text: # Python 3 program to create Blockchain
  
  # For timestamp
  import datetime
  
  # Calculating the hash
  # in order to add digital
  # fingerprints to the blocks
  import hashlib
  
  # To store data
  # in our blockchain
  import json
  
  # Flask is for creating the web
  # app and jsonify is for
  # displaying the blockchain
  from flask import Flask, jsonify
  
  
  class Blockchain:
  
      # This function is created
      # to create the very first
      # block and set it's hash to "0"
      def __init__(self):
          self.chain = []
          self.create_block(proof=1, previous_hash='0')
  
      # This function is created
      # to add further blocks
      # into the chain
      def create_block(self, proof, previous_hash):
          block = {'index': len(self.chain) + 1,
                   'timestamp': str(datetime.datetime.now()),
                   'proof': proof,
                   'previous_hash': previous_hash}
          self.chain.append(block)
          return block
  
      # This function is created
      # to display the previous block
      def print_previous_block(self):
          return self.chain[-1]
  
      # This is the function for proof of work
      # and used to successfully mine the block
      def proof_of_work(self, previous_proof):
          new_proof = 1
          check_proof = False
  
          while check_proof is False:
              hash_operation = hashlib.sha256(
                  str(new_proof ** 2 - previous_proof ** 2).encode()).hexdigest()
              if hash_operation[:5] == '00000':
                  check_proof = True
              else:
                  new_proof += 1
  
          return new_proof
  
      def hash(self, block):
          encoded_block = json.dumps(block, sort_keys=True).encode()
          return hashlib.sha256(encoded_block).hexdigest()
  
      def chain_valid(self, chain):
          previous_block = chain[0]
          block_index = 1
  
          while block_index < len(chain):
              block = chain[block_index]
              if block['previous_hash'] != self.hash(previous_block):
                  return False
  
              previous_proof = previous_block['proof']
              proof = block['proof']
              hash_operation = hashlib.sha256(
                  str(proof ** 2 - previous_proof ** 2).encode()).hexdigest()
  
              if hash_operation[:5] != '00000':
                  return False
              previous_block = block
              block_index += 1
  
          return True
  
  
  # Creating the Web
  # App using flask
  app = Flask(__name__)
  
  # Create the object
  # of the class blockchain
  blockchain = Blockchain()
  
  
  # Mining a new block
  @app.route('/mine_block', methods=['GET'])
  def mine_block():
      previous_block = blockchain.print_previous_block()
      previous_proof = previous_block['proof']
      proof = blockchain.proof_of_work(previous_proof)
      previous_hash = blockchain.hash(previous_block)
      block = blockchain.create_block(proof, previous_hash)
  
      response = {'message': 'A block is MINED',
                  'index': block['index'],
                  'timestamp': block['timestamp'],
                  'proof': block['proof'],
                  'previous_hash': block['previous_hash']}
  
      return jsonify(response), 200
  
  
  # Display blockchain in json format
  @app.route('/get_chain', methods=['GET'])
  def display_chain():
      response = {'chain': blockchain.chain,
                  'length': len(blockchain.chain)}
      return jsonify(response), 200
  
  
  # Check validity of blockchain
  @app.route('/valid', methods=['GET'])
  def valid():
      valid = blockchain.chain_valid(blockchain.chain)
  
      if valid:
          response = {'message': 'The Blockchain is valid.'}
      else:
          response = {'message': 'The Blockchain is not valid.'}
      return jsonify(response), 200
  
  
  # Run the flask server locally
  app.run(host='127.0.0.1', port=5000)
  