BUