svmlightcorpus.py: Add sequence serialization of corpus

Current version of serialization support only lists, but this
adds support for any type of sequence.

Closes: #2113

gensim/corpora/svmlightcorpus.py

@@ -111,6 +111,9 @@ def save_corpus(fname, corpus, id2word=None, labels=False, metadata=False):
         """
         logger.info("converting corpus to SVMlight format: %s", fname)
 
+        if labels is not False:
+            # Cast any sequence (incl. a numpy array) to a list, to simplify the processing below.
+            labels = list(labels)
         offsets = []
         with utils.smart_open(fname, 'wb') as fout:
             for docno, doc in enumerate(corpus):

gensim/test/test_corpora.py

@@ -23,7 +23,7 @@
                             ucicorpus, malletcorpus, textcorpus, indexedcorpus, wikicorpus)
 from gensim.interfaces import TransformedCorpus
 from gensim.utils import to_unicode
-from gensim.test.utils import datapath, get_tmpfile
+from gensim.test.utils import datapath, get_tmpfile, common_corpus
 
 
 class DummyTransformer(object):
@@ -382,6 +382,17 @@ def setUp(self):
         self.corpus_class = svmlightcorpus.SvmLightCorpus
         self.file_extension = '.svmlight'
 
+    def test_serialization(self):
+        path = get_tmpfile("svml.corpus")
+        labels = [1] * len(common_corpus)
+        second_corpus = [(0, 1.0), (3, 1.0), (4, 1.0), (5, 1.0), (6, 1.0), (7, 1.0)]
+        self.corpus_class.serialize(path, common_corpus, labels=labels)
+        serialized_corpus = self.corpus_class(path)
+        self.assertEqual(serialized_corpus[1], second_corpus)
+        self.corpus_class.serialize(path, common_corpus, labels=np.array(labels))
+        serialized_corpus = self.corpus_class(path)
+        self.assertEqual(serialized_corpus[1], second_corpus)
+
 
 class TestBleiCorpus(CorpusTestCase):
     def setUp(self):