Blockchain.py

from hashlib import sha256
import time
from json import dumps

# to define each blocks


class Block:
    def __init__(self, data, prevHash=None):
        self.data = data
        # timestamp of block creation
        self.time = time.time()
        # to help in guessing the target during mining
        self.nonce = 0
        # hash key of previous block
        self.prevHash = prevHash
        self.hash = self.calc_hash()

    def calc_hash(self):
        hash_object = None
        input = self.data+str(self.time)+str(self.nonce)
        if self.prevHash is not None:
            input += self.prevHash
        # to produce 256 bit hex digest using SHA algorithm
        hash_object = sha256(input.encode('utf-8'))
        digest = hash_object.hexdigest()
        return digest
    # to mine current block as proof-of-work

    def mine_block(self):
        # to mine until target is reached
        while self.hash[:BlockChain.mining_difficulty] != BlockChain.target:
            self.nonce += 1
            self.hash = self.calc_hash()
        print("Block mined : {0} Nonce : {1}".format(self.hash, self.nonce))

# to define block chain


class BlockChain:
    # mining difficulty. For Bitcoin it is around 3,007,383,866,430
    mining_difficulty = 5
    target = "0"*mining_difficulty

    def __init__(self, data):
        self.block_chain = list()
        # genesis block
        gen_block = Block(data)
        self.block_chain.append(gen_block)
    # to check if the blockchain is valid

    def checkChainValid(self):
        i = 1
        while i < len(self.block_chain):
            prev_block = self.block_chain[i-1]
            cur_block = self.block_chain[i]
            if cur_block.hash != cur_block.calc_hash():
                print("Current hash mismatch !")
                return False
            elif cur_block.prevHash != prev_block.hash:
                print("Previous hash mismatch !")
                return False
            elif cur_block.hash[:BlockChain.mining_difficulty] != BlockChain.target:
                print("Current block {0} is not mined !".format(
                    cur_block.hash))
                return False
            i += 1
        return True

    def getBlockChain(self):
        return self.block_chain


# driver/test code
block_chain_obj = BlockChain("Genesis block")
block_chain = block_chain_obj.getBlockChain()
print("Mining block #0")
block_chain[0].mine_block()

block_chain.append(Block("Hello world", block_chain[0].hash))
print("Mining block #1")
# block_chain[1].mine_block()

block_chain.append(Block("I am a block", block_chain[1].hash))
print("Mining block #2")
block_chain[2].mine_block()

if block_chain_obj.checkChainValid():
    # produce json dump of valid blockchain
    print(dumps([block.__dict__ for block in block_chain],
                indent=4, sort_keys=True))
else:
    print("BlockChain is invalid !")