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Deep Learning Cookbook by Douwe Osinga

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Chapter 4. Building a Recommender System Based on Outgoing Wikipedia Links

Recommender systems are traditionally trained on previously collected ratings from users. We want to predict ratings from users, so starting with historical ratings feels like a natural fit. However, this requires us to have a substantial set of ratings before we can get going and it doesn’t allow us to do a good job on new items for which we don’t have ratings yet. Moreover, we deliberately ignore the metainformation that we have on items.

In this chapter you’ll explore how to build a simple movie recommender system based solely on outgoing Wikipedia links. You’ll start by extracting a training set from Wikipedia and then train embeddings based on these links. You’ll then implement a simple support vector machine classifier to give recommendations. Finally, you’ll explore how you can use your newly trained embeddings to predict review scores for the movies.

The code in this chapter can be found in these notebooks:

04.1 Collect movie data from Wikipedia
04.2 Build a recommender system based on outgoing Wikipedia links

4.1 Collecting the Data

Problem

You want to obtain a dataset for training for a specific domain, like movies.

Solution

Parse a Wikipedia dump and extract only the pages that are movies.

Note

The code in this recipe shows how to fetch and extract training data from Wikipedia, which is a very useful skill. However, downloading and processing a full dump takes a rather long time. The data directory of the notebook folder contains the top 10,000 movies pre-extracted that we’ll use in the rest of the chapter, so you don’t need to run the steps in this recipe.

Let’s start by downloading a recent dump from Wikipedia. You can easily do this using your favorite browser, and if you don’t need the very latest version, you should probably pick a nearby mirror. But you can also do it programmatically. Here’s how to get the latest dump pages:

index = requests.get('https://dumps.wikimedia.org/backup-index.html').text
soup_index = BeautifulSoup(index, 'html.parser')
dumps = [a['href'] for a in soup_index.find_all('a')
             if a.has_attr('href') and a.text[:-1].isdigit()]

We’ll now go through the dumps and find the newest one that has actually finished processing:

for dump_url in sorted(dumps, reverse=True):
    print(dump_url)
    dump_html = index = requests.get(
        'https://dumps.wikimedia.org/enwiki/' + dump_url).text
    soup_dump = BeautifulSoup(dump_html, 'html.parser')
    pages_xml = [a['href'] for a in soup_dump.find_all('a')
                 if a.has_attr('href')
                 and a['href'].endswith('-pages-articles.xml.bz2')]
    if pages_xml:
        break
    time.sleep(0.8)

Note the sleep to stay under the rate limiting of Wikipedia. Now let’s fetch the dump:

wikipedia_dump = pages_xml[0].rsplit('/')[-1]
url = url = 'https://dumps.wikimedia.org/' + pages_xml[0]
path = get_file(wikipedia_dump, url)
path

The dump we retrieved is a bz2-compressed XML file. We’ll use sax to parse the Wikipedia XML. We’re interested in the <title> and the <page> tags so our Content​Handler looks like this:

class WikiXmlHandler(xml.sax.handler.ContentHandler):
    def __init__(self):
        xml.sax.handler.ContentHandler.__init__(self)
        self._buffer = None
        self._values = {}
        self._movies = []
        self._curent_tag = None

    def characters(self, content):
        if self._curent_tag:
            self._buffer.append(content)

    def startElement(self, name, attrs):
        if name in ('title', 'text'):
            self._curent_tag = name
            self._buffer = []

    def endElement(self, name):
        if name == self._curent_tag:
            self._values[name] = ' '.join(self._buffer)

        if name == 'page':
            movie = process_article(**self._values)
            if movie:
                self._movies.append(movie)

For each <page> tag this collects the contents of the title and of the text into the self._values dictionary and calls process_article with the collected values.

Although Wikipedia started out as a hyperlinked text-based encyclopedia, over the years it has developed into a more structured data dump. One way this is done is by having pages link back to so-called category pages. These links function as tags. The page for the film One Flew Over the Cuckoo’s Nest links to the category page “1975 films,” so we know it is a movie from 1975. Unfortunately, there is no such thing as a category page for just movies. Fortunately, there is a better way: Wikipedia templates.

Templates started out as a way to make sure that pages that contain similar information have that information rendered in the same way. The “infobox” template is very useful for data processing. Not only does it contain a list of key/value pairs applicable to the subject of the page, but it also has a type. One of the types is “film,” which makes the task of extracting all movies a lot easier.

