Re: Consistency in Python

[Brendon Towle]

Date: 25 Aug 2006 04:22:37 -0700 From: "Paul Boddie" <[EMAIL PROTECTED]> Subject: Re: Consistency in Python To: python-list@python.org Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; charset="iso-8859-1" Paul McGuire wrote: But with mutators that return self, a client could write any of these: bx = Box().length(100).width(50).height(20) bx = Box().width(50).height(20).length(100) bx = Box().width(50).length(100).height(20) ...etc... and the results are the same. This is very convenient, and I've often thought about doing such things in my own APIs, but there can sometimes be subtle problems introduced into programs when you decide to change the nature of such a mutator, or if you have another type/class whose mutators are of a different nature, such that the width method produces a new Box object (which is quite similar to what the questioner seemed to have in mind) instead of mutating (or failing to mutate) the existing object. Indeed, it may be important to know whether you're creating new objects or not, and without metadata the most convenient way to communicate this is quite probably to define a rigid interface (after all, why should width or append return an object?) which cannot be implemented on immutable things. So, it's possible that the above answers the question I have and I'm just not clever enough to see the answer. But: I had a case where I wanted to extract and return the top N elements of a list (where "top" meant "smallest numeric value"; the alternative for "largest numeric value" is a trivial modification). To me, the obvious answer was: def topNElements(lst, n):     return lst.sort()[:n] But, of course, that fails with a TypeError, because lst.sort() returns None, which is not subscriptable. The function really needs to be: def topNElements(lst, n):     lst.sort()     return lst[:n] It gets worse if I want to include an invariant in the list before I sort it: def bogusTopNElementsWithInvariant(lst, n):     return lst.append(INVARIANT).sort()[:n] def topNElementsWithInvariant(lst, n)     lst.append(INVARIANT)     lst.sort()     return lst[:n] So, my question is: Someone obviously thought that it was wise and proper to require the longer versions that I write above. Why? B. -- Brendon Towle, PhD Cognitive Scientist +1-412-690-2442x127 Carnegie Learning, Inc. The Cognitive Tutor Company ® Helping over 375,000 students in 1000 school districts succeed in math.

-- 
http://mail.python.org/mailman/listinfo/python-list