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Discussion on: Creating a blockchain in 60 lines of Python

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theflanman profile image
Connor Flanigan •

Interesting article. I wanted to take a stab at cleaning it up a little to utilize more pythonic syntax.

from hashlib import sha256
from time import time
import json


class Block:

    def __init__(self, **kwargs):
        """
        Create a new block for the Blockchain

        :param timestamp: Timestamp of the block, defaults to the time the block object is created
        :param data: Data to store in the block, defaults to an empty list
        :param prevHash: Hash of the previous block, defaults to None.  Should always be specefied except for the genesis block.
        """
        self.timestamp = kwargs.get('timestamp', time())
        self.data = kwargs.get('data', [])
        self.prevHash = kwargs.get('prevHash', None)
        self._hash = None
        self.nonce = 0

    @property
    def hash(self):
        """
        Return the (non-python) hash of the block

        :return: The bytes of the hash of this block
        """
        if self._hash is None:
            hashFun = sha256()
            hashFun.update(self.encode(self.prevHash))
            hashFun.update(self.encode(self.timestamp))
            hashFun.update(self.encode(self.data))
            hashFun.update(self.encode(self.nonce))
            self._hash = hashFun.hexdigest()
        return self._hash

    def rehash(self):
        """
        Mark this block to re-calculate the hash the next time it's grabbed.
        """

        self._hash = None

    @staticmethod
    def encode(val):
        """
        Generate a UTF-8 bytes object to represent any object

        :param val: Value to encode
        :return: UTF-8 encoded byte representation of val
        """
        return str(val).encode('utf-8')

    def mine(self, difficulty):
        """
        Mine this block until a valid hash is found, based on leading zeros

        Basically, it loops until our hash starts with
        the string 0...000 with length of <difficulty>.

        :param difficulty:  length of the leading zeros to mine for
        """

        while self.hash[:difficulty] != '0' * difficulty:
            # We increases our nonce so that we can get a whole different hash.
            self.nonce += 1
            # Update our new hash with the new nonce value.
            self.rehash()


class Blockchain:

    def __init__(self):
        """
        initialize the blockchain with an empty, unmined "genesis" block.
        """
        self.chain = [Block()]
        self.blockTime = 30000
        self.difficulty = 1

    def __iter__(self):
        for c in self.chain:
            yield c

    def __getitem__(self, item):
        return self.chain[item]

    def append(self, data, **kwargs):
        """
        Add a new block to the blockchain from a new piece of data and an optional timestamp.

        :param data: Data to add to the new block.
        :param timestamp: UTC timecode of the new block
        """

        # Since we are adding a new block, prevHash will be the hash of the old latest block
        block = Block(data=data, prevHash=self[-1].hash, timestamp=kwargs.get('timestamp', time()))
        block.mine(self.difficulty)

        # Since now prevHash has a value, we must reset the block's hash
        self.chain.append(block)

        if time() - self[-1].timestamp < self.blockTime:
            self.difficulty += 1
        else:
            self.difficulty -= 1

    def isValid(self):
        """
        Iterates over the pairs of sequential blocks to validate that their previous hashes are set correctly

        :return: `True` if Valid, `False` otherwise
        """

        for prevBlock, currentBlock in zip(self[:-1], self[1:]):

            # Check validation
            if prevBlock.hash != currentBlock.prevHash:
                return False

        return True

    def __repr__(self):
        return json.dumps(
            [{k: getattr(item, k) for k in ['data', 'timestamp', 'nonce', 'hash', 'prevHash']}
             for item in self],
            indent=4
        )


if __name__ == '__main__':
    chain = Blockchain()
    chain.append({"from": "John", "to": "Bob", "amount": 100})
    chain.append({"from": "bob", "to": "john", "amount": 50})

    print(chain)
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imjoseangel profile image
Jose Angel Munoz • • Edited

Thanks @theflanman for your contribution. It is always cool to see code like yours to learn.

Do you think adding typing could be a good addition to your code? like:

    def __init__(self, **kwargs) -> None:
        pass
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Also I ♥ to see how you are using @property decorator in the example. I have prepared a small snippet to see the effect when using it:

class Test():
    def __init__(self, **kwargs) -> None:
        pass

    @property
    def hash(self) -> None:
        return sha256().hexdigest()

    def __repr__(self) -> str:
        return self.hash


def main():
    print(Test())
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I really like how you are using the iterator in the __repr__ and other dunder methods.

Have Fun!

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theflanman profile image
Connor Flanigan •

The consensus we've reached at work has mostly been that typing should really only be bothered to help users and devs where it's not clear what expected input and output is. Things like __init__ methods or other dunder methods are expected to return a certain type of object so they don't need it.

What I might recommend is typing your inputs a little more. As an example, explicitly use ints to represent timestamps, and use the function time.time_ns() for timestamps, then use structs to generate more accurate representations to hash.

class Block:

    def __init__(self, data: Optional[List[Any]] = None, timestamp: Optional[float]=None, prevHash: Optional[bytes] = None):
        """
        Create a new block for the Blockchain

        :param timestamp: Timestamp of the block, defaults to the time the block object is created
        :param data: Data to store in the block, defaults to an empty list
        :param prevHash: Hash of the previous block, defaults to None.  Should always be specefied except for the genesis block.
        """
        if datais None:
            self.data= []
        else:
            self.data= data

        if timestamp is None:
            self.timestamp = time_ns()
        else:
            self.timestamp = timestamp

        if prevHashis None:
            self.prevHash= b''
        else:
            self.prevHash= prevHash

        self._hash = None
        self.nonce = 0

    @property
    def hash(self) -> bytes:
        """
        Return the (non-python) hash of the block

        :return: The bytes of the hash of this block
        """
        if self._hash is None:
            hashFun = sha256()
            hashFun.update(self.prevHash)
            hashFun.update(struct.pack('@l', self.timestamp))
            hashFun.update(self.encode(self.data))
            hashFun.update(struct.pack('@l', self.nonce))
            self._hash = hashFun.hexdigest()
        return self._hash
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Here I've specified that data is a list of anything, timestamp is an int, and prevHash is bytes, and that Block.hash generates bytes. This informs a user/developer that's interfacing with it that this is what the methods are expecting to take or should be expected to return. The only thing I might also add for type hinting in the Block class is specifying the difficulty parameter in Block.mine() is an integer, as well as a check that it's greater than or equal to 1, raising an exception if it isn't.