Blockchain Example

Blockchain.py

import datetime
import hashlib
import random

class Block:
    blockNo = 0
    data = None
    next = None
    hash = None
    nonce = 0
    previous_hash = 0x0                 # Formally a pointer to the previous block hash.  
    timestamp = datetime.datetime.now() # Class records the time when the block object gets created.  

#     Initiate Blockchain with a data
    def __init__(self, data):
        self.data = data
        
#     Using hashlib.SHA256() for hashing block data.
    def hash(self):
        h = hashlib.sha256()
        h.update((str(self.blockNo) +
                  str(self.data) +
                  str(self.hash) +
                  str(self.nonce) +
                  str(self.previous_hash) +
                  str(self.timestamp)
                  ).encode('utf-8'))
        return h.hexdigest()

    def __str__(self):
        return "BlockNo: " + str(self.blockNo) + 
    "\ndata: " + str(self.data) + 
    "\nhash: " + str(self.hash()) + 
    "\nnonce: " + str(self.nonce) + 
    "\npreviousHash: " + str(self.previous_hash)

class Blockchain:
#   Level of difficulty can be set higher if needed. The higher the value, the more time the algorithm will need 
#   in order to mine the underlying block as target = 2 ** (256 - difficulty)
#   maxNonce is the maximum amount of trials per block.
    difficulty = 20
    maxNonce = 2**32
    target = 2 ** (256 - difficulty)

    block = Block("Genesis")
    dummy = head = block

    def add(self, block):
        block.previous_hash = self.block.hash()
        block.blockNo = self.block.blockNo + 1

        self.block.next = block
        self.block = self.block.next
        
#   Mine selected Block with hashlib.sha256()
    def mineBlock(self, block):
        for n in range(maxNonce):
            if target >= int(block.hash(), 16):
                self.add(block)
                print(block)
                break
            else:
                block.nonce += 1

    def printBlocks(self):
        while self.head is not None:
            print(self.head)
            self.head = self.head.next

        self.head = self.dummy

    def printBlock(self, blockNumber):
        while self.head.blockNo <= blockNumber-1:
            self.head = self.head.next
        print(self.head)
        self.head = self.dummy