BUG: ensure reduction output matches input along non-reduction axes.

Previously, things like
`np.add.reduce(np.ones((3, 3)), axis=0, out=np.empty(1))`
worked, even though the output should have shape (3,).

2 files changed, 38 insertions, 2 deletions

diff --git a/numpy/core/src/umath/reduction.c b/numpy/core/src/umath/reduction.c
index 4ce8d8ab7..79c302755 100644
--- a/numpy/core/src/umath/reduction.c
+++ b/numpy/core/src/umath/reduction.c
@@ -84,10 +84,12 @@ allocate_reduce_result(PyArrayObject *arr, const npy_bool *axis_flags,
  * The return value is a view into 'out'.
  */
 static PyArrayObject *
-conform_reduce_result(int ndim, const npy_bool *axis_flags,
+conform_reduce_result(PyArrayObject *in, const npy_bool *axis_flags,
                       PyArrayObject *out, int keepdims, const char *funcname,
                       int need_copy)
 {
+    int ndim = PyArray_NDIM(in);
+    npy_intp *shape_in = PyArray_DIMS(in);
     npy_intp strides[NPY_MAXDIMS], shape[NPY_MAXDIMS];
     npy_intp *strides_out = PyArray_STRIDES(out);
     npy_intp *shape_out = PyArray_DIMS(out);
@@ -118,6 +120,16 @@ conform_reduce_result(int ndim, const npy_bool *axis_flags,
                     return NULL;
                 }
             }
+            else {
+                if (shape_out[idim] != shape_in[idim]) {
+                    PyErr_Format(PyExc_ValueError,
+                            "output parameter for reduction operation %s "
+                            "has a non-reduction dimension not equal to "
+                            "the input one.", funcname);
+                    return NULL;
+                }
+            }
+
         }
 
         Py_INCREF(out);
@@ -138,6 +150,13 @@ conform_reduce_result(int ndim, const npy_bool *axis_flags,
                         "does not have enough dimensions", funcname);
                 return NULL;
             }
+            if (shape_out[idim_out] != shape_in[idim]) {
+                PyErr_Format(PyExc_ValueError,
+                        "output parameter for reduction operation %s "
+                        "has a non-reduction dimension not equal to "
+                        "the input one.", funcname);
+                return NULL;
+            }
             strides[idim] = strides_out[idim_out];
             shape[idim] = shape_out[idim_out];
             ++idim_out;
@@ -240,7 +259,7 @@ PyArray_CreateReduceResult(PyArrayObject *operand, PyArrayObject *out,
 
         /* Steal the dtype reference */
         Py_XDECREF(dtype);
-        result = conform_reduce_result(PyArray_NDIM(operand), axis_flags,
+        result = conform_reduce_result(operand, axis_flags,
                                        out, keepdims, funcname, need_copy);
     }
 
diff --git a/numpy/core/tests/test_ufunc.py b/numpy/core/tests/test_ufunc.py
index 7109de776..4c04aceb3 100644
--- a/numpy/core/tests/test_ufunc.py
+++ b/numpy/core/tests/test_ufunc.py
@@ -1888,6 +1888,23 @@ class TestUfunc(object):
         assert_equal(y_base[1,:], y_base_copy[1,:])
         assert_equal(y_base[3,:], y_base_copy[3,:])
 
+    @pytest.mark.parametrize('output_shape',
+                             [(), (1,), (1, 1), (1, 3), (4, 3)])
+    @pytest.mark.parametrize('f_reduce', [np.add.reduce, np.minimum.reduce])
+    def test_reduce_wrong_dimension_output(self, f_reduce, output_shape):
+        # Test that we're not incorrectly broadcasting dimensions.
+        # See gh-15144 (failed for np.add.reduce previously).
+        a = np.arange(12.).reshape(4, 3)
+        out = np.empty(output_shape, a.dtype)
+        assert_raises(ValueError, f_reduce, a, axis=0, out=out)
+        if output_shape != (1, 3):
+            assert_raises(ValueError, f_reduce, a, axis=0, out=out,
+                          keepdims=True)
+        else:
+            check = f_reduce(a, axis=0, out=out, keepdims=True)
+            assert_(check is out)
+            assert_array_equal(check, f_reduce(a, axis=0, keepdims=True))
+
     def test_no_doc_string(self):
         # gh-9337
         assert_('\n' not in umt.inner1d_no_doc.__doc__)