Why print() Is Not Enough

1print() shows the answer, then forgets it

Think about what print(2 + 3) actually does. Python solves 2 + 3, gets 5, throws the 5 on the screen — and then lets it go. The answer is displayed, not kept.

It is like a friend who does a sum in their head, tells you the answer, and then instantly forgets it. If you ask again a minute later, they have to work the whole thing out from the beginning.

2Where this becomes a real problem

A tiny program can get away with it. A real program cannot. Suppose you work out the total marks of a student near the top of a long program, and further down you need that total three more times — once to find the percentage, once to print the report, once to decide pass or fail.

If the total only ever went into a print(), it is gone. Your only choice is to write the whole calculation out again, everywhere you need it:

print(37 + 45 + 41)
print((37 + 45 + 41) / 3)
print((37 + 45 + 41) - 100)

123
41.0
23

This program works. But three things about it are bad:

• The computer does the same work again and again. Those three numbers are added three separate times. Here that is nothing. In a program that repeats a heavy calculation a thousand times, it is very slow.
• You have to repeat yourself. The same long sum is typed out three times, and every copy is a chance to make a typing mistake.
• One change means many changes. If a mark is corrected from 41 to 44, you must remember to fix it in all three places. Miss one, and your program quietly prints a wrong answer.

3The fix: work it out once, and keep it

What we really want is a way to say to Python: “solve this, and hold on to the answer for me — I will need it later.” Python gives us exactly that. You give the answer a name, and from then on you use the name:

total = 37 + 45 + 41
print(total)
print(total / 3)
print(total - 100)

123
41.0
23

Same three lines of output. But now the sum is worked out once, the answer is stored under the name total, and every line after that simply asks for total. Correct a mark and you change it in one place only.

4Watch both programs run

Press Run both. The two programs print exactly the same thing — so watch the memory strip in the middle instead. On the left the answer puffs away each time. On the right it is worked out once and stays in a box.

That box is called a variable, and it is the whole idea of this chapter. It is a place in the computer's memory where your program parks a value, gives it a name, and picks it up again whenever it likes.

5Recap