Python TDD assert statements not working like i thought

Question

I am just confused about why this is happening. When I run my assert it seems that each test is not creating its own object. so when i get to the last assert statement the test fails because the other assert are still in the list. Any help would be great thank you My Class code:

from datetime import date

class Transaction():
    """
    Transaction
    """
    balance = 0.0
    timestamp = date.today()
    def __init__(self, amount, dt=None):
        self.balance = amount
        self.transactions = []
        if dt is None:
            self.timestamp = date.today()
        else:
            self.timestamp = dt
    def __repr__(self):
        return '{self.__class__.__name__}({self.balance:,.2f}, {self.timestamp})'.format(self=self)
    def __str__(self):
        return f'{self.timestamp}: ${self.balance:,.2f}'

class Account():
    """
    Account Class
    """
    balance = 0.0
    transaction = Transaction(balance)
    def __init__(self):
        self.balance = 0.0
    def deposit(self, amount):
        self.balance += +amount
        self.transaction.transactions.append(+amount)
    def withdraw(self, amount):
        self.balance -= amount
        self.transaction.transactions.append(-amount)
    def get_balance(self):
        if len(self.transaction.transactions) < 1:
            return 0
        return sum(self.transaction.transactions)

My Code for pytest:

def test_append_transaction():
    account = Account()
    account.deposit(200)
    assert account.transaction.transactions == [200]

def test_deposit():
    user = Account()
    user.deposit(300)
    assert user.balance == +300.00

def test_append_withdraw():
    account = Account()
    account.withdraw(50)
    assert account.transaction.transactions == [-50]

def test_withdraw():
    account = Account()
    account.withdraw(50)
    assert account.balance == -50.0

Answer 1

Your tests are failing because your code is wrong - i.e. they are failing because you've written your tests correctly, in a way which is able to detect bugs, and they detected a bug in your code.

Yes, each test function does create a new Account instance, as you can clearly see in the test functions themselves. However, each Account instance does not have its own distinct Transaction instance, because you made this a class attribute instead of an instance attribute.

To fix the bug in your Account class, you should initialise

        self.transaction = Transaction(self.balance)

in the __init__ method, so that each Account instance has a reference to a distinct Transaction instance.