ATM machine (deposits/withdrawals) using OOP

[Drone4four]

I’m trying to write a rudimentary banking ATM machine using Python OOP. The exercise I am working on calls for all kinds of advanced features but I am breaking down the task into smaller pieces so it is easier to test and ask questions and build from there.

Right now I am just trying to initialize a starting balance of $0.00 and make basic deposits and withdrawals. If the user tries to withdraw more than their available balance, Python needs to throw a ValueError indicating “Transaction declined.”

Here is my script and test case so far:

class BankAccount:
   starting_balance = 0.00 # USD
 
   def __init__(self, first_name, last_name):
       self.first_name = first_name
       self.last_name = last_name
 
   def deposit(self, starting_balance, amount):
       balance = starting_balance + amount
       return balance
  
   def withdraw(self, balance, amount):
       try:
           balance = balance - amount
       except balance <= 0.00:
           raise ValueError('Transaction declined. Insufficient funds. Deposit some money first.')
           withdraw(self, balance, amount)
       else:
           return balance

Here is me testing the script in my Python REPL:

$ bpython
bpython version 0.22.1 on top of Python 3.10.2 /usr/bin/python
>>> import script
>>> BA = script.BankAccount('Winston', 'Smith')
>>> BA.first_name
'Winston'
>>> BA.last_name
'Smith'
>>> BA.deposit(starting_balance, 100)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    BA.deposit(starting_balance, 100)
NameError: name 'starting_balance' is not defined
>>> BA.deposit(0.00, 100.0)
100.0
>>> BA.withdraw(balance, 25.00)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    BA.withdraw(balance, 25.00)
NameError: name 'balance' is not defined
>>> BA.withdraw(100.00, 25.00)
75.0
>>> BA.withdraw(100.00, 125.00)
-25.0
>>> 

There are more things that don’t work than what does work.

Here is what works:

• I’m able to import the script and instantiate the class by passing in a first and last name.
  
• When I invoke the deposit() method including a $0.00 balance and a $100.00 bill, the method returns the new $100.0 balance.
  
• When I invoke the withdraw() method and pass in the $25.00 amount, the method returns $75.00.
  

Here is what doesn’t work:

• For starters the global constant class attribute variable starting_balance is defined as $0.00 so I would think that the default value of $0.00 can be passed into the deposit() method as the first argument. When I try doing this, Python says it’s not defined (when it clearly is). So anyways, to proceed I pass in $0.00.
  
• When I attempt to withdraw $25.00 from the new balance, Python says “balance is not defined” even though the balance was returned in the previous deposit() transaction. It’s as if the balance value is not retained. In order to proceed, I pass in the balance of $100.00 and withdraw $25.00 which successfully returns $75.00
  
• But when I proceed to withdraw $125.00 (amount greater than available balance), Python returns $-25.00 which I tried to prevent and catch with the try/exception mechanism. So this doesn’t work as intended.
  

My questions are:

1. How do I declare the global constant class attribute variable properly so that it is 0.00 and don’t have to redundantly enter 0.00 when invoking deposit()?
   
2. How do I retain the balance attribute after the deposit() method is called so that I can pass it along when I later attempt to withdraw from that same balance?
   
3. How would you better formulate the try/except mechanism to catch transactions which don’t allow the user to go below $0.00?
   

This script is part of a non-credit exercise for Fred Baptiste’s Udemy course, Python Object Oriented Programming Deep Dive. I’ve also leveraged resources online such as: "What do __init__ and self do in Python?" and "Python Class Attributes: An Overly Thorough Guide".