On Sun, 23 Mar 2008 18:56:51 -0700, John Machin wrote: >> Python knows the truth value of built-in types like dicts without >> actually converting them to bools, or for that matter calling __len__ >> or __nonzero__ on them. > > What the 2.5.1 interpreter does is call PyObject_IsTrue, which checks to > see if the built_in or extension type is a mapping (not just a dict) > with a length method and if so calls it; similarly for sequences: > > else if (v->ob_type->tp_as_mapping != NULL && > v->ob_type->tp_as_mapping->mp_length != NULL) > res = (*v->ob_type->tp_as_mapping->mp_length)(v); > else if (v->ob_type->tp_as_sequence != NULL && > v->ob_type->tp_as_sequence->sq_length != NULL) > res = (*v->ob_type->tp_as_sequence->sq_length)(v); > > Was that what you meant by "without ... calling __len__"?
What I meant was that the interpreter *didn't* do was lookup the name '__len__' via the normal method resolution procedure. There's no need to search the dict's __dict__ attribute looking for an attribute named __len__, or indeed any sort of search at all. It's a direct pointer lookup to get to mp_length. I didn't mean to imply that Python magically knew whether a dict was empty without actually checking to see if it were empty. Apologies for being less than clear. Also, since I frequently criticize others for confusing implementation details with Python the language, I should criticize myself as well. What I described is specific to the CPython implementation -- there's no requirement for other Python versions to do the same thing. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
