Anyway, I understand from Nick's response that len() is conceived for collections which have a size computable in constant time (I suppose structures that maintain some inner attribute to store their length). I also believe that "length" sounds more like an attribute name than a name for something which really does some computation ("count" sounds more appropriate for that). On the other hand, it doesn't seem to be a general concern not to provide construction which can lead to hidden complexity cost (like "in" used on a list vs a set).