Welcome to the Bank of JMU!

Where we work hard not to lose any of your money!

(At least...not on purpose.)

• All money stored as integers (Ex: 123 cents)
• All input/output shown as floats (Ex: "$1.23")

Starter Code

Download: jmu_bank.py

Functions defined so far:

• money()
• open_account()
• deposit()
• withdraw()

Exercise 1

Fix the bug in money()

• This function is used to format money for output
• Ex: money(1234500) should return "$12,345.00"
• Hint #1: Assign the variables dollars and cents
• Hint #2: Format with :,d and :02d (in an f-string)

Global Variables

import random

accounts = []  # account numbers
balances = []  # money, in cents

numbers = list(range(10000, 100000))
random.shuffle(numbers)

• range is a built-in sequence type
• numbers is a list of all 5-digit positive integers
• Can you guess what random.shuffle() does?

Using the Bank

>>> open_account()
  Opening new account
  Opened account #81017
81017

>>> deposit(81017, 123.4)
  Depositing $123.40 into account #81017
  New balance is $123.40
123.4

>>> withdraw(81017, 23.4)
  Withdrawing $23.40 from account #81017
  New balance is $100.00
100.0

Exercise 2

Define a test_bank() function:

• Open an account
• Deposit into account
• Withdraw from account
  
  

Add these lines at the very end:

if __name__ == "__main__":
    test_bank()

Exercise 3

Run with Thonny's debugger:

• Set a breakpoint on the line that calls test_bank()
• Step through the entire program, one line at a time
• Pay attention to variables when functions are called
  
  

If you haven't already, enable View > Variables (for globals)

Local Variables

• When a function is called, a new "frame" is created
• New variables are created/visible within that frame
• When a function returns, the variables are deleted
  
  

def deposit(acct, amount):
    amount = int(round(amount * 100))
    index = accounts.index(acct)
    balances[index] += amount
    return balances[index] / 100

• Parameters: acct and amount
• Local variable: index

Assigning Parameters

What does this code print?

def change(x):
    x = 456

x = 123
change(x)
print(x)

View in Python Tutor

• Assignment creates/updates a local variable!

Modifying Parameters

def change(scores):
    lowest = min(scores)
    scores.remove(lowest)

scores = [95, 53, 88]
change(scores)
print(scores)

View in Python Tutor

• This example changes an object
• Previous slide changed a variable

Modifying Globals

Global variables in jmu_bank.py

• numbers (list): available account numbers
• accounts (list): numbers of open accounts
• balances (list): balances of open accounts
  
  

def open_account():
    acct = numbers.pop(0)
    accounts.append(acct)
    balances.append(0)
    return acct

Exercise 4

Write a close_account(acct) function

Ex: Account 12345 has a balance of $6.78

• At the start, print "Closing account #12345"
• Get the index for the account (and balance)
• Pop the index from accounts and balances
• Print "Closed account #12345 with $6.78"
• Return the account's balance (as a float)

Docstrings

def deposit(acct, amount):
    """Deposit money into an account.

    Args:
        acct (int): account number
        amount (float): money to deposit

    Returns:
        float: updated account balance
    """

• One-line summary after """
• Args: section (if applicable)
• Returns: section (if applicable)

help() Function

When in the shell, help() is always there!

The help() function shows the docstring

Works on modules, functions, keywords, etc.

Can also run help() without an argument!

Exercise 5

Write a docstring for your close_account()

Run !flake8 jmu_bank.py and fix any errors

Try calling help(close_account) in the shell

Exercise 6 (optional)

If time permits, write this function:

def transfer(acct_from, acct_to, amount):
    """Transfer money from one account to another."""

• At the start, print "Starting transfer"
• At the end, print "Transfer complete"
• Withdraw acct_from, deposit acct_to
• Return True, indicating success
• Finish the docstring, run flake8