Why does python `any` return a bool instead of the value?

Solution 1:

This very issue came up up on the Python developer's mailing list in 2005, when Guido Van Rossum proposed adding any and all to Python 2.5.

Bill Janssen requested that they be implemented as

def any(S):
    for x in S:
        if x:
            return x
    return S[-1]

def all(S):
    for x in S:
        if not x:
            return x
    return S[-1]

Raymond Hettinger, who implemented any and all, responded specifically addressing why any and all don't act like and and or:

Over time, I've gotten feedback about these and other itertools recipes. No one has objected to the True/False return values in those recipes or in Guido's version.

Guido's version matches the normal expectation of any/all being a predicate. Also, it avoids the kind of errors/confusion that people currently experience with Python's unique implementation of "and" and "or".

Returning the last element is not evil; it's just weird, unexpected, and non-obvious. Resist the urge to get tricky with this one.

The mailing list largely concurred, leaving the implementation as you see it today.

Solution 2:

and and or can be sensibly defined in a way that they always return one of their operands. However, any and all cannot sensibly be defined always to return a value from their input sequence: specifically they cannot do so when the list is empty. Both any and all currently have a well defined result in this situation: any returns False and all returns True. You would be forced to sometimes return a boolean value and sometimes return an item from the sequence, which makes for an unpleasant and surprising interface. Much better to be simple and consistent.

Solution 3:

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), we can alternatively explicitly capture a witness of an any expression or a counterexample of an all expression:

To quote a couple examples from the PEP description:

if any(len(long_line := line) >= 100 for line in lines):
  print("Extremely long line:", long_line)

if all((nonblank := line).strip() == '' for line in lines):
  print("All lines are blank")
else:
  print("First non-blank line:", nonblank)