Is `a<b<c` valid python?

Question

I was curious to see if I could use this a<b<c as a conditional without using the standard a<b and b<c. So I tried it out and my test results passed.

a = 1
b = 2
c = 3

assert(a<b<c) # In bounds test
assert(not(b<a<c)) # Out of bounds test
assert(not(a<c<b)) # Out of bounds test

Just for good measure I tried more numbers, this time in the negative region. Where a, b, c = -10, -9, -8. The test passed once again. Even the test suit at a higher range works a, b, c = 10, 11, 12. Or even a, b, c = 10, 20, 5.

And the same experiment done in C++. This was my mentality going into it:

#include <iostream>

using namespace std;

int main()
{
    int a,b,c;
    a=10;
    b=20;
    c=5;
    cout << ((a<b<c)?"True":"False") << endl; // Provides True (wrong)
    cout << ((a<b && b<c)?"True":"False") << endl; // Provides False (proper answer)
    return 0;
}

I originally though that this implementation would be invalid since in every other language I have come across would evaluate a boolean before it would reach c. With those languages, a<b would evaluate to a boolean and continuing the evaluation, b<c, would be invalid since it would attempt to evaluate a boolean against a number (most likely throwing a compile time error or falsifying the intended comparison). This is a little unsettling to me for some reason. I guess I just need to be reassured that this is part of the syntax. It would also be helpful to provide a reference to where this feature is provided in the Python documentation so I can see to what extent they provide features like this.

Sukrit Kalra Sukrit Kalra · Accepted Answer · 2013-09-12T03:56:30

This is documented here.

Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.

And, as an example,

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

Is `a<b<c` valid python?

2 Answers