Python Inequality Chaining


April 2, 2020

In Python the comparison a <= b == c < d does the mathematically correct thing. This is a handy notational trick.

This wasn’t obvious to me because a lot of programming languages treat these associatively, so that a <= b < c may resolve to (a <= b) < c. This is very dangerous if boolean (True or False) are coerced to integers (1 or 0) because it may look like it works but give the wrong results.

However Python’s documentation explains that a chained comparison like a <= b < c is translated to (a <= b) and (b < c), which is exactly what you expect (with b only evaluated once, in case it has side effects). This is a neat trick that can make code a bit easier to read (though you have to be careful if you’re switching between languages!)