What is a Python program?

A small puzzle

Let's start with a small puzzle. Here is a program with 3 lines. Line 1 prints Hello, line 2 prints Line two, and line 3 is missing its closing bracket.

Were you surprised? To understand what happened, we need to know what a Python program really is. Let's try to understand.

Python means two things

First, what is Python? When programmers say Python, they mean two things. English is a good way to see both.

English has words, and grammar rules for putting them together. "Get me water" makes sense, but "Water get me" does not. The words are the same; one change in order, and the meaning is gone.

Python, the language, works the same way: a set of words, and the rules for writing them. These grammar rules are what we call syntax.

The second thing is the interpreter. In everyday English, an interpreter is the person who stands between two people who don't share a language.

Python's interpreter does the same job. Like a broker, it stands between you and the computer. It reads what you wrote, and gets the computer to do it.

So when you press Run, you are not talking to the computer directly. You are handing your program to the interpreter.

So, what is a Python program?

A Python program is just text. You type it, and save it in a file whose name ends in .py, like hello.py. That file is your program.

Nothing special happens while you type. A .py file is plain text, the same as a note on your phone. Everything happens when you press Run.

What happens when you press Run

Follow one program from your keyboard to the screen, in three moves.

The interpreter first reads the whole file, from the first line to the last, and checks the grammar. Then it turns your code into small, simple steps. Only then does it start running those steps, top to bottom.

These simple steps are what we call bytecode. You never see them, and you don't need to.

The careful cook

Say you are cooking from a new recipe. A careful cook reads the whole recipe first. If one sentence in it makes no sense, he doesn't start cooking at all.

If the recipe reads fine, he starts at step 1 and cooks step by step, top to bottom.

Python's interpreter is that careful cook. Reading the whole recipe first is what we call compiling. Cooking it step by step is what we call interpreting.

The puzzle, answered

Now back to the puzzle. Line 3 is missing its bracket, so the grammar check fails, and the interpreter never starts running. Not one line runs, so nothing is printed, not even Hello.

A grammar mistake is what we call a syntax error. One syntax error anywhere in the file, and nothing runs.

What it means: all 3 lines run, because the grammar check passed and the cook started cooking.

How is this different from C?

So what does a compiler do in a language like C? It stops after the first job. It turns your whole program into a separate program file, an .exe on Windows, which you can run later as many times as you like.

You don't need C to learn Python. It is here only so that the word compiler means something to you the next time you meet it.

Where the interpreter comes from

Where do you get this interpreter? Normally, you download it from python.org and install it on your computer. That download is the interpreter. It is called CPython, because it is written in C.

To make learning easy, we have already put the interpreter on our website. Every Run button on this page is that same Python, rebuilt to run inside your browser. Open the page, type your code and press Run, with nothing to install.

Python 3, and what comes with it

One more thing before you search the internet for help. Python has two major versions, Python 2 and Python 3. Python 2 was retired on 1 January 2020, so we use Python 3 and skip old code written for Python 2.

Python also comes with a box of ready-made tools for dates, maths, files and much more. This box is what we call the standard library. Here are two of its tools, used in two lines.

What you should see: 12.0, the square root of 144, and then today's date.

Outside that box, there is a public shelf of extra tools written by other people. It is called PyPI, and it held over 900,000 of them this month. We will learn to use that shelf later, when our programs need it.

The takeaway

1. A Python program is text, in a .py file, written to Python's grammar.
2. The interpreter compiles the whole file first, and checks that grammar.
3. Then it interprets the program, step by step, top to bottom.

That leaves you with a question. If every line follows the grammar, does that mean the program does what you wanted?

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