Merge pull request #18115 from seberg/half-promotion

BUG: Fix promotion of half and string

2 files changed, 21 insertions, 4 deletions

diff --git a/numpy/core/src/multiarray/dtypemeta.c b/numpy/core/src/multiarray/dtypemeta.c
index 4c11723e7..b1cd074a0 100644
--- a/numpy/core/src/multiarray/dtypemeta.c
+++ b/numpy/core/src/multiarray/dtypemeta.c
@@ -375,7 +375,10 @@ default_builtin_common_dtype(PyArray_DTypeMeta *cls, PyArray_DTypeMeta *other)
 {
     assert(cls->type_num < NPY_NTYPES);
     if (!other->legacy || other->type_num > cls->type_num) {
-        /* Let the more generic (larger type number) DType handle this */
+        /*
+         * Let the more generic (larger type number) DType handle this
+         * (note that half is after all others, which works out here.)
+         */
         Py_INCREF(Py_NotImplemented);
         return (PyArray_DTypeMeta *)Py_NotImplemented;
     }
@@ -398,9 +401,8 @@ static PyArray_DTypeMeta *
 string_unicode_common_dtype(PyArray_DTypeMeta *cls, PyArray_DTypeMeta *other)
 {
     assert(cls->type_num < NPY_NTYPES);
-    if (!other->legacy || other->type_num > cls->type_num ||
-        other->type_num == NPY_OBJECT) {
-        /* Let the more generic (larger type number) DType handle this */
+    if (!other->legacy || (!PyTypeNum_ISNUMBER(other->type_num) &&
+            (cls->type_num == NPY_STRING && other->type_num == NPY_UNICODE))) {
         Py_INCREF(Py_NotImplemented);
         return (PyArray_DTypeMeta *)Py_NotImplemented;
     }
diff --git a/numpy/core/tests/test_half.py b/numpy/core/tests/test_half.py
index ae9827bc7..1b6fd21e1 100644
--- a/numpy/core/tests/test_half.py
+++ b/numpy/core/tests/test_half.py
@@ -67,6 +67,21 @@ class TestHalf:
         j = np.array(i_f16, dtype=int)
         assert_equal(i_int, j)
 
+    @pytest.mark.parametrize("string_dt", ["S", "U"])
+    def test_half_conversion_to_string(self, string_dt):
+        # Currently uses S/U32 (which is sufficient for float32)
+        expected_dt = np.dtype(f"{string_dt}32")
+        assert np.promote_types(np.float16, string_dt) == expected_dt
+        assert np.promote_types(string_dt, np.float16) == expected_dt
+
+        arr = np.ones(3, dtype=np.float16).astype(string_dt)
+        assert arr.dtype == expected_dt
+
+    @pytest.mark.parametrize("string_dt", ["S", "U"])
+    def test_half_conversion_from_string(self, string_dt):
+        string = np.array("3.1416", dtype=string_dt)
+        assert string.astype(np.float16) == np.array(3.1416, dtype=np.float16)
+
     @pytest.mark.parametrize("offset", [None, "up", "down"])
     @pytest.mark.parametrize("shift", [None, "up", "down"])
     @pytest.mark.parametrize("float_t", [np.float32, np.float64])