonify(response), 201

@app.route('/chain', methods=['GET'])
def full_chain():
    response = {
        'chain': blockchain.chain,
        'length': len(blockchain.chain),
    }

    return jsonify(response), 200

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)