Functions — reusable recipe cards

You will be able to

• ✓ Define a function with def and a colon
• ✓ Pass arguments and use return
• ✓ Call the same function with different inputs

def names a procedure

def greet(name): starts a function. The indented block is the recipe. greet is the name you will call later. name is a parameter — a labelled bag that exists only while the function runs.

Calling greet("Diya") puts "Diya" into name and runs the body. Calling greet("Kabir") reuses the same body. That is the entire point: one definition, many uses.

return vs print

print shows a human. return hands a value back to the caller so the rest of the program can store it, print it, or send it into another function. A GST helper should return the amount; the cashier program decides whether to print a receipt.

If you only print inside a function, you cannot add two results together. return 118 lets you write bill = add_gst(100) + delivery.

def square(n):
    return n * n

print("Area of 5x5 tile", square(5))

Parameters are local

A parameter does not overwrite a variable of the same name outside the function in the way beginners fear — think of it as a new bag on a new desk. When the function ends, the desk is cleared.

Name parameters after meaning: amount, days, width. Avoid f(x, y, z) unless you are writing pure maths. Future you will thank present you.

Words that matter

Parameter

A named input listed in the def line.

Argument

The actual value you pass when calling.

return

Sends a value back to the caller.

Common mistakes

Example program — Rectangle area for art class

Call area twice. One definition, two posters.

Your turn — GST helper

Write add_gst(amount) that returns amount * 118 / 100. Print add_gst(100) so 118 appears.

Self-assessment

Check your understanding before you mark the lesson complete.