Lazy function evaluation in any() / all()

Logical operators in Python are lazy. With the following definition:

def func(s):
    print(s)
    return True

calling the or operator

>>> func('s') or func('t')
's'
True

only evaluates the first function call, because or recognizes that the expression evaluates to
True, regardless of the return value of the second function call. and behaves analogously.

However, when using any() (analogously: all()) in the following way:

>>> any([func('s'), func('t')])
's'
't'
True

all function calls are evaluated, because the inner list is constructed first, before any starts to iterate over the boolean values of its items.

That way we lose the power of any being short-circuit, which means that it breaks as soon as the first element of the iterable is truish. If the function calls are expensive, evaluating all the functions up front is a big loss and is a waste of this ability of any. In some sense, one could call this a Python gotcha, because it might be unexpected for users trying to leverage this feature of any, and because any is often thought as being just another syntactic way of chaining a sequence of or statements. But any is just short-circuit, not lazy, and that is a difference here.

any accepts an iterable. So, there should be a way of creating an iterator which does not evaluate its elements up front but pass them unevaluated to any and lets them evaluate inside of any only, in order to achieve a fully lazy evaluation.

So, the question is: How can we use any with truly lazy function evaluation? That means: How can we make an iterator of function calls which any can consume, without evaluating all the function calls in advance?

We can use a generator expression, passing the functions and their arguments separately and evaluating only in the generator like so:

>>> any(func(arg) for arg in ('s', 't'))
's'
True

For different functions with different signatures, this could look like the following:

any(
    f(*args)
    for f, args in [(func1, ('s',)), (func2, (1, 't'))]
)

That way, any will stop iterating over the generator as soon as one function call evaluates to True, and that means that the function evaluation is fully lazy.

Another neat way to postpone the function evaluation is to use lambda expressions, like so:

>>> any(
...     f() 
...     for f in [lambda: func('s'), lambda: func('t')]
... )
's'
True

It is sad that any() and all() don’t have the logical functionality and are limited by this somewhat artificial constraint. Instead, a once-thru loop construction can be handy, particularly if there are intermediate results that need to be generated and used. This is related also to a function with early returns.

    for _ in range(1):
        val = func1()
        if not val:
            break
        val1 = intermediate_func1(val)
        if not func3(val1):
            break
        val2 = intermediate_func2(val1)
        if not func4(val2):
            break
        result = the_really_expensive_function(val, val2)
        if result:
            return True, result
    return False, None

similar construct using a function and early returns.

def example():
    val = func1()
    if not val:
        return False, None
    val1 = intermediate_func1(val)
    if not func3(val1):
        return False, None
    val2 = intermediate_func2(val1)
    if not func4(val2):
        return False, None
    result = the_really_expensive_function(val, val2)
    if result:
        return True, result
    return False, None

What I wanted to use (but can’t; and this is only feasible with := operator):

if all(
        val := func1(),
        func3(val1 := intermediate_func1(val)),
        func4(val2 := intermediate_func2(val1)),
        result := the_really_expensive_function(val, val2),
        ):
    return True, result
return False, None

Maybe in the future this will be feasible.