Yes, the library designer have to anticipate what you could possibly need to alter and expose it as optional parameter. Just looking at how Python's urllib/urll

Yes, the library designer have to anticipate what you could possibly need to alter and expose it as optional parameter. Just looking at how Python's urllib/urllib2 have evolved with versions shows this is no easy task (otherwise the developers of the standard lib would have get it exactly right the first time). And it gets tougher when you actually want to modify behavior (like handling http redirects). The support for modifying behavior in popular languages is basically events, inheritance / interface implementation, and composing pre-built objects to get the desired behavior. This leaves me wanting to have a better way.