🧱 What You'll Build
A basic blockchain in Python that:
Stores a chain of blocks
Each block contains data, a timestamp, and a hash
Uses SHA-256 for cryptographic hashing
Validates the chain to ensure integrity
✅ Prerequisites
Python 3.x installed
Basic understanding of Python syntax
Familiarity with concepts like lists and dictionaries
🧰 Step 1: Import Required Libraries
import hashlib
import time
📦 Step 2: Create the Block Class
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.calculate_hash()
def calculate_hash(self):
block_string = f"{self.index}{self.timestamp}{self.data}{self.previous_hash}"
return hashlib.sha256(block_string.encode()).hexdigest()
⛓️ Step 3: Create the Blockchain Class
class Blockchain:
def __init__(self):
self.chain = [self.create_genesis_block()]
def create_genesis_block(self):
return Block(0, time.time(), "Genesis Block", "0")
def get_latest_block(self):
return self.chain[-1]
def add_block(self, new_data):
latest_block = self.get_latest_block()
new_block = Block(
index=latest_block.index + 1,
timestamp=time.time(),
data=new_data,
previous_hash=latest_block.hash
)
self.chain.append(new_block)
def is_chain_valid(self):
for i in range(1, len(self.chain)):
curr = self.chain[i]
prev = self.chain[i - 1]
if curr.hash != curr.calculate_hash():
print(f"Block {i} has been tampered with.")
return False
if curr.previous_hash != prev.hash:
print(f"Block {i}'s previous hash doesn't match Block {i-1}'s hash.")
return False
return True
🧪 Step 4: Test Your Blockchain
# Create the blockchain
my_blockchain = Blockchain()
# Add some blocks
my_blockchain.add_block("First block after Genesis")
my_blockchain.add_block("Second block")
my_blockchain.add_block("Third block")
# Display the blockchain
for block in my_blockchain.chain:
print(f"Index: {block.index}")
print(f"Timestamp: {block.timestamp}")
print(f"Data: {block.data}")
print(f"Hash: {block.hash}")
print(f"Previous Hash: {block.previous_hash}\n")
# Validate the blockchain
print("Blockchain valid?", my_blockchain.is_chain_valid())
🧠 What’s Happening Behind the Scenes?
Genesis Block: The first block, manually created with no predecessor.
Hashing: SHA-256 ensures that any change in data invalidates the hash.
Chain Integrity: Each block refers to the previous one using previous_hash.
Validation: Checks that no blocks have been tampered with.
🛠️ Optional Add-Ons
You can extend this basic blockchain by adding:
Proof-of-work (mining)
A simple P2P network
Block timestamps in readable format
JSON output for blocks
✅ Summary
You've just built a basic blockchain in Python—a great starting point to understand how technologies like Bitcoin and Ethereum work at a fundamental level.
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