Handling for iterables without 0-th element, fixes #2556 (#2629)

* Handling for iterables without 0-th element, fixes #2556

* Improved accessing the first element for the case of big datasets

gensim/sklearn_api/d2vmodel.py

@@ -159,7 +159,7 @@ def fit(self, X, y=None):
             The trained model.
 
         """
-        if isinstance(X[0], doc2vec.TaggedDocument):
+        if isinstance([i for i in X[:1]][0], doc2vec.TaggedDocument):
             d2v_sentences = X
         else:
             d2v_sentences = [doc2vec.TaggedDocument(words, [i]) for i, words in enumerate(X)]

gensim/test/test_d2vmodel.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2010 Radim Rehurek <radimrehurek@seznam.cz>
+# Licensed under the GNU LGPL v2.1 - http://www.gnu.org/licenses/lgpl.html
+
+"""
+Automated tests for checking D2VTransformer class.
+"""
+
+import unittest
+import logging
+from gensim.sklearn_api import D2VTransformer
+from gensim.test.utils import common_texts
+
+
+class IteratorForIterable:
+    """Iterator capable of folding into list."""
+    def __init__(self, iterable):
+        self._data = iterable
+        self._index = 0
+
+    def __next__(self):
+        if len(self._data) > self._index:
+            result = self._data[self._index]
+            self._index += 1
+            return result
+        raise StopIteration
+
+
+class IterableWithoutZeroElement:
+    """
+    Iterable, emulating pandas.Series behaviour without 0-th element.
+    Equivalent to calling `series.index += 1`.
+    """
+    def __init__(self, data):
+        self.data = data
+
+    def __getitem__(self, key):
+        if key == 0:
+            raise KeyError("Emulation of absence of item with key 0.")
+        return self.data[key]
+
+    def __iter__(self):
+        return IteratorForIterable(self.data)
+
+
+class TestD2VTransformer(unittest.TestCase):
+    def TestWorksWithIterableNotHavingElementWithZeroIndex(self):
+        a = IterableWithoutZeroElement(common_texts)
+        transformer = D2VTransformer(min_count=1, size=5)
+        transformer.fit(a)
+
+
+if __name__ == '__main__':
+    logging.basicConfig(format='%(asctime)s : %(levelname)s : %(message)s', level=logging.DEBUG)
+    unittest.main()