Functions

🧱 Why functions?

A function is a named block of code that does one job. Modular programming breaks a big program into functions, so each part is easier to write, test, fix and reuse. It is like a school: each department does its own job, and together they run everything.

🧰 Built-in and user-defined

Built-in functions come with Python: print(), input(), len(), int(), max(), round(). User-defined functions are the ones you write yourself with def. Once defined, you can call your function as many times as you like.

✍️ Defining a function

def area(length, width): return length * width. This defines a function named area with two parameters. Calling area(3, 4) passes 3 and 4 in as arguments, and the function returns 12. Inside a real program, the body goes on its own indented lines under the def line.

↩️ Parameters and return values

Parameters are the names in the definition (length, width); arguments are the actual values you pass in (3, 4). return sends a result back to the caller and ends the function. A function without return gives back the special value None.

🔭 Scope: local and global

A variable created inside a function is local: it exists only while the function runs and can't be used outside it. A variable created outside all functions is global. Prefer passing values in as parameters and returning results; it keeps functions independent and easy to test.

Key points

• A function is a named block of code that does one job.
• Modular programs are easier to write, test, fix and reuse.
• Python has built-in functions like print and len, and you can write your own with def.
• Parameters are names in the definition; arguments are the values you pass in.
• return sends a result back and ends the function.
• Variables made inside a function are local to it.
• Pass values in and return results, so functions stay independent.

Try it for real: Write a fees function

• If you can, open Python and write a function fees_due(paid, total) that returns the balance. Test it with fees_due(15000, 40000).
• Write a function is_even(n) that returns True if n is even. Hint: use %.