Comment 33 for bug 525808

This depends on whether or not the called function wants to alter the behavior (or rather the context dictionary values) of the calling function and thus seems optional and depends on the case.

I'd just like to point out that the context is not passed with a "pointer" or anything like that, it's mostly a semantic issue but you can read up on it here: http://effbot.org/zone/call-by-object.htm