Bank ATM program in Python For homework, I have to code a program in Python that effectively simulates a bank ATM.

print('Welcome to Northen Frock Bank ATM')
restart=('Y')
chances = 3
balance = 67.14
while chances &gt;= 0:
    pin = int(input('Please Enter You 4 Digit Pin: '))
    if pin == (1234):
        print('You entered you pin Correctly\n')
        while restart not in ('n','NO','no','N'):
            print('Please Press 1 For Your Balance\n')
            print('Please Press 2 To Make a Withdrawl\n')
            print('Please Press 3 To Pay in\n')
            print('Please Press 4 To Return Card\n')
            option = int(input('What Would you like to choose?'))
            if option == 1:
                print('Your Balance is Â£',balance,'\n')
                restart = input('Would You you like to go back? ')
                if restart in ('n','NO','no','N'):
                    print('Thank You')
                    break
            elif option == 2:
                option2 = ('y')
                withdrawl = float(input('How Much Would you like to      
withdraw? \nÂ£10/Â£20/Â£40/Â£60/Â£80/Â£100 for other enter 1: '))
                if withdrawl in [10, 20, 40, 60, 80, 100]:
                    balance = balance - withdrawl
                    print ('\nYour Balance is now Â£',balance)
                    restart = input('Would You you like to go back? ')
                    if restart in ('n','NO','no','N'):
                        print('Thank You')
                        break
                elif withdrawl != [10, 20, 40, 60, 80, 100]:
                    print('Invalid Amount, Please Re-try\n')
                    restart = ('y')
                elif withdrawl == 1:
                    withdrawl = float(input('Please Enter Desired amount:'))    

            elif option == 3:
                Pay_in = float(input('How Much Would You Like To Pay In? '))
                balance = balance + Pay_in
                print ('\nYour Balance is now Â£',balance)
                restart = input('Would You you like to go back? ')
                if restart in ('n','NO','no','N'):
                    print('Thank You')
                    break
            elif option == 4:
                print('Please wait whilst your card is Returned...\n')
                print('Thank you for you service')
                break
            else:
                print('Please Enter a correct number. \n')
                restart = ('y')
    elif pin != ('1234'):
        print('Incorrect Password')
        chances = chances - 1
        if chances == 0:
            print('\nNo more tries')
            break


This program basically does what it is designed to do, but I want to learn how 
to use the def() and do it this way.
The best way to do this would be to take each of your sections, and give them each a function of their own. Also, when it's possible, separate user input functions from purely logic to improve readability!

For example, it could start looking like this:

def verify_pin(pin):
    if pin == '1234':
        return True
    else:
        return False

def log_in():
    tries = 0
    while tries &lt; 4:
        pin = input('Please Enter Your 4 Digit Pin: ')
        if verify_pin(pin):
            print("Pin accepted!")
            return True
        else:
            print("Invalid pin")
            tries += 1
    print("To many incorrect tries. Could not log in")
    return False

def start_menu():
    print("Welcome to the atm!")
    if log_in():
        # you will need to make this one yourself!
        main_menu()
    print("Exiting Program")

start_menu()


Whenever you can avoid nesting, you should. This makes it easier to read.

In this case you can clearly see logging in will open the main_menu (which you will have to write), and entering a correct pin is the only way to log in.