In looking over the code for ZWiki/ZC, I see a lot of places with the following construct: apply(foo.__call__, some, arguments) Why not just say foo(some, arguments)? Examples of the construct from ZWikiPage.py: apply(self.aq_parent.standard_wiki_page.__call__, (None, REQUEST, REQUEST.RESPONSE)) or apply(DTMLDocument.__call__,(self, self.aq_parent, REQUEST, REQUEST.RESPONSE)) [OK, in this case the question technically is why use apply(foo.__call__, (a, b, c)) instead of foo(a,b,c)? Hmmm... x.foo(a, b, c) -> foo.__call__(x, a, b, c).] These requests are mostly, if not entirely, directed at WikiPages/Headers/etc, whose definition is, in part, class ZWikiPage(DTMLDocument): #, CatalogAware): def __call__(self, client=None, REQUEST={}, RESPONSE=None, **kw): """Render a zwiki page, with standard header & footer """ I thought the use of keyword argument dictionaries in apply might explain this, but, as the two opening examples show, not all cases have keywords. Second, even if they do one could say aDocument(self, REQUEST, REQUEST.RESPONSE, kw). I understand from Johan that this is "inherited" code (in the non OO sense), so I thought I'd throw this out as a general question. Is there some subtlety of the interaction of Zope (acquisition, perhaps, or extension classes in general) and python that makes foo(x) and foo.__call__(x) have different meanings? Is it something about standard_zwiki_page, which I notice is not a regular python variable? I don't need the answer to this to do something; I'm just trying to understand how things work. Thanks.