Does that change if the condition isn't just "bool(name)"? For example:

    if y > 0 given y = f(x):
        ...

That's the situation where I strongly prefer the postfix operator spelling, since it's pretty clear how I should pronounce it (i.e. "if y is greater than zero, given y is set to f-of-x, then ..."). By contrast, while a variety of  plausible suggestions have been made, I still don't really know how to pronounce "if (y := f(x)) > 0:)" in a way that's going to be clear to an English-speaking listener (aside from pronouncing it the same way as I'd pronounce the version using "given", but that then raises the question of "Why isn't it written the way it is pronounced?").

I do agree with Tim that the name repetition would strongly encourage the use of short names rather than long ones (since you're always typing them at least twice), such that we'd probably see code like:

    while not probable_prime(n) given (n =
                            highbit | randrange(1, highbit, 2)):
        pass

Rather than the more explicit:

    while not probable_prime(candidate) given (candidate =
                            highbit | randrange(1, highbit, 2)):
        pass

However, I'd still consider both of those easier to follow than:

    while not probable_prime(candidate := highbit | randrange(1, highbit, 2)):
        pass

since it's really unclear to me that "candidate" in the latter form is a positional argument being bound to a name in the local environment, and *not* a keyword argument being passed to "probable_prime".

I've also been pondering what the given variant might look like as a generally available postfix operator, rather than being restricted to if/elif/while clauses, and I think that would have interesting implications for the flexibility of its usage in comprehensions, since there would now be *three* places where "given" could appear (as is already the case for the inline binding operator spelling):

- in the result expression

- in the iterable expression

- in the filter expression

That is:

    [(x, y, x - y) given y = f(x) for x in data]

Cheers,

Nick.