結局は http://d.hatena.ne.jp/methane/20090702/1246556675 なんだけど、ソースを提示しておく。 まず、Python FAQにはこんな風に書いてある。 http://www.python.org/doc/faq/general/#why-does-python-use-methods-for-some-functionality-e-g-list-index-but-functions-for-other-e-g-len-list The major reason is history. Functions were used for those operations that were generic for a group of types and which were intended to work even for objects t