On Mon, Feb 23, 2009 at 10:43 AM, Tim Chase <python.l...@tim.thechases.com> wrote: <snip> > # swap list contents...not so much... >>>> m,n = [1,2,3],[4,5,6] >>>> m[:],n[:] = n,m >>>> m,n > ([4, 5, 6], [4, 5, 6])
Pseudo-C-Python expansion: #evaluate RHS. simply *take pointers* since the RHS is just plain variables ptr_n = &n ptr_m = &m #perform "assignments" to LHS #remember that x[y] = z is really a method call to __setitem__ in disguise #let's pseudo-expand the method calls #also remember that the assignments must be serialized (Python isn't magically parallel) m._items = (*ptr_n)._items #uh-oh! this just clobbered n._items! since we only kept a pointer to n, n._items is now gone forever! n._items = (*ptr_m)._items #uh-oh! this now has no real effect since we clobbered m._items #badness! > The first two work as expected but the 3rd seems to leak some internal > abstraction. It seems to work if I force content-copying: > >>>> m[:],n[:] = n[:],m[:] Pseudo-C-Python expansion: #evaluate RHS. it's more complicated this time. this matters significantly! #remember that x[y] is really a *method call* in disguise to __getitem__ #let's pseudo-expend the method-calls too #note that we *make copies* instead of just taking pointers this time! n_cpy = copy_list(n) m_cpy = copy_list(m) #same principles as before m._items = n_cpy._items #but we have a copy of m._items in m_cpy, so m._items isn't lost forever! n._items = m_cpy._items #yay, it works! #huzzah <snip> > Is this a bug, something Python should smack the programmer for trying, or > just me pushing the wrong edges? :) Little from column A, little from column B. Using the swapping assignment works slightly unintuitively with list slices because copying doesn't magically take place; you need to do this copying explicitly whereas it's sorta done for you with plain variable swapping. Just keep in mind that container contents work "by-reference" rather than "by-value", so to speak. Cheers, Chris -- Follow the path of the Iguana... http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
