How a program runs

You have written a few programs now. This page is a short pause to build a picture of what the computer is doing with them. You will not need any hardware knowledge, and there is nothing to memorise. The picture just makes the next few pages, and a lot of future errors, make sense.

Two parts

For our purposes a computer has two parts that matter.

Memory is where values live while a program runs. Think of it as a huge set of shelves. When your program says price = 40, Python puts the value 40 on a free shelf and sticks a label on it that reads price.

The processor is the part that does things. It reads your program one instruction at a time, starting from the top, does exactly what that instruction says, and moves on. It never stops to wonder what you meant. (Soon you will meet lines that send it somewhere else, such as calling a function or repeating a loop. Even then it only ever does one instruction at a time.)

Everything you have seen so far is these two parts working together: the processor steps through your lines, and each line puts values on shelves, reads values off shelves, or shows a value on the screen.

Names are labels, not shelves

This is the one idea on the page worth slowing down for. A name does not contain a value. It points at one. Run this:

price = 40
print(price)
price = 45
print(price)

On the first line, 40 goes on a shelf and the price label is attached to it. On the third line, 45 goes on a different shelf, and the label is moved to point at the new one. Nobody refers to the old 40 any more, so Python is free to reuse that shelf.

The same thing happens with price = price + 5: the processor reads the value price points at, adds 5, puts the result on a new shelf, and moves the label. The name on the left and the name on the right are the same label at two moments in time.

Why does this matter? Because two labels can point at the same shelf. With numbers and text you will never notice, since those values cannot be changed once made. Later you will meet values that can be changed in place, and then two labels on one shelf is a surprise waiting to happen. When you get there, come back to this picture.

Why order matters

Because the processor does one line at a time and never looks ahead, a name has to be given a value before the line that uses it runs. Run this:

print(total)
total = 100

The processor reaches print(total), looks for a shelf labelled total, finds none, and stops with a NameError. The assignment on the next line is no help; the processor has not got there yet. Swap the two lines and it works.

Why it never gets bored

A processor carries out hundreds of millions of instructions every second. That is the whole reason programming is worth doing: a few lines that work once will work a million times, and the computer will not get tired or make a mistake on the ten-thousandth repeat.

It also never gets bored, which is a problem when you tell it to repeat forever. Later, in the section on loops, you will write a program that never finishes, on purpose, and this site will stop it after ten seconds. That is not the processor failing. It is the processor doing exactly what it was told.

What this page is not

There is a lot more to a real computer: how values are stored as electrical states, what happens when memory runs out, how several programs share one processor. None of it changes anything on the pages ahead, so none of it is here. The shelves-and-stepper picture is enough to carry you through this whole course.

Next: Functions.