[Python-Dev] set.copy documentation string (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Thu Dec 29 00:43:38 CET 2005

Noam Raphael wrote:

Perhaps it bothers the programmer with something that shouldn't bother him. I mean that I might do help(set.copy), and then think, "Oh, it returns a shallow copy. Wait a minute - 'shallow' means that I get a new object, which references the same objects as the old one. Perhaps I should use another function, which does deep copying? Let me think about it - no. All members of a set are immutable, so it doesn't matter." I think that in this case, the fact that the copy is shallow is an implementation detail, since there's no sense in making a deep copy of a set.

If "makes no sense" means "would not make a difference", then you are wrong. Objects in a set are not necessarily unmodifiable; they just have to be hashable. Watch this:

class A: ... def hash(self):return 0 ... def cmp(self, other):return 0 ... a1 = A() import copy a2 = copy.deepcopy(a1) a1.x = 10 a2.x = 20 a1.x 10 s = set([a1]) a2 in s True

So even though a1 and a2 are distinct objects with different, modifiable state, they are treated as equal in the set. A deepcopy of the set (if it existed) would do something different than a plain copy.

I know it's just a word, and that it doesn't matter a lot. But why not make things a tiny bit better?

Things would get worse. The code would become underspecified.

Regards, Martin

More information about the Python-Dev mailing list