Lesson 14 of 16 · 52 min · UG / professional

Functions: return, None and a one-line spec

def names a contract. return hands a value back. Forget return and callers get nothing useful (None on a laptop). A first-line string is a docstring — the spec the next intern reads.

Lab recipe cards

Functions are reusable procedures.

You will be able to

✓ Define def, pass arguments, return values
✓ Know a missing return is None on a laptop
✓ Put a one-line docstring as the spec

A contract with a name

def late_fee(days): names a contract. Parameters are the names in the def line; arguments are what you pass. return hands a value back and ends the call. Forget return and the laptop gives None — callers then print inside the function to ‘see’ the answer, and tests die.

The first string in the body is a docstring. Tools and help() read it. Write the wall-notice example there: late_fee(0) is 0. Polymorphic idea: the same function can take ints or (later) decimals if the operations exist — do not type-declare everything; do document what you meant.

Call by object reference

Passing a list does not copy it. If the function appends, the caller sees it. Pass a snapshot if the function should not edit the source. Functions are objects — you can assign f = late_fee and call f(4).

Words that matter

def: Define a named function.
return: Send a value back and end this call.
docstring: First string in the body — the spec.
None: The laptop value when you did not return.

Common mistakes

Avoid: Printing the fee inside the function and returning nothing.

Do this: return the fee; print at the edge.

Run it step by step

Each box is a real program. Press Run, change a number, Run again — the output must follow your code.

1. Step 1 — one input, one output

net_pay(100) is 88 after PF 12.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

2. Step 2 — compose two contracts

GST after PF.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

Example program — A rule two callers share

Change 12 in one place.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

Your turn — Hours to pay

pay(hours) returns hours * 120. Print pay(3) so 360 appears.

Python sandboxlesson://workspace

console

Edit the example, press Run, then Build if you want a compile check.

build

Press Build to compile.

Self-assessment

Check your understanding before you mark the lesson complete.

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