For each movie we want to extract the name, the outgoing links and, just because we can, the properties stored in the infobox. The aptly named mwparserfromhell does a decent job of parsing Wikipedia:

def process_article(title, text):
    rotten = [(re.findall('\d\d?\d?%', p),
        re.findall('\d\.\d\/\d+|$', p), p.lower().find('rotten tomatoes'))
        for p in text.split('\n\n')]
    rating = next(((perc[0], rating[0]) for perc, rating, idx in rotten
        if len(perc) == 1 and idx > -1), (None, None))
    wikicode = mwparserfromhell.parse(text)
    film = next((template for template in wikicode.filter_templates()
                 if template.name.strip().lower() == 'infobox film'),
                 None)
    if film:
        properties = {param.name.strip_code().strip():
                      param.value.strip_code().strip()
                      for param in film.params
                      if param.value.strip_code().strip()
                     }
        links = [x.title.strip_code().strip()
                 for x in wikicode.filter_wikilinks()]
        return (title, properties, links) + rating

We can now feed the bzipped dump into the parser:

parser = xml.sax.make_parser()
handler = WikiXmlHandler()
parser.setContentHandler(handler)
for line in subprocess.Popen(['bzcat'],
                             stdin=open(path),
                             stdout=subprocess.PIPE).stdout:
  try:
    parser.feed(line)
  except StopIteration:
    break

Finally, let’s save the results so next time we need the data, we don’t have to process for hours:

with open('wp_movies.ndjson', 'wt') as fout:
  for movie in handler._movies:
    fout.write(json.dumps(movie) + '\n')

Discussion

Wikipedia is not only a great resource to answer questions about almost any area of human knowledge; it also is the starting point for many deep learning experiments. Knowing how to parse the dumps and extract the relevant bits is a skill useful for many projects.

At 13 GB the dumps are sizeable downloads. Parsing the Wikipedia markup language comes with its own challenges: the language has grown organically over the years and doesn’t seem to have a strong underlying design. But with today’s fast connections and some great open source libraries to help with the parsing, it has all become quite doable.

In some situations the Wikipedia API might be more appropriate. This REST interface to Wikipedia allows you to search and query in a number of powerful ways and only fetch the articles that you need. Getting all the movies that way would take a long time given the rate limiting, but for smaller domains it is an option.

If you end up parsing Wikipedia for many projects, it might be worth it to first import the dump into a database like Postgres so you can query the dataset directly.

4.2 Training Movie Embeddings

Problem

How can you use link data between entities to produce suggestions like “If you liked this, you might also be interested in that”?

Solution

Train embeddings using some metainformation as connectors. This recipe builds on the previous one by using the movies and links extracted there. To make the dataset a bit smaller and less noisy, we’ll work with only the top 10,000 movies determined by popularity on Wikipedia.

We’ll treat the outgoing links as the connectors. The intuition here is that movies that link to the same page are similar. They might have the same director or be of the same genre. As the model trains, it learns not only which movies are similar, but also which links are similar. This way it can generalize and discover that a link to the year 1978 has a similar meaning as a link to 1979, which in turn helps with movie similarity.

We’ll start by counting the outgoing links as a quick way to see whether what we have is reasonable:

link_counts = Counter()
for movie in movies:
    link_counts.update(movie[2])
link_counts.most_common(3)
[(u'Rotten Tomatoes', 9393),
 (u'Category:English-language films', 5882),
 (u'Category:American films', 5867)]

Our model’s task is to determine whether a certain link can be found on the Wikipedia page of a movie, so we need to feed it labeled examples of matches and nonmatches. We’ll keep only links that occur at least three times and build a list of all valid (link, movie) pairs, which we’ll store for quick lookups later. We keep the same handy as a set for quick lookups later:

top_links = [link for link, c in link_counts.items() if c >= 3]
link_to_idx = {link: idx for idx, link in enumerate(top_links)}
movie_to_idx = {movie[0]: idx for idx, movie in enumerate(movies)}
pairs = []
for movie in movies:
    pairs.extend((link_to_idx[link], movie_to_idx[movie[0]])
                  for link in movie[2] if link in link_to_idx)
pairs_set = set(pairs)

We are now ready to introduce our model. Schematically, we take both the link_id and the movie_id as a number and feed those into their respective embedding layers. The embedding layer will allocate a vector of embedding_size for each possible input. We then set the dot product of these two vectors to be the output of our model. The model will learn weights such that this dot product will be close to the label. These weights will then project movies and links into a space such that movies that are similar end up in a similar location:

def movie_embedding_model(embedding_size=30):
    link = Input(name='link', shape=(1,))
    movie = Input(name='movie', shape=(1,))
    link_embedding = Embedding(name='link_embedding',
        input_dim=len(top_links), output_dim=embedding_size)(link)
    movie_embedding = Embedding(name='movie_embedding',
        input_dim=len(movie_to_idx), output_dim=embedding_size)(movie)
    dot = Dot(name='dot_product', normalize=True, axes=2)(
        [link_embedding, movie_embedding])
    merged = Reshape((1,))(dot)
    model = Model(inputs=[link, movie], outputs=[merged])
    model.compile(optimizer='nadam', loss='mse')
    return model

model = movie_embedding_model()

We’ll feed the model using a generator. The generator yields batches of data made up of positive and negative examples.

