Bugs item #1080424, was opened at 2004-12-07 00:36 Message generated for change (Comment added) made by tim_one You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1080424&group_id=5470
Category: Python Interpreter Core >Group: Not a Bug >Status: Closed >Resolution: Invalid Priority: 6 Submitted By: Matthias Klose (doko) Assigned to: Nobody/Anonymous (nobody) Summary: Inplace set merge produces wrong results Initial Comment: [forwarded from http://bugs.debian.org/284490] the inplace set merge can produce wrong results compared to the a = a | b non in-place (and slower) version. Please see the attached tarball: running the "test" script shows the problem. ---------------------------------------------------------------------- >Comment By: Tim Peters (tim_one) Date: 2004-12-07 02:46 Message: Logged In: YES user_id=31435 I can pretty much guarantee this isn't a bug in Python, but is in some aspect of your algorithm that *relies* on not sharing mutable sets. For example, if I leave Debtags1.py's self.items[t] |= add_elements alone but add this right after it: self.items[t] = self.items[t].copy() then Debtags1.py produces the same output as Debtags.py. Same thing with Debtags2.py: adding that line also makes Debtag2.py's output the same. That proves the problem isn't in the implementations of "|=" or .update(). It strongly suggests that you're mutating a shared set that you're not expecting to mutate, or aren't expecting is shared. For example, your driver does s = db.elset(sys.argv[1]) for t in sys.argv[2:]: s &= db.elset(t) and that mutates the set in self.items[sys.argv[1]]. If you don't intend that computing output will mutate the sets in db, then that code is confused. That's not the source of your differing output, but "something like it" probably is. In fact, the problem is probably here: self.items[t] = add_elements That can assign the same add_elements as the value associated with *many* distinct values of t. Then you try to update those later in place, but "those" is a single shared set. Changing the value associated with one of the t's then changes the value associated with all of the t's that originally got assigned the same "add_elements" set. If I go back to the original Debtags1.py, and replace self.items[t] = add_elements with self.items[t] = add_elements.copy() then the later updates-in-place do no harm, and it produces the output you said you expected. If you don't understand this, here's a dead simple example: >>> x = set([1]) >>> y = x # y and x are the *same* set now >>> x |= set([2]) # so mutating x in place ... >>> x set([1, 2]) >>> y # ... also mutates the set bound to y set([1, 2]) >>> ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1080424&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
