What a Program Really Is

You have used programs every day for years: a browser, a music player, the app that tells you when the bus arrives. Each of them began as a person writing instructions. A program is exactly that: a list of instructions that a computer follows, one after another, exactly as written.

The key word is exactly. A computer has no common sense and never guesses what you meant. Picture a recipe handed to someone who has never cooked and takes every word literally. "Add salt" with no amount leaves them stuck. A computer in that position does not ask. It stops, and prints an error saying what it could not do. That sounds harsh, but it is the reason programming can be learned at all: the rules are fixed, and the machine applies them the same way every time.

Instructions, Followed Literally

A computer does what the text says, not what you intended. When the two differ, the result is called a bug: the program runs, but it does the wrong thing, or it stops with an error. Every programmer, at every level, spends a large part of the day in that gap between intention and instruction. The skill you are about to learn is not avoiding the gap. It is closing it quickly.

This is also why a single wrong character can stop a program. A missing bracket is not a small typo to the computer. It is an instruction that no longer makes sense, and the computer has no way to decide what you probably meant.

Why We Need a Language

Deep down, the processor inside your computer understands only numbers. Nobody writes programs in raw numbers any more. Instead we write in a programming language: a precise written form that a person can read, with strict rules about spelling and punctuation. A separate program then turns that text into action. For Python, that program is called the interpreter. It reads your instructions and carries them out, one by one.

How text becomes a running program, and what happens inside the machine while it runs, is a story of its own. Computing Foundations from Zero tells it in its fourth chapter. This book takes the next step and has you write the text.

What Python Looks Like

Here are three lines of Python. Do not type them yet; just read them. The first line gives the name price to the number 12. The second asks whether the price is less than 15. The third, which only runs if the answer is yes, prints a short message. Read aloud, it is almost an English sentence: if the price is under fifteen, say it is a bargain.

price = 12
if price < 15:
    print("That is a bargain.")

Every line in that example is something this book will teach properly: giving a value a name in Chapter 2, making decisions in Chapter 3, and printing text as early as this chapter's last topic.

What This Book Will Do

By the last chapter you will write and maintain a script of a couple of hundred lines. It will read a file, keep its data between runs, fetch information from the internet, cope with bad input without crashing, and come with tests that prove it works. Along the way you will learn to read other people's Python, which is at least as important as writing your own.

The method is the loop in this chapter's figure: write some instructions, run them, read what happens, and change them. Nothing more mysterious than that. The rest is practice, and learning to read the messages the computer sends back.