[MRG+1] BUG: MultiLabelBinarizer.fit_transform sometimes returns an invalid CSR matrix (scikit-learn#7750)

* BUG: MultiLabelBinarizer makes invalid CSR matrix

See scipy/scipy#6719 for context.

The gist is that the `inverse` array may have a different dtype than `yt.indices`, which causes trouble down the line because, in those cases, `yt.indices` and `yt.indptr` have different dtypes.

Alternately, we could insert `yt.check_format(full_check=False)` after modifying the sparse matrix members.

* Fixing for old numpy

Older versions don't support kwargs for `astype`

* Adding tests

* line-wrapping

* adding comment to tests

[ci skip]

* added rationale comment

[ci skip]

Author: perimosocordiae

sklearn/preprocessing/label.py

@@ -732,7 +732,9 @@ def fit_transform(self, y):
         class_mapping = np.empty(len(tmp), dtype=dtype)
         class_mapping[:] = tmp
         self.classes_, inverse = np.unique(class_mapping, return_inverse=True)
-        yt.indices = np.take(inverse, yt.indices)
+        # ensure yt.indices keeps its current dtype
+        yt.indices = np.array(inverse[yt.indices], dtype=yt.indices.dtype,
+                              copy=False)
 
         if not self.sparse_output:
             yt = yt.toarray()

sklearn/preprocessing/tests/test_label.py

@@ -226,6 +226,8 @@ def test_sparse_output_multilabel_binarizer():
             got = mlb.fit_transform(inp())
             assert_equal(issparse(got), sparse_output)
             if sparse_output:
+                # verify CSR assumption that indices and indptr have same dtype
+                assert_equal(got.indices.dtype, got.indptr.dtype)
                 got = got.toarray()
             assert_array_equal(indicator_mat, got)
             assert_array_equal([1, 2, 3], mlb.classes_)
@@ -236,6 +238,8 @@ def test_sparse_output_multilabel_binarizer():
             got = mlb.fit(inp()).transform(inp())
             assert_equal(issparse(got), sparse_output)
             if sparse_output:
+                # verify CSR assumption that indices and indptr have same dtype
+                assert_equal(got.indices.dtype, got.indptr.dtype)
                 got = got.toarray()
             assert_array_equal(indicator_mat, got)
             assert_array_equal([1, 2, 3], mlb.classes_)