FIX trigger indptr/indices copy when data are copied in astype (scipy…

…#18192)

* FIX trigger indptr/indices copy when data are copied in astype

* TST add unit tests

* iter

* Add comment regarding object memview support

scipy/sparse/_csparsetools.pyx.in

@@ -176,7 +176,15 @@ def _lil_get_lengths_{{NAME}}(object[:] input,
 
 {{define_dispatch_map('_LIL_GET_LENGTHS_DISPATCH', '_lil_get_lengths', IDX_TYPES)}}
 
-def lil_flatten_to_array(object[:] input,
+# We define the fuse type below because Cython does not currently allow to
+# declare object memory views (cf. https://github.com/cython/cython/issues/2485)
+# We can track the support of object memory views in
+# https://github.com/cython/cython/pull/4712
+ctypedef fused obj_fused:
+    object
+    double
+
+def lil_flatten_to_array(const obj_fused[:] input,
                          cnp.ndarray output):
     return _LIL_FLATTEN_TO_ARRAY_DISPATCH[output.dtype](input, output)
 
@@ -311,7 +319,7 @@ def _lil_fancy_set_{{PYIDX}}_{{PYVALUE}}(cnp.npy_intp M, cnp.npy_intp N,
 
 
 def lil_get_row_ranges(cnp.npy_intp M, cnp.npy_intp N,
-                       object[:] rows, object[:] datas,
+                       const obj_fused[:] rows, const obj_fused[:] datas,
                        object[:] new_rows, object[:] new_datas,
                        object irows,
                        cnp.npy_intp j_start,

scipy/sparse/_data.py

@@ -68,9 +68,11 @@ def __itruediv__(self, other):  # self /= other
     def astype(self, dtype, casting='unsafe', copy=True):
         dtype = np.dtype(dtype)
         if self.dtype != dtype:
-            return self._with_data(
-                self._deduped_data().astype(dtype, casting=casting, copy=copy),
-                copy=copy)
+            matrix = self._with_data(
+                self.data.astype(dtype, casting=casting, copy=True),
+                copy=True
+            )
+            return matrix._with_data(matrix._deduped_data(), copy=False)
         elif copy:
             return self.copy()
         else:

scipy/sparse/tests/test_base.py

@@ -1294,6 +1294,24 @@ def check_equal_but_not_same_array_attribute(attribute):
                 for attribute in ('offsets', 'data'):
                     check_equal_but_not_same_array_attribute(attribute)
 
+    @sup_complex
+    def test_astype_immutable(self):
+        D = array([[2.0 + 3j, 0, 0],
+                   [0, 4.0 + 5j, 0],
+                   [0, 0, 0]])
+        S = self.spmatrix(D)
+        if hasattr(S, 'data'):
+            S.data.flags.writeable = False
+        if hasattr(S, 'indptr'):
+            S.indptr.flags.writeable = False
+        if hasattr(S, 'indices'):
+            S.indices.flags.writeable = False
+        for x in supported_dtypes:
+            D_casted = D.astype(x)
+            S_casted = S.astype(x)
+            assert_equal(S_casted.dtype, D_casted.dtype)
+
+
     def test_asfptype(self):
         A = self.spmatrix(arange(6,dtype='int32').reshape(2,3))