Think in Python / CORE PATH

Functions & explicit contracts

A function turns named inputs into a useful result with a clear contract.

3h planning budget · Sohila Khaled Abbas · Reviewed 2026-11-01

Think of a reusable Power Query function, but make data dependencies explicit.

Understand the idea

A function turns named inputs into a useful result with a clear contract. Think of a reusable Power Query function, but make data dependencies explicit.

Work through the practice with a tiny fixture, predict the result before running it, and record one boundary case. Use the exit check as evidence before moving on.

What to learn

Separate input/output from transformation logic.
Use return values, parameters and small docstrings.
Specify valid inputs and the result unit.

A small original example

def net_revenue(gross, discount):
    if not 0 <= discount <= gross:
        raise ValueError("discount outside gross amount")
    return gross - discount

Practice on your own data

Task 03: calculate net revenue from gross and discount piastres.

Practice this lesson in the Python lab →

EXIT CHECK

net_revenue(10000, 1500) is 8500; a discount larger than gross is rejected.

Common pitfalls

A missing return produces None.
A function that mutates input may surprise its callers.

Prerequisites

Variables, types & missing values

Boot.dev chapter 3 · 43:06 ↗

Video and playlist companions

Choose one explanation for the concept, then attempt the practice. These are topic mappings, not a claim that every linked lesson covers every outcome.