Steve D'Aprano <steve+pyt...@pearwood.info> writes: > Every few years, the following syntax comes up for discussion, with > some people saying it isn't obvious what it would do, and others > disagreeing and saying that it is obvious. So I thought I'd do an > informal survey. > > What would you expect this syntax to return? > > [x + 1 for x in (0, 1, 2, 999, 3, 4) while x < 5]
I agree that the syntax is confusing. The comprehension encourages thinking in sets: an operation that takes a collection as input, and emits a different collection, through one conceptual operation. Adding ‘while’ in there encourages thinking not in terms of a single set-based operation, but an iteration of separate operations. That confuses the model, and I no longer have a coherent model about which to reason what the syntax might mean. > For comparison, what would you expect this to return? (Without > actually trying it, thank you.) > > [x + 1 for x in (0, 1, 2, 999, 3, 4) if x < 5] Though ‘for’ is used elsewhere in Python to mean iteration, ‘for’ also has strong connotation in mathematics for set-based operations (“the result is foo for all bar, if baz”). So the same confusion doesn't occur: this is a comprehension which is about set-based thinking, which is supported by all the semantic connotations of the syntax. So I think ‘while’ should not be added to the syntax of comprehension expressions. I agree with those who think it is not obvious what it would mean. -- \ “I love to go down to the schoolyard and watch all the little | `\ children jump up and down and run around yelling and screaming. | _o__) They don't know I'm only using blanks.” —Emo Philips | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
