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?