Implementing a Blockchain in Python

blockchain.py

## Import statements
import hashlib
import datetime

class Block:
  '''
  All blocks contain the following data:
  block_height -> index of block on chain
  data -> data in the block
  hash -> hash of this block
  previous_hash -> hash of the previous block
  timestamp -> time the block was added to the chain
  nonce -> number that miners are trying to solve for
    they get rewarded. Default = 0
  '''
  # Block instance attributes
  def __init__(self, index, data, previous_hash, timestamp, nonce=0):
    self.index = index
    self.timestamp = timestamp
    self.data = data
    self.previous_hash = previous_hash
    self.nonce = nonce
    self.hash = self.hash_block()

  # Block description
  def __str__(self):
    return(f"This is a block")
    
    '''
    defining a function that computes the hash of
    the block based on the attributes.
    Takes in no inputs. 
    Creates a string of all the class variables.
    Computes SHA256 hash of the string.
    '''
  def hash_block(self):
    #block_data = str(self.index) + self.timestamp + self.data + self.previous_hash + self.nonce
    block_data = str(self.index) + str(self.timestamp) + self.data + self.previous_hash
    # encoding block data to utf8 - needed before hashing
    block_data = block_data.encode('utf8')
    # create a sha256 hash object
    hash = hashlib.sha256()
    # feeding blockdata to the hash object
    hash.update(block_data)
    # return the output as the digest of the hash
    return(hash.hexdigest())
    


def test_blockchain(index, time, data, previous_hash):
  new_block = Block(index, data, previous_hash, time)
  #check_string = "2def27922fc1c67254a9cdb0c660b91abf9b135ad38fc13c7c77007448b824a0"
  check_string = "687102d55b610a7a7271fdf2ac15c746009fa4909a6d8a375cc1bca72a83dc47"
  # print(str(new_block.hash))
  if(str(new_block.hash) == check_string):
    print_statement = "Pass"
  else:
    print_statement = "Fail"
  print(print_statement)

time = '2021-05-22 04:20:20'
data = "Jason's blockchain on Python"
previous_hash = '323fdb2771a9e5cc34ebc3d44c8888fa4bca31ce4a276523ba29cf3da0668b58'
index = 0


# create a new Block
# block1 = Block(index, data, previous_hash, time )
# print(block1.hash)

test_blockchain(index, time, data, previous_hash)

'''
Implementing other functions for the blockchain
1. create_genesis_block() - first block in the chain
3. next_block() - 
'''

def create_genesis_block():
  '''
  This function creates the first block in the chain
  '''
  index = 0 # since it is the first block
  timestamp = datetime.datetime.now() # depends on when it is called
  data = "Jason's Genesis Block"
  previous_hash = "0"
  return(Block(index, data, previous_hash, timestamp))

def next_block(last_block, nonce=0):
  '''
  uses the last block (object)
  returns a new instance of class Block where
  index = index of last_block + 1
  timestamp = Now
  data = "This is block {index}"
  previous_hash = hash of last_block
  '''
  index = last_block.index + 1
  #timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  timestamp = datetime.datetime.now()
  data = f"This is block {index}"
  previous_hash = last_block.hash
  return(Block(index, data, previous_hash, timestamp, nonce))

def test_next_block(genesis_block):
  block_1 = next_block(genesis_block)
  if block_1.index == 1 and block_1.data == "This is block 1" and block_1.previous_hash == genesis_block.hash and str(type(block_1.timestamp)) == "<class 'datetime.datetime'>":
    print("Pass")
  else:
    print("Fail")

# testing genesis block creation
genesis_block = create_genesis_block()
test_next_block(genesis_block)