The pydoc documentation for filter reads:

    filter(function or None, sequence) -> list, tuple, or string
    
    Return those items of sequence for which function(item) is true.  If
    function is None, return the items that are true.  If sequence is a tuple
    or string, return the same type, else return a list.

It would be nicer to know (e.g. when offline and with no local access to the HTML documentation) that filter() can actually be used more generally with an iterable:

    filter(function or None, **iterable**) -> list, tuple, or string
    
    Return those items of **iterable** for which…

It was fixed in Python 3, and it is too late for 2.7.