snakecoin.py

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: {}\n".format(block_to_add.hash) 

Thanks @JohnStuartRutledge ! Works great!

Thanks to @aunyks @JohnStuartRutledge
I'm using Python 3.6, however something was not working out and I was getting:
...
Block #5 added to blockchain
Hash: <bound method Block.hash_block of <main.Block object at 0x7efe6c902588>>

It appears that the line:
print('Hash: {}\n'.format(block.hash))

needs to be
print('Hash: {}\n'.format(block.hash()))

in order to obtain:
Block #5 added to blockchain
Hash: 80963e3493ffed59bb9ec6653621328d53dfc81994ae15f2c29b29eb7d0bb1c1

block.hash gives us the method object itself,
whereas, we want to actually call block.hash() instead

great

Similar implementation and packaged with tests.
https://github.com/koshikraj/justblockchain

Also, check out the extended implementation with block validation with server interface.
https://github.com/koshikraj/pynaivechain

Thx to all very good stuff:

Block #20 added to blockchain
Hash: 2c4fd089fd70c17ec4eb73b2555cada39f2d8d98b45e74bc3dea99b550a9c6a4 :-)

@rogel-appdev were you able to resolve the issue? I am facing the same one.

I found the solution.
Add import statements to your files.

Python 3

import hashlib as hasher
import datetime as date

class Block: # Defining what a GitCoin consists of
    def __init__(self,index,timeStamp,data,previousHash):
        self.index = index
        self.timeStamp = timeStamp
        self.data = data
        self.previousHash = previousHash
        self.hash = self.hashBlock()
    
    def hashBlock(self):
        sha = hasher.sha256() # sha256 is a hashing algorithm
        sha.update(str(self.index).encode('utf-8') + str(self.timeStamp).encode('utf-8') + str(self.data).encode('utf-8') + str(self.previousHash).encode('utf-8'))
        return sha.hexdigest() # returns the combined hash value in Hexadecimals

def createGenesisBlock(): # Pre-Defining a block with index 0 and an arbitrary previous hash 
    return Block(0, date.datetime.now, "Genesis Block", 0)

def createNextBlock(prevBlock): # Creating all the later blocks in the blockchain
    currIndex = prevBlock.index + 1
    currTimeStamp = date.datetime.now()
    currData = "Block" + str(currIndex)
    currHash = prevBlock.hash
    return Block(currIndex, currTimeStamp, currData, currHash)

blockChain = [createGenesisBlock()] # Creating the BlockChain
prevBlock = blockChain[0] # Adding the Genesis block

blocksToCreate = 20 # No. of blocks to be added after the Genesis Block

for i in range(0,blocksToCreate):
    blockToAdd = createNextBlock(prevBlock)
    blockChain.append(blockToAdd)
    prevBlock = blockToAdd
    print("Block %s has been added to the blockchain!" % (blockToAdd.index))
    print("Hash: %s\n" % (blockToAdd.hash))

Can we do better than this? Repeated code is never a good sign.

str(self.index).encode('utf-8') + str(self.timeStamp).encode('utf-8') + str(self.data).encode('utf-8') + str(self.previousHash).encode('utf-8')

For example:

def encode(x):
     return str(x).encode('utf-8')

def concat(elements: list):
     return ''.join([encode(x) for x in elements])

# the expression above becomes
concat([self.index, self.timeStamp, self.data, self.previousHash])    

Or is this loosing some readbility?

In case it helps anyone, here is the working Python 3 version I got working that includes a few tweaks like encoding the class attributes to utf-8.

Thanks to OP for the tutorial!

from datetime import datetime
import hashlib as hasher


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 __str__(self):
        return 'Block #{}'.format(self.index)

    def hash_block(self):
        sha = hasher.sha256()
        seq = (str(x) for x in (
               self.index, self.timestamp, self.data, self.previous_hash))
        sha.update(''.join(seq).encode('utf-8'))
        return sha.hexdigest()


def make_genesis_block():
    """Make the first block in a block-chain."""
    block = Block(index=0,
                  timestamp=datetime.now(),
                  data="Genesis Block",
                  previous_hash="0")
    return block


def next_block(last_block, data=''):
    """Return next block in a block chain."""
    idx = last_block.index + 1
    block = Block(index=idx,
                  timestamp=datetime.now(),
                  data='{}{}'.format(data, idx),
                  previous_hash=last_block.hash)
    return block


def test_code():
    """Test creating chain of 20 blocks."""
    blockchain = [make_genesis_block()]
    prev_block = blockchain[0]
    for _ in range(0, 20):
        block = next_block(prev_block, data='some data here')
        blockchain.append(block)
        prev_block = block
        print('{} added to blockchain'.format(block))
        print('Hash: {}\n'.format(block.hash))


# run the test code
test_code()

Thanks OP! And to JohnStuartRutledge. This worked for me.

