Hierarchical Dirichlet Process Gensim topic number independent of corpus size

Question

I am using the Gensim HDP module on a set of documents.

>>> hdp = models.HdpModel(corpusB, id2word=dictionaryB)
>>> topics = hdp.print_topics(topics=-1, topn=20)
>>> len(topics)
150
>>> hdp = models.HdpModel(corpusA, id2word=dictionaryA)
>>> topics = hdp.print_topics(topics=-1, topn=20)
>>> len(topics)
150
>>> len(corpusA)
1113
>>> len(corpusB)
17

Why is the number of topics independent of corpus length?

Answer 1

@Aaron's code above is broken due to gensim API changes. I rewrote and simplified it as follows. Works as of June 2017 with gensim v2.1.0

import pandas as pd

def topic_prob_extractor(gensim_hdp):
    shown_topics = gensim_hdp.show_topics(num_topics=-1, formatted=False)
    topics_nos = [x[0] for x in shown_topics ]
    weights = [ sum([item[1] for item in shown_topics[topicN][1]]) for topicN in topics_nos ]

    return pd.DataFrame({'topic_id' : topics_nos, 'weight' : weights})

Answer 2

@Aron's and @Roko Mijic's approaches neglect the fact that the function show_topics returns by default the top 20 words of each topic only. If one returns all the words that compose a topic, all the approximated topic probabilities in that case will be 1 (or 0.999999). I experimented with the following code, which is an adaptation of @Roko Mijic's:

def topic_prob_extractor(gensim_hdp, t=-1, w=25, isSorted=True):
    """
    Input the gensim model to get the rough topics' probabilities
    """
    shown_topics = gensim_hdp.show_topics(num_topics=t, num_words=w ,formatted=False)
    topics_nos = [x[0] for x in shown_topics ]
    weights = [ sum([item[1] for item in shown_topics[topicN][1]]) for topicN in topics_nos ]
    if (isSorted):
        return pd.DataFrame({'topic_id' : topics_nos, 'weight' : weights}).sort_values(by = "weight", ascending=False);
    else:
        return pd.DataFrame({'topic_id' : topics_nos, 'weight' : weights});

A better, yet I'm not sure if 100% valid, approach is the one mentioned here. You can get the topics' true weights (alpha vector) of the HDP model as:

alpha = hdpModel.hdp_to_lda()[0];

Examining the topics' equivalent alpha values is more logical than tallying up the weights of the first 20 words of each topic to approximate its probability of usage in the data.

Answer 3

There is apparently a bug in Gensim(version 3.8.3), in which giving -1 to show_topics doesn't return anything at all. So I have tweaked the answers by Roko Mijic and aaron.

def topic_prob_extractor(gensim_hdp):
    shown_topics = gensim_hdp.show_topics(num_topics=gensim_hdp.m_T, formatted=False)
    topics_nos = [x[0] for x in shown_topics ]
    weights = [ sum([item[1] for item in shown_topics[topicN][1]]) for topicN in topics_nos ]
    return pd.DataFrame({'topic_id' : topics_nos, 'weight' : weights})

Answer 4

@user3907335 is exactly correct here: HDP will calculate as many topics as the assigned truncation level. However, it may be the case that many of these topics have basically zero probability of occurring. To help with this in my own work, I wrote a handy little function that performs a rough estimate of the probability weight associated with each topic. Note that this is a rough metric only: it does not account for the probability associated with each word. Even so, it provides a pretty good metric for which topics are meaningful and which aren't:

import pandas as pd
import numpy as np 

def topic_prob_extractor(hdp=None, topn=None):
    topic_list = hdp.show_topics(topics=-1, topn=topn)
    topics = [int(x.split(':')[0].split(' ')[1]) for x in topic_list]
    split_list = [x.split(' ') for x in topic_list]
    weights = []
    for lst in split_list:
        sub_list = []
        for entry in lst: 
            if '*' in entry: 
                sub_list.append(float(entry.split('*')[0]))
        weights.append(np.asarray(sub_list))
    sums = [np.sum(x) for x in weights]
    return pd.DataFrame({'topic_id' : topics, 'weight' : sums})

I assume that you already know how to calculate an HDP model. Once you have an hdp model calculated by gensim you call the function as follows:

topic_weights = topic_prob_extractor(hdp, 500)

Answer 5

I think you misunderstood the operation performed by the called method. Directly from the documentation you can see:

Alias for show_topics() that prints the top n most probable words for topics number of topics to log. Set topics=-1 to print all topics.

You trained the model without specifying the truncation level on the number of topics and the default one is 150. Calling the print_topics with topics=-1 you'll get the top 20 words for each topic , in your case 150 topics.

I'm still a newbie of the library, so maybe I' wrong

Answer 6

I haven't used gensim for HDPs, but is it possible that most of the topics in the smaller corpus have extremely low probability of occurring ? Can you trying printing the topic probabilities? Maybe, the length of the topics array doesn't necessarily mean that all those topics were actually found in the corpus.

Answer 7

Deriving the average coherence of HDP topics from their coherence at the individual text level is a way to order (and potentially truncate) them. The following function does just that:

def order_subset_by_coherence(dirichlet_model, bow_corpus, num_topics=10, num_keywords=10):
    """
    Orders topics based on their average coherence across the corpus

    Parameters
    ----------
        dirichlet_model : gensim.models.hdpmodel.HdpModel
        bow_corpus : list of lists (contains (id, freq) tuples)
        num_topics : int (default=10)
        num_keywords : int (default=10)

    Returns
    -------
        ordered_topics: list of lists containing topic tokens
    """
    shown_topics = dirichlet_model.show_topics(num_topics=150, # return all topics
                                               num_words=num_keywords,
                                               formatted=False)
    model_topics = [[word[0] for word in topic[1]] for topic in shown_topics]
    topic_corpus = dirichlet_model.__getitem__(bow=bow_corpus, eps=0) # cutoff probability to 0 

    topics_per_response = [response for response in topic_corpus]
    flat_topic_coherences = [item for sublist in topics_per_response for item in sublist]

    significant_topics = list(set([t_c[0] for t_c in flat_topic_coherences])) # those that appear
    topic_averages = [sum([t_c[1] for t_c in flat_topic_coherences if t_c[0] == topic_num]) / len(bow_corpus) \
                      for topic_num in significant_topics]

    topic_indexes_by_avg_coherence = [tup[0] for tup in sorted(enumerate(topic_averages), key=lambda i:i[1])[::-1]]
    significant_topics_by_avg_coherence = [significant_topics[i] for i in topic_indexes_by_avg_coherence]
    ordered_topics = [model_topics[i] for i in significant_topics_by_avg_coherence][:num_topics] # truncate if desired

    return ordered_topics

A version of this function that includes an output of the averages coherences associated with the topics for keyword (tag) generation for a corpus can be found in this answer. A similar process for keywords for individual texts can further be found in this answer.