On Sat, 5 Mar 2022 at 12:12, Chris Angelico <rosuav@gmail.com> wrote:
On Sat, 5 Mar 2022 at 22:22, Stephen J. Turnbull <stephenjturnbull@gmail.com> wrote:
Python has one, and you've already mentioned it:
for thing in (x for x in this_collection if is_interesting(x)):
It's noticably verbose, but it's an exact translation of your statement of the abstract concept above. It has all the benefits of the proposed syntax except compactness[1].
It is extremely verbose, considering that the filtering part of a comprehension uses the same variable name as iteration, doesn't need anything to be repeated, and is just another clause. A simple representation of that in a statement loop would be "for thing in this_collection if is_interesting(thing):", which doesn't repeat itself at all (the variable name "thing" is kinda essential to the concept of filtration here, so I don't count that); and repetition isn't simply about number of characters on the line, it's about reducing the potential for errors. (Plus, the genexp adds a significant amount of run-time overhead.)
So you're right, I stand (partly) corrected: there IS a very clunky way to spell this concept.
What Python needs is a non-clunky way to express this.
for thing in filter(is_interesting, this_collection): ... That seems pretty non-clunky. Is the issue here that "filter" is not sufficiently well-known? Or that you don't want to name the is_interesting function, and lambdas are "too clunky"? This feels like another case where the general dislike of lambda results in people wanting special-case syntax so they can avoid either writing a throwaway function, or using lambda. If we had "placeholder" expressions (see, for example https://pypi.org/project/placeholder/) so we could do things like from placeholder import _ for thing in filter(_%3==0, the_list): ... would that be sufficiently "non-clunky"? There are many ways of achieving this sort of result. Clearly, from the fact that this request comes up repeatedly, there's *something* unsatisfying about all of them, but I'm not entirely clear what particular problem is uniquely solved by new "for x in collection if condition" syntax, and not by any of the other possibilities? Personally, I don't mind having the if on a separate line, I find the generator expression tolerable but a bit verbose, and I'm glad "filter" and the placeholder library exist in case I need them, but I've never really found the existing options sufficiently annoying that I've wanted a for...if statement. Paul