We sample the positive samples from the pairs array and then fill it up with negative examples. The negative examples are randomly picked and we make sure they are not in the pairs_set. We then return the data in a format that our network expects, an input/output tuple:

def batchifier(pairs, positive_samples=50, negative_ratio=5):
    batch_size = positive_samples * (1 + negative_ratio)
    batch = np.zeros((batch_size, 3))
    while True:
        for idx, (link_id, movie_id) in enumerate(
                random.sample(pairs, positive_samples)):
            batch[idx, :] = (link_id, movie_id, 1)
        idx = positive_samples
        while idx < batch_size:
            movie_id = random.randrange(len(movie_to_idx))
            link_id = random.randrange(len(top_links))
            if not (link_id, movie_id) in pairs_set:
                batch[idx, :] = (link_id, movie_id, -1)
                idx += 1
        np.random.shuffle(batch)
        yield {'link': batch[:, 0], 'movie': batch[:, 1]}, batch[:, 2]

Time to train the model:

positive_samples_per_batch=512

model.fit_generator(
    batchifier(pairs,
               positive_samples=positive_samples_per_batch,
               negative_ratio=10),
    epochs=25,
    steps_per_epoch=len(pairs) // positive_samples_per_batch,
    verbose=2
)

Training times will depend on your hardware, but if you start with the 10,000 movie dataset they should be fairly short, even on a laptop without GPU acceleration.

We can now extract the movie embeddings from our model by accessing the weights of the movie_embedding layer. We normalize them so we can use the dot product as an approximation of the cosine similarity:

movie = model.get_layer('movie_embedding')
movie_weights = movie.get_weights()[0]
lens = np.linalg.norm(movie_weights, axis=1)
normalized = (movie_weights.T / lens).T

Now let’s see if the embeddings make some sense:

def neighbors(movie):
    dists = np.dot(normalized, normalized[movie_to_idx[movie]])
    closest = np.argsort(dists)[-10:]
    for c in reversed(closest):
        print(c, movies[c][0], dists[c])

neighbors('Rogue One')
29 Rogue One 0.9999999
3349 Star Wars: The Force Awakens 0.9722805
101 Prometheus (2012 film) 0.9653338
140 Star Trek Into Darkness 0.9635347
22 Jurassic World 0.962336
25 Star Wars sequel trilogy 0.95218825
659 Rise of the Planet of the Apes 0.9516557
62 Fantastic Beasts and Where to Find Them (film) 0.94662267
42 The Avengers (2012 film) 0.94634
37 Avatar (2009 film) 0.9460137

Discussion

Embeddings are a useful technique, and not just for words. In this recipe we’ve trained a simple network and produced embeddings for movies with reasonable results. This technique can be applied any time we have a way to connect items. In this case we used the outgoing Wikipedia links, but we could also use incoming links or the words that appear on the page.

The model we trained here is extremely simple. All we do is ask it to come up with an embedding space such that the combination of the vector for the movie and the vector for the link can be used to predict whether or not they will co-occur. This forces the network to project movies into a space such that similar movies end up in a similar location. We can use this space to find similar movies.

In the Word2vec model we use the context of a word to predict the word. In the example of this recipe we don’t use the context of the link. For outgoing links it doesn’t seem like a particularly useful signal, but if we were using incoming links, it might have made sense. Pages linking to movies do this in a certain order, and we could use the context of the links to improve our embedding.

Alternatively, we could use the actual Word2vec code and run it over any of the pages that link to movies, but keep the links to movies as special tokens. This would then create a mixed movie and word embedding space.

4.3 Building a Movie Recommender

Problem

How can you build a recommender system based on embeddings?

Solution

Use a support vector machine to separate the positively ranked items from the negatively ranked items.

