Python blockchain

import hashlib as hasher
import datetime as date

# Define what a Snakecoin block is
class Block:
def __init__(self, index, timestamp, data, previous_hash):
self.index = index
self.timestamp = timestamp
self.data = data
self.previous_hash = previous_hash
self.hash = self.hash_block()

def hash_block(self):
sha = hasher.sha256()
sha.update(str(self.index) + str(self.timestamp) + str(self.data) + str(self.previous_hash))
return sha.hexdigest()

# Generate genesis block
def create_genesis_block():
# Manually construct a block with
# index zero and arbitrary previous hash
return Block(0, date.datetime.now(), "Genesis Block", "0")

# Generate all later blocks in the blockchain
def next_block(last_block):
this_index = last_block.index + 1
this_timestamp = date.datetime.now()
this_data = "Hey! I'm block " + str(this_index)
this_hash = last_block.hash
return Block(this_index, this_timestamp, this_data, this_hash)

# Create the blockchain and add the genesis block
blockchain = [create_genesis_block()]
previous_block = blockchain[0]

# How many blocks should we add to the chain
# after the genesis block
num_of_blocks_to_add = 20

# Add blocks to the chain
for i in range(0, num_of_blocks_to_add):
block_to_add = next_block(previous_block)
blockchain.append(block_to_add)
previous_block = block_to_add
# Tell everyone about it!
print "Block #{} has been added to the blockchain!".format(block_to_add.index)
print "Hash: {} ".format(block_to_add.hash)