blockchain.py

from collections import defaultdict
from block import Block 

NUM_ACCOUNTS = 2 

# the Blockchain class holds the chain itself and a ledger of address balances. 
class Blockchain():
    
    #constructor
    def __init__(self):
        self.chain = [] 
        self.state = defaultdict(int)
        self.current_block = 0
    
    # gets the balance of an address
    def get_balance(self, addr): 
        return self.state[addr]
    
    # adds a new block to the Blockchain
    def add_block(self, b):
        if not self.validate_block(b):
            return False 
        self.chain.append(b)
        self.update_state(b)
        self.current_block = b.num
        return True
        
    # gets a Block from the Blockchain given the block number 'num'  
    def get_block(self, num):
        for b in self.chain: 
            if b.num == num:
                return b
    
    # validates that a Block meets the criteria to be part of the chain
    def validate_block(self, b):
        if not b: 
            return False 
        
        if not isinstance(b, Block): 
            return False 
        
        if b.prev != self.current_block: 
            return False 

        if b.num <= self.current_block: 
            return False 
        return True

    # updates the Blockchain's ledger of address balances given a list of transactions from an incoming Block
    def update_state(self, b):
        for tx in b.transactions: 
            if self.state[tx.sender] == 0: 
                print("[transaction #%s]: failed due to low balance"%(tx.id))
                continue
            self.state[tx.sender] -= 1    
            self.state[tx.receiver] += 1 
            print("[transaction #%s]: success [%s -> %s]"%(tx.id, tx.sender, tx.receiver))

    # (helper function) gives an address some balance 
    def faucet(self, addr): 
        self.state[addr] += 100