Runtime difference between set.discard and set.rem

Question

The official Python 2.7 docs for these methods sounds nearly identical, with the sole difference seeming to be that remove() raises a KeyError while discard does not.

I'm wondering if there is a difference in execution speed between these two methods. Failing that, is there any meaningful difference (barring KeyError) between them?

Answer 1

Raising an exception in one case is a pretty meaningful difference. If trying to remove an element from a set that is not there would be an error, you better use set.remove() rather than set.discard().

The two methods are identical in implementation, except that compared to set_discard() the set_remove() function adds the lines:

if (rv == DISCARD_NOTFOUND) {
    set_key_error(key);
    return NULL;
}

This raises the KeyError. As this is slightly more work, set.remove() is a teeniest fraction slower; your CPU has to do one extra test before returning. But if your algorithm depends on the exception then the extra branching test is hardly going to matter.