Why does range in python "stop short"?

Question

OK, teaching python to the kids. We just wrote our first little program:

b = 0
for a in range (1, 10)
    b = b + 1
    print a, b

Stops at 9, 9. They asked "why is that" and I can't say that I know the answer.

My code always involves files, and my "for row in reader" doesn't stop one line short, so I don't actually know. In mathematical notation, this behavior would be [1,10). Technically (1,10) would be 2,3,4,5,6,7,8,9, and indeed I want [1,10].

Answer 1

It's just usually more useful than the alternatives.

• range(10) returns exactly 10 items (if you want to repeat something 10 times)
• range(10) returns exactly the indices in a list of 10 items
• range(0, 5) + range(5, 10) == range(0, 10), which makes it easier to reason about ranges