How to Write Function in Python

In all programming and scripting languages, a function is a block of code that only runs when it is called.

We can pass data known as parameters into a function. A function can return data as a result. It saves the time of a developer.

For example, custom functions run every time it called and allowed to run the code in more than one place.

Let’s Create a Simple ATM Script

The project is broken into three sections:

• Collect customer input
• Calculate the ending balance
• Display the results to the customer

Input rule:

D = Deposit
W = Withdrawal
B = Account Balance
Q = Quit

Final Output:

Beginning Balance: 500.50
Deposit Amount: 200
Withdrawal Amount: 100

ATM Script:

import sys
account_balance = float(500.25)

def printbalance():
  print('Your current balance:\n %.2f' %(account_balance))
def deposit(amount):
  print("Deposit was $%.2f, current balance is $%.2f" %(deposit_amount, account_balance + deposit_amount))

def withdrawal(amount):
  if amount > account_balance:
  print("$%.2f is greater than your account balance of $%.2f" %(amount, account_balance))
  else:
    print("Withdrawal amount was $%.2f, current balance is $%.2f" %(amount, account_balance - amount))

if (userchoice == 'D'):
  deposit_amount = float(input("How much would you like to deposit today?\n"))
  deposit(deposit_amount)
elif (userchoice == 'W'):
  withdrawal_amount = float(input("How much would you like to withdraw today?\n"))
  withdrawal(withdrawal_amount)
elif (userchoice == 'B'):
  printbalance()
else:
  print("Thank you for banking with us.")

Explanation line by line

This is an account balance.

import sys
account_balance = float(500.25)

This is a custom function called “printbalance.” This function returns the account balance.

def printbalance():
  print('Your current balance:\n %.2f' %(account_balance))

This is a custom function called “deposit.” This function calculates a new account balance and return deposit amount that user input and calculated the new account balance.

def deposit(amount):
  print("Deposit was $%.2f, current balance is $%.2f" %(deposit_amount, account_balance + deposit_amount))

This is a custom function called “withdrawal.” This function calculates a new account balance. Returns the withdrawal amount and calculates the new account balance.

def withdrawal(amount):

If the withdrawal amount is greater than the account balance,

if amount > account_balance:

return the following message: “withdrawal amount” is greater than your account balance of “account balance”

 print("$%.2f is greater than your account balance of $%.2f" %(amount, account_balance))

If the withdrawal amount is less than or equal to the account balance then calculate a new account balance,

else:

return withdrawal amount that user input and calculated the new account balance.

 print("Withdrawal amount was $%.2f, current balance is $%.2f" %(amount, account_balance - amount))

Ok, now let’s collect customer input. Prompting the user to input the request.

userchoice = input ("What would you like to do?\n")

If a user wants to deposit,

if (userchoice == 'D'):

prompting the user to input the amount of money.

 deposit_amount = float(input("How much would you like to deposit today?\n"))

And then, call the deposit function and send the input amount of money.

 deposit(deposit_amount)

If a user wants to withdrawal,

elif (userchoice == 'W'):

prompting the user to input the amount of money.

 withdrawal_amount = float(input("How much would you like to withdraw today?\n"))

And then, call the withdrawal function and send the input amount of money.

  withdrawal(withdrawal_amount)

If a user wants to know account balance,

elif (userchoice == 'B'):

call the print balance function, no parameters/no input need to send.

  printbalance()

If a user wants to quit,

else:

output the message.

  print("Thank you for banking with us.")

Wrapping Up

I hope you enjoy creating a simple ATM script as much as I enjoyed doing this! You can include these types of projects into your portfolio by creating your own site or simply uploaded to Dropbox and share it with your clients.