Personally, I'd want to see mutator methods return `self` so you can do more than one mutation in a statement, but the convention is that all the mutator methods return `None`.

On Thu, Jun 25, 2020, 10:28 AM Ben Avrahami <avrahami.ben@gmail.com> wrote:
Hey all,
Often I've found this kind of code:

seen = set()
for i in iterable:
  if i in seen:
    ...  # do something in case of duplicates
  else:
    seen.add(i)
    ... # do something in case of first visit

This kind of code appears whenever one needs to check for duplicates in case of a user-submitted iterable, or when we loop over a recursive iteration that may involve cycles (graph search or the like). This code could be improved if one could ensure an item is in the set, and get whether it was there before in one operation. This may seem overly specific, but dicts do do this:

seen = {}
for i in iterable:
  if seen.set_default(i, some_value) is not None:
    ...  # do something in case of duplicates
  else:
    ... # do something in case of first visit

I think the set type would benefit greatly from its add method having a return value. set.add would return True if the item was already in the set prior to insertion, and False otherwise.

Looking at the Cpython code, the set_add_entry already detects existing entries, adding a return value would require no additional complexity.

Any thoughts?
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-leave@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/6WYNYNG5J5HBD3PA7PW75RP4PMLOMH4C/
Code of Conduct: http://python.org/psf/codeofconduct/