Comment by pjscott | Hacker News Reader

Comment by pjscott | Hacker News Reader

Nice post overall, but I take issue with a couple of the examples. First, the YoutubeSearch class has no reason to be a class. It wants to be a function:

    def youtube_search(term):
        url = 'https://gdata.youtube.com/feeds/api/videos'
        params = {'q': term, 'alt': 'json', 'orderby': 'relevance', 'v': '2'}
        r = requests.get(url, params=params)
        r.raise_for_status()         # 4xx and 5xx replies raise exception
        for video in r.json.get('feed').get('entry'):
            yield {
                'title': video.get('title').get('$t'),
                'url': video.get('link')[0].get('href'),
            }

The ExpressiveList's __sub__() method, as written in the article, requires O(n*k) time to remove k things from a list of length n. By using a set of things to remove, we can get that down to O(n+k):

    class ExpressiveList(list):
        def __sub__(self, other):
            if isinstance(other, list):
                other_set = set(other)
                return ExpressiveList(x for x in self if x not in other_set)
            else:
                return ExpressiveList(x for x in self if x != other)

What's worse about the __sub__ method is that it will work in unpredictable ways, for instance

  >>> ExpressiveList(item, [2], [3]) - item

will either be [[2], [3]] if item isn't a list, or something different if item is a list. This goes against the "Explicit is better than implicit" philosophy of python, and can cause all sorts of headaches later.

The primary reason to make YoutubeSearch a class is, in the future, implementing searching other video sites and taking advantage of inheritance in the implementation. That isn't demonstrated here, but the plumbing of "doing a request", "providing an iterator" and such could be promoted to the base class and subclasses are only responsible for knowing their domain.

YAGNI and besides you can still provide two layers of access, one being convenience functions on top of the class implementation.

When you actually want to implement those other sites, you can replace the function by a class with a __call__ method.

There's no practical difference between a method and a class in this case.

Having YoutubeSearch be a class doesn't really bother me, especially since the point of the post was to demonstrate the use of __ methods, and you can't do that if you're using a method instead.

However, I discourage promoting inheritance (except in the guise of mixin, and even then...) as a way to factorize common functionalities. Composition is usually much better suited.

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