"Boolean" operations in Python (ie: the and/or operators)

Question

This method searches for the first group of word characters (ie: [a-zA-Z0-9_]), returning the first matched group or None in case of failure.

def test(str):
    m = re.search(r'(\w+)', str)
    if m:
        return m.group(1)
    return None

The same function can be rewritten as:

def test2(str):
    m = re.search(r'(\w+)', str)
    return m and m.group(1)

This works the same, and is documented behavior; as this page clearly states:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

However, being a boolean operator (it even says so on the manual), I expected and to return a boolean. As a result, I was astonished when I found out (how) this worked.

What are other use cases of this, and/or what is the rationale for this rather unintuitive implementation?

Answer 1

Basically a and b returns the operand that has the same truth value as the whole expression.

It might sound a bit confusing but just do it in your head: If a is False, then b does not matter anymore (because False and anything will always be False), so it can return a right away.

But when a is True then only b matters, so it returns b right away without even looking.

This is a very common and very basic optimization many languages do.

Answer 2

What are other use cases of this,

Conciseness (and therefore clarity, as soon as you get used to it, since after all it does not sacrifice readability at all!-) any time you need to check something and either use that something if it's true, or another value if that something is false (that's for and -- reverse it for or -- and I'm very deliberately avoiding the actual keywords-or-the-like True and False, since I'm talking about every object, not just bool!-).

Vertical space on any computer screen is limited, and, given the choice, it's best spent on useful readability aids (docstrings, comments, strategically placed empty lines to separate blocks, ...) than in turning, say, a line such as:

inverses = [x and 1.0/x for x in values]

into six such as:

inverses = []
for x in values:
    if x:
        inverses.append(1.0/x)
    else:
        inverses.append(x)

or more cramped versions thereof.

and/or what is the rationale for this rather unintuitive implementation?

Far from being "unintuitive", beginners regularly were tripped up by the fact that some languages (like standard Pascal) did not specify the order of evaluation and the short-circuiting nature of and and or; one of the differences between Turbo Pascal and the language standard, which back in the day made Turbo the most popular Pascal dialect of all times, was exactly that Turbo implemented and and or much like Python did later (and the C language did earlier...).