Uhh how would I create a wallet for this file

I modified line 15 as below and it worked just fine:

sha.update(str(self.index).encode('utf-8') + str(self.timestamp).encode('utf-8') + str(self.data).encode('utf-8') + str(self.previous_hash).encode('utf-8'))

:Thank you +1:

sha.update(str(self.index).encode('utf-8') + str(self.timestamp).encode('utf-8') + str(self.data).encode('utf-8') + str(self.previous_hash).encode('utf-8'))

You're right !

worked python3 version:

import hashlib
import time


class Block:
    def __init__(self, index: int, timestamp: int, data: str, previous_hash):
        """
        :param index: block index
        :param timestamp: millisecond
        :param data: block real data
        :param previous_hash: previous block 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 = hashlib.sha256()
        seq = [
            str(x) for x in [self.index, self.timestamp, self.data, self.previous_hash]
        ]
        sha.update("".join(seq).encode("utf8"))
        return sha.hexdigest()

    def __str__(self):
        return f"Block:index={self.index}&timestamp={self.timestamp}&data={self.data}&hash={self.hash}"


def get_cur_millisecond():
    return int(round(time.time() * 1000))


def create_genesis_block():
    return Block(0, get_cur_millisecond(), "snake coin genesis block", "0")


def next_block(previous_block: Block):
    index = previous_block.index + 1
    return Block(
        index=index,
        timestamp=get_cur_millisecond(),
        data=f"this is block #{index}",
        previous_hash=previous_block.hash,
    )


def make_test_block_chain():
    block_chain = [create_genesis_block()]
    previous_block = block_chain[0]
    for _ in range(20):
        block = next_block(previous_block)
        block_chain.append(block)
        previous_block = block
        # broadcast: tell everyone about it!
        print(f"Block #{block.index} has been added to the blockchain! detail: {block}")
        time.sleep(1)


if __name__ == "__main__":
    make_test_block_chain()

worked python3 version:

import hashlib
import time


class Block:
    def __init__(self, index: int, timestamp: int, data: str, previous_hash):
        """
        :param index: block index
        :param timestamp: millisecond
        :param data: block real data
        :param previous_hash: previous block 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 = hashlib.sha256()
        seq = [
            str(x) for x in [self.index, self.timestamp, self.data, self.previous_hash]
        ]
        sha.update("".join(seq).encode("utf8"))
        return sha.hexdigest()

    def __str__(self):
        return f"Block:index={self.index}&timestamp={self.timestamp}&data={self.data}&hash={self.hash}"


def get_cur_millisecond():
    return int(round(time.time() * 1000))


def create_genesis_block():
    return Block(0, get_cur_millisecond(), "snake coin genesis block", "0")


def next_block(previous_block: Block):
    index = previous_block.index + 1
    return Block(
        index=index,
        timestamp=get_cur_millisecond(),
        data=f"this is block #{index}",
        previous_hash=previous_block.hash,
    )


def make_test_block_chain():
    block_chain = [create_genesis_block()]
    previous_block = block_chain[0]
    for _ in range(20):
        block = next_block(previous_block)
        block_chain.append(block)
        previous_block = block
        # broadcast: tell everyone about it!
        print(f"Block #{block.index} has been added to the blockchain! detail: {block}")
        time.sleep(1)


if __name__ == "__main__":
    make_test_block_chain()

Tested this out and it worked great! Thanks!

import hashlib as hasher
import datetime as date

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)).encode())   #changed line
        return sha.hexdigest()

def create_genesis_block():
    return Block(0, date.datetime.now(), "Genesis Block", 0)

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)

blockchain = [create_genesis_block()]
previous_block = blockchain[0]

num_of_blocks_to_add = 20

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
    print ("Block #{} has been added to the blockchain!".format(block_to_add.index))
    print ("Hash: {}\n".format(block_to_add.hash))    

For anyone in Python 3, just changed the line 14. Tested and it worked