BUG: remove fast scalar power for arrays with object dtype (#11050)

Fixes #11047 

If objects want to optimize `x**2` as `x*x`, they can do it inside their own `__pow__`, rather than relying on numpy to do it.

2 files changed, 35 insertions, 1 deletions

diff --git a/numpy/core/src/multiarray/number.c b/numpy/core/src/multiarray/number.c
index 915d743c8..14389a925 100644
--- a/numpy/core/src/multiarray/number.c
+++ b/numpy/core/src/multiarray/number.c
@@ -476,7 +476,9 @@ fast_scalar_power(PyArrayObject *a1, PyObject *o2, int inplace,
     double exponent;
     NPY_SCALARKIND kind;   /* NPY_NOSCALAR is not scalar */
 
-    if (PyArray_Check(a1) && ((kind=is_scalar_with_conversion(o2, &exponent))>0)) {
+    if (PyArray_Check(a1) &&
+            !PyArray_ISOBJECT(a1) &&
+            ((kind=is_scalar_with_conversion(o2, &exponent))>0)) {
         PyObject *fastop = NULL;
         if (PyArray_ISFLOAT(a1) || PyArray_ISCOMPLEX(a1)) {
             if (exponent == 1.0) {
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 46bdc19a7..3ca201edd 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -3317,7 +3317,39 @@ class TestBinop(object):
         with assert_raises(NotImplementedError):
             a ** 2
 
+    def test_pow_array_object_dtype(self):
+        # test pow on arrays of object dtype
+        class SomeClass(object):
+            def __init__(self, num=None):
+                self.num = num
+
+            # want to ensure a fast pow path is not taken
+            def __mul__(self, other):
+                raise AssertionError('__mul__ should not be called')
+
+            def __div__(self, other):
+                raise AssertionError('__div__ should not be called')
+            
+            def __pow__(self, exp):
+                return SomeClass(num=self.num ** exp)
+
+            def __eq__(self, other):
+                if isinstance(other, SomeClass):
+                    return self.num == other.num
+
+            __rpow__ = __pow__
+
+        def pow_for(exp, arr):
+            return np.array([x ** exp for x in arr])
+
+        obj_arr = np.array([SomeClass(1), SomeClass(2), SomeClass(3)])
 
+        assert_equal(obj_arr ** 0.5, pow_for(0.5, obj_arr))
+        assert_equal(obj_arr ** 0, pow_for(0, obj_arr))
+        assert_equal(obj_arr ** 1, pow_for(1, obj_arr))
+        assert_equal(obj_arr ** -1, pow_for(-1, obj_arr))
+        assert_equal(obj_arr ** 2, pow_for(2, obj_arr))
+        
 class TestTemporaryElide(object):
     # elision is only triggered on relatively large arrays