CODESARRAY

Short note, by Codesarray, , 2 min read

Python chained comparison: 0 < x < 10 just works

In most languages that line is a bug. In Python it is one test that means exactly what it says, and this note shows how Python reads it.

In most languages, 0 < x < 10 is a bug: the first comparison gives a boolean, and that boolean is then compared with 10. In Python it is a chained comparison, and it is just how you write it.

One chain, two tests

x = 5
print(0 < x < 10)
print(0 < x and x < 10)
x = 15
print(0 < x < 10)
print(1 < 3 > 2)
True
True
False
True

0 < x < 10 is exactly the same test as 0 < x and x < 10. With x at 5 both are True. Set x to 15 and the chain is False, because the second half fails. The last line shows that the operators do not even have to point the same way: 1 < 3 > 2 asks whether 3 is bigger than both its neighbours, and it is.

How Python reads a chain

Python reads a < b < c as a < b and b < c, with one difference from writing it out yourself: the middle value is evaluated once. If b is a function call, it is called one time, not two.

The same rule applies to every comparison operator, so x == y == z asks whether all three values are equal, and a <= b < c is a half-open range check. Every link in the chain has to be True for the whole chain to be True.

What to take from this

  • Write a range check the way you would on paper: 0 < x < 10.
  • It means 0 < x and x < 10, with x checked once.
  • Any comparisons chain, including ==, and the directions can mix.

This is a short note on Control flow, lesson 3 of the Python beginner track.

This article goes with the video Chained comparisons. Watch it on YouTube.