Sparse2Corpus: update __getitem__ to work on slices, lists and ellips…

…is (#3247)

* Sparse2Corpus: update __getitem__ to work on slices, ellipsis, and iterable

* Sparse2Corpus: update __getitem__ to work on slices, ellipsis, and iterable

* Update CHANGELOG.md

Co-authored-by: Michael Penkov <m@penkov.dev>

CHANGELOG.md

@@ -6,6 +6,7 @@ Changes
 * [#3194](https://github.com/RaRe-Technologies/gensim/pull/3194): Added random_seed parameter to make LsiModel reproducible, by [@parashardhapola](https://github.com/parashardhapola)
 * [#3251](https://github.com/RaRe-Technologies/gensim/pull/3251): Apply new convention of delimiting instance params in str function, by [@menshikh-iv](https://github.com/menshikh-iv)
 * [#3227](https://github.com/RaRe-Technologies/gensim/pull/3227): Fix FastText doc-comment example for `build_vocab` and `train` to use correct argument names, by [@HLasse](https://github.com/HLasse)
+* [#3247](https://github.com/RaRe-Technologies/gensim/pull/3247): Sparse2Corpus: update __getitem__ to work on slices, lists and ellipsis, by [@PrimozGodec](https://github.com/PrimozGodec)
 * [#3250](https://github.com/RaRe-Technologies/gensim/pull/3250): Make negative ns_exponent work correctly, by [@menshikh-iv](https://github.com/menshikh-iv)
 * [#3258](https://github.com/RaRe-Technologies/gensim/pull/3258): Adding another check to _check_corpus_sanity for compressed files, adding test, by [@dchaplinsky](https://github.com/dchaplinsky)
 

gensim/matutils.py

@@ -597,23 +597,30 @@ def __iter__(self):
     def __len__(self):
         return self.sparse.shape[1]
 
-    def __getitem__(self, document_index):
-        """Retrieve a document vector from the corpus by its index.
+    def __getitem__(self, key):
+        """
+        Retrieve a document vector or subset from the corpus by key.
 
         Parameters
         ----------
-        document_index : int
-            Index of document
+        key: int, ellipsis, slice, iterable object
+            Index of the document retrieve.
+            Less commonly, the key can also be a slice, ellipsis, or an iterable
+            to retrieve multiple documents.
 
         Returns
         -------
-        list of (int, number)
-            Document in BoW format.
-
+        list of (int, number), Sparse2Corpus
+            Document in BoW format when `key` is an integer. Otherwise :class:`~gensim.matutils.Sparse2Corpus`.
         """
-        indprev = self.sparse.indptr[document_index]
-        indnow = self.sparse.indptr[document_index + 1]
-        return list(zip(self.sparse.indices[indprev:indnow], self.sparse.data[indprev:indnow]))
+        sparse = self.sparse
+        if isinstance(key, int):
+            iprev = self.sparse.indptr[key]
+            inow = self.sparse.indptr[key + 1]
+            return list(zip(sparse.indices[iprev:inow], sparse.data[iprev:inow]))
+
+        sparse = self.sparse.__getitem__((slice(None, None, None), key))
+        return Sparse2Corpus(sparse)
 
 
 def veclen(vec):

gensim/test/test_matutils.py

@@ -7,7 +7,9 @@
 import logging
 import unittest
 import numpy as np
+from numpy.testing import assert_array_equal
 from scipy import sparse
+from scipy.sparse import csc_matrix
 from scipy.special import psi  # gamma function utils
 
 import gensim.matutils as matutils
@@ -266,6 +268,44 @@ def test_return_norm_zero_vector_gensim_sparse(self):
         self.assertEqual(norm, 1.0)
 
 
+class TestSparse2Corpus(unittest.TestCase):
+    def setUp(self):
+        self.orig_array = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
+        self.s2c = matutils.Sparse2Corpus(csc_matrix(self.orig_array))
+
+    def test_getitem_slice(self):
+        assert_array_equal(self.s2c[:2].sparse.toarray(), self.orig_array[:, :2])
+        assert_array_equal(self.s2c[1:3].sparse.toarray(), self.orig_array[:, 1:3])
+
+    def test_getitem_index(self):
+        self.assertListEqual(self.s2c[1], [(0, 2), (1, 5), (2, 8)])
+
+    def test_getitem_list_of_indices(self):
+        assert_array_equal(
+            self.s2c[[1, 2]].sparse.toarray(), self.orig_array[:, [1, 2]]
+        )
+        assert_array_equal(self.s2c[[1]].sparse.toarray(), self.orig_array[:, [1]])
+
+    def test_getitem_ndarray(self):
+        assert_array_equal(
+            self.s2c[np.array([1, 2])].sparse.toarray(), self.orig_array[:, [1, 2]]
+        )
+        assert_array_equal(
+            self.s2c[np.array([1])].sparse.toarray(), self.orig_array[:, [1]]
+        )
+
+    def test_getitem_range(self):
+        assert_array_equal(
+            self.s2c[range(1, 3)].sparse.toarray(), self.orig_array[:, [1, 2]]
+        )
+        assert_array_equal(
+            self.s2c[range(1, 2)].sparse.toarray(), self.orig_array[:, [1]]
+        )
+
+    def test_getitem_ellipsis(self):
+        assert_array_equal(self.s2c[...].sparse.toarray(), self.orig_array)
+
+
 if __name__ == '__main__':
     logging.basicConfig(format='%(asctime)s : %(levelname)s : %(message)s', level=logging.DEBUG)
     unittest.main()