The previous recipe let us cluster movies and make suggestions like “If you liked Rogue One, you should also check out Interstellar.” In a typical recommender system we want to show suggestions based on a series of movies that the user has rated. As we did in Chapter 3, we can use an SVM to do just this. Let’s take the best and worst movies according to Rolling Stone from 2015 and pretend they are user ratings:

best = ['Star Wars: The Force Awakens', 'The Martian (film)',
        'Tangerine (film)', 'Straight Outta Compton (film)',
        'Brooklyn (film)', 'Carol (film)', 'Spotlight (film)']
worst = ['American Ultra', 'The Cobbler (2014 film)',
         'Entourage (film)', 'Fantastic Four (2015 film)',
         'Get Hard', 'Hot Pursuit (2015 film)', 'Mortdecai (film)',
         'Serena (2014 film)', 'Vacation (2015 film)']
y = np.asarray([1 for _ in best] + [0 for _ in worst])
X = np.asarray([normalized_movies[movie_to_idx[movie]]
                for movie in best + worst])

Constructing and training a simple SVM classifier based on this is easy:

clf = svm.SVC(kernel='linear')
clf.fit(X, y)

We can now run the new classifier over all the movies in our dataset and print the best five and the worst five:

estimated_movie_ratings = clf.decision_function(normalized_movies)
best = np.argsort(estimated_movie_ratings)
print('best:')
for c in reversed(best[-5:]):
    print(c, movies[c][0], estimated_movie_ratings[c])

print('worst:')
for c in best[:5]:
    print(c, movies[c][0], estimated_movie_ratings[c])
best:
(6870, u'Goodbye to Language', 1.24075226186855)
(6048, u'The Apu Trilogy', 1.2011876298842317)
(481, u'The Devil Wears Prada (film)', 1.1759994747169913)
(307, u'Les Mis\xe9rables (2012 film)', 1.1646775074857494)
(2106, u'A Separation', 1.1483743944891462)
worst:
(7889, u'The Comebacks', -1.5175929012505527)
(8837, u'The Santa Clause (film series)', -1.4651252650867073)
(2518, u'The Hot Chick', -1.464982008376793)
(6285, u'Employee of the Month (2006 film)', -1.4620595013243951)
(7339, u'Club Dread', -1.4593221506016203)

Discussion

As we saw in the previous chapter, we can use support vector machines to efficiently construct a classifier that distinguishes between two classes. In this case, we have it distinguish between good movies and bad movies based on the embeddings that we have previously learned.

Since an SVM finds one or more hyperplanes that separate the “good” examples from the “bad” examples, we can use this as the personalization function—the movies that are the furthest from the separating hyperplane and on the right side are the movies that should be liked best.

4.4 Predicting Simple Movie Properties

Problem

You want to predict simple movie properties, like Rotten Tomatoes ratings.

Solution

Use a linear regression model on the learned vectors of the embedding model to predict movie properties.

Let’s try this for Rotten Tomatoes ratings. Luckily they are already present in our data in movie[-2] as a string of the form N%:

rotten_y = np.asarray([float(movie[-2][:-1]) / 100
                       for movie in movies if movie[-2]])
rotten_X = np.asarray([normalized_movies[movie_to_idx[movie[0]]]
                       for movie in movies if movie[-2]])

This should get us data for about half our movies. Let’s train on the first 80%:

TRAINING_CUT_OFF = int(len(rotten_X) * 0.8)
regr = LinearRegression()
regr.fit(rotten_X[:TRAINING_CUT_OFF], rotten_y[:TRAINING_CUT_OFF])

Now let’s see how we’re doing on the last 20%:

error = (regr.predict(rotten_X[TRAINING_CUT_OFF:]) -
         rotten_y[TRAINING_CUT_OFF:])
'mean square error %2.2f' % np.mean(error ** 2)
mean square error 0.06

That looks really impressive! But while it is a testament to how effective linear regression can be, there is an issue with our data that makes predicting the Rotten Tomatoes score easier: we’ve been training on the top 10,000 movies, and while popular movies aren’t always better, on average they do get better ratings.

We can get an idea of how well we’re doing by comparing our predictions with just always predicting the average score:

error = (np.mean(rotten_y[:TRAINING_CUT_OFF]) - rotten_y[TRAINING_CUT_OFF:])
'mean square error %2.2f' % np.mean(error ** 2)
'mean square error 0.09'

Our model does perform quite a bit better, but the underlying data makes it easy to produce a reasonable result.

Discussion

Complex problems often need complex solutions, and deep learning can definitely give us those. However, starting with the simplest thing that could possibly work is often a good approach. It gets us started quickly and gives us an idea of whether we’re looking in the right direction: if the simple model doesn’t produce any useful results at all it’s not that likely that a complex model will help, whereas if the simple model does work there’s a good chance that a more complex model can help us achieve better results.

Linear regression models are as simple as they come. The model tries to find a set of factors such that the linear combination of these factors and our vectors approach the target value as closely as possible. One nice aspect of these models compared to most machine learning models is that we can actually see what the contribution of each of the factors is.

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