API: Disallow strings in logical ufuncs (numpy#21024)

* API: Disallow strings in logical ufuncs

This restores pre 1.22 NumPy behaviour for strings passed into logical
ufuncs.  Logical ufuncs should be able to cast inputs to booleans
without modifying their results.
Thus they do this now (in NumPy 1.22+).  The problem is that string to
bool casts are very strange in NumPy currently.

This should be reverted/removed once string to bool casts are well
defined.  But until then, it seems more reasonable to just reject
strings.

Closes numpygh-20898

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

numpy/core/src/umath/dispatching.c

@@ -1018,6 +1018,11 @@ logical_ufunc_promoter(PyUFuncObject *NPY_UNUSED(ufunc),
         /* bail out, this is _only_ to give future/deprecation warning! */
         return -1;
     }
+    if ((op_dtypes[0] != NULL && PyTypeNum_ISSTRING(op_dtypes[0]->type_num))
+            || PyTypeNum_ISSTRING(op_dtypes[1]->type_num)) {
+        /* bail out on strings: currently casting them to bool is too weird */
+        return -1;
+    }
 
     for (int i = 0; i < 3; i++) {
         PyArray_DTypeMeta *item;

numpy/core/tests/test_ufunc.py

@@ -2134,7 +2134,7 @@ def test_logical_ufuncs_mixed_object_signatures(self, ufunc, signature):
             [np.logical_and, np.logical_or, np.logical_xor])
     def test_logical_ufuncs_support_anything(self, ufunc):
         # The logical ufuncs support even input that can't be promoted:
-        a = np.array('1')
+        a = np.array(b'1', dtype="V3")
         c = np.array([1., 2.])
         assert_array_equal(ufunc(a, c), ufunc([True, True], True))
         assert ufunc.reduce(a) == True
@@ -2150,6 +2150,23 @@ def test_logical_ufuncs_support_anything(self, ufunc):
         out = np.zeros((), dtype=a.dtype)
         assert ufunc.reduce(a, out=out) == 1
 
+    @pytest.mark.parametrize("ufunc",
+            [np.logical_and, np.logical_or, np.logical_xor])
+    def test_logical_ufuncs_reject_string(self, ufunc):
+        """
+        Logical ufuncs are normally well defined by working with the boolean
+        equivalent, i.e. casting all inputs to bools should work.
+
+        However, casting strings to bools is *currently* weird, because it
+        actually uses `bool(int(str))`.  Thus we explicitly reject strings.
+        This test should succeed (and can probably just be removed) as soon as
+        string to bool casts are well defined in NumPy.
+        """
+        with pytest.raises(TypeError, match="contain a loop with signature"):
+            ufunc(["1"], ["3"])
+        with pytest.raises(TypeError, match="contain a loop with signature"):
+            ufunc.reduce(["1", "2", "0"])
+
     @pytest.mark.parametrize("ufunc",
              [np.logical_and, np.logical_or, np.logical_xor])
     def test_logical_ufuncs_out_cast_check(self, ufunc):