transactions.py

import random
random.seed(0)

def randomTransaction(maxValue=3):
    # This will create valid transactions in the range of (1,maxValue)
    sign      = int(random.getrandbits(1))*2 - 1   # This will randomly choose -1 or 1
    amount    = random.randint(1,maxValue)
    alicePays = sign * amount
    bobPays   = -1 * alicePays
    # By construction, this will always return transactions that respect the conservation of tokens.
    # However, note that we have not done anything to check whether these overdraft an account
    return {u'Ada':alicePays,u'Charlie':bobPays}

def updateState(txn, state):
    # Inputs: txn, state: dictionaries keyed with account names, holding numeric values for transfer amount (txn) or account balance (state)
    # Returns: Updated state, with additional users added to state if necessary
    # NOTE: This does not not validate the transaction- just updates the state!

    # If the transaction is valid, then update the state
    state = state.copy() # As dictionaries are mutable, let's avoid any confusion by creating a working copy of the data.
    for key in txn:
        if key in state.keys():
            state[key] += txn[key]
        else:
            state[key] = txn[key]
    return state

# This function validates each piece of data based on the business logic of the
#   application. 
def isValidData(txn,state):
    # Assume that the transaction is a dictionary keyed by account names

    # Check that the sum of the deposits and withdrawals is 0
    if sum(txn.values()) is not 0:
        return False

    # Check that the transaction does not cause an overdraft
    for key in txn.keys():
        if key in state.keys():
            acctBalance = state[key]
        else:
            acctBalance = 0
        if (acctBalance + txn[key]) < 0:
            return False

    return True