Modules and the standard library

So far every program has lived in one file. That is fine while you are learning, but real programs do not fit in one file — at a few thousand lines, even the person who wrote it cannot find the bit they want.

Modules are how you split code across files. In Python a .py file simply *is* a module. Put some functions in calc.py and another file can use them by writing import calc. No declaration, no config file. Put the file there and it is already a module.

There are three ways to bring one in. import calc takes the whole thing and you call calc.add(). from calc import add takes just what you need and you call add() directly. And as gives it a short name, like import numpy as np. All three turn up mixed together in other people's code, so we will look at each.

The second half is the treat. Python ships with a standard library — a pile of tools already there, no installing, no setting up, just import and go. People say Python comes with batteries included, and it is one of the reasons it is used so widely.

We cover four. math gives you square roots, pi and the rest. random covers anything whose answer changes each run, like a dice roll or a draw. datetime does dates and times, so you never have to work out what date it is in thirty days. And json is the shared way applications pass data to each other — the common tongue of the web.

The point is not to memorise those four. It is to know that whatever you want to do, somebody has probably already made it. Look before you write. Once that habit sticks, the range of what you can build widens fast.

One aside: Python also has pip, which fetches tools people around the world have made. That is for your own computer, and it cannot run inside this course's browser sandbox. The slides show you the steps, so come back to them when you are working on a PC.