On Mon, Jun 20, 2011 at 6:42 PM, Spencer Pearson <speeze.pear...@gmail.com> wrote: > I was recently trying to implement a dict-like object which would do > some fancy stuff when it was modified, and found that overriding the > __setitem__ method of an instance did not act the way I expected. The > help documentation (from help(dict.__setitem__)) claims that > "d.__setitem__(k,v)" is equivalent to "d[k]=v", but I've produced this > code that, on Python 2.6, acts differently in the two cases.
Technically, the strict equivalence is only one-way, as you've shown; but one generally avoids calling the __magic__ methods directly, so this subtle distinction is seldom used intentionally. <snip> > I would expect the two setitems to both call print_args, but that's > not what happens. In the first case, it calls print_args, but in the > second case, the __setitem__ declared in MyDict is called instead. > > The documentation at > http://docs.python.org/reference/datamodel.html#specialnames > says that for new-style classes, "x[i]" is equivalent to > "type(x).__getitem__(x, i)". I assume that "x[i]=y" has similarly been > changed to be equivalent to "type(x).__setitem__(x, i, y)", since that > would produce the results that I'm getting. Is the help documentation > for dict.__setitem__ just outdated, or am I missing some subtlety > here? The sentence of the docs in question begins with "For instance,"; hence, __setitem__ is just the arbitrarily-chosen example used in this part of the docs. But the point applies equally to all the special methods. See the very last section of the same webpage: http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes > Also: when I say "d.f(*args)", am I correct in thinking that d checks > to see if it has an instance attribute called "f", and if it does, > calls f(*args); and if it doesn't, checks whether its parent class > (and then its grandparent, and so on) has a class attribute called > "f", and if it does, calls f(x, *args)? See the first paragraph under the "Classes" entry on http://docs.python.org/reference/datamodel.html#objects-values-and-types for perfect accuracy. Cheers, Chris -- http://mail.python.org/mailman/listinfo/python-list
