Add malletmodel2ldamodel transformation function (#766)

gensim/models/wrappers/ldamallet.py

@@ -44,6 +44,7 @@
 
 from gensim import utils, matutils
 from gensim.utils import check_output
+from gensim.models.ldamodel import LdaModel
 
 logger = logging.getLogger(__name__)
 
@@ -350,3 +351,27 @@ def read_doctopics(self, fname, eps=1e-6, renorm=True):
                     if total_weight:
                         doc = [(id_, float(weight) / total_weight) for id_, weight in doc]
                 yield doc
+
+
+def malletmodel2ldamodel(mallet_model, gamma_threshold=0.001, iterations=50):
+    """
+    Function to convert mallet model to gensim LdaModel. This works by copying the
+    training model weights (alpha, beta...) from a trained mallet model into the
+    gensim model.
+
+    Args:
+    ----
+    mallet_model : Trained mallet model
+    gamma_threshold : To be used for inference in the new LdaModel.
+    iterations : number of iterations to be used for inference in the new LdaModel.
+
+    Returns:
+    -------
+    model_gensim : LdaModel instance; copied gensim LdaModel
+    """
+    model_gensim = LdaModel(
+        id2word=mallet_model.id2word, num_topics=mallet_model.num_topics,
+        alpha=mallet_model.alpha, iterations=iterations,
+        gamma_threshold=gamma_threshold)
+    model_gensim.expElogbeta[:] = mallet_model.wordtopics
+    return model_gensim

gensim/test/test_ldamallet_wrapper.py

@@ -95,6 +95,18 @@ def testSparseTransform(self):
                             (i, sorted(vec), sorted(expected)))
         self.assertTrue(passed)
 
+    def testMallet2Model(self):
+        if not self.mallet_path:
+            return
+        passed = False
+        tm1 = ldamallet.LdaMallet(self.mallet_path, corpus=corpus, num_topics=2, id2word=dictionary)
+        tm2 = ldamallet.malletmodel2ldamodel(tm1)
+        for document in corpus:
+            self.assertEqual(tm1[document][0], tm2[document][0])
+            self.assertEqual(tm1[document][1], tm2[document][1])
+            logging.debug('%d %d', tm1[document][0], tm2[document][0])
+            logging.debug('%d %d', tm1[document][1], tm2[document][1])
+
 
     def testPersistence(self):
         if not self.mallet_path: