bank.py

class Account():
	
	def __init__(self):
	    self.balance = 0
	    self.currency = 'CAD'
	    self.overdraft = False
	    
	def make_deposit(self, amount):
	    """imagine a boat load of business logic that would make this useful"""
        pass

	def make_withdrawl(self, amount):
	    """imagine a boat load of business logic that would make this useful"""
        pass
        
	def calculate_bank_fees(self):
	    """imagine a boat load of business logic that would make this useful"""
        pass

class Banker():
    
    def transfer(self, from_account, to_account, amount):
        return False #again this would be actual business logic

builders.py

from bank import Account

def anAccount():
    return AccountBuilder(100, 'CAD', False)

class AccountBuilder:
    
    def __init__(self, balance, currency, overdraft):
        self.balance = balance
        self.currency = currency
        self.overdraft = overdraft
        
    def with_overdraft(self):
        return AccountBuilder(self.balance, self.currency, True)
    
    def with_no_overdraft(self):
        return AccountBuilder(self.balance, self.currency, False)
        
    def with_balance(self, balance):
        return AccountBuilder(balance, self.currency, self.overdraft)
        
    def with_currency(self, currency):
        return AccountBuilder(self.balance, currency, self.overdraft)
        
    def build(self):
        account = Account()
        account.balance = self.balance
        account.currency = self.currency
        account.overdraft = self.overdraft
        return account

test-banker.py

from bank import Banker
import builders

class TestBanker():
    
    def test_cannot_transfer_between_account_with_different_currency(self):
        account1 = builders.anAccount().with_currency('USD').with_balance(200).build()
        account2 = builders.anAccount().with_currency('CAD').build()
        
        sut = Banker()
        assert sut.transfer(account1, account2, 100) == False
        
    def test_cannot_transfer_more_money_than_you_have_if_no_overdraft(self):
        account1 = builders.anAccount().with_balance(100).build()
        account2 = builders.anAccount().build()
        
        sut = Banker()
        assert sut.transfer(account1, account2, 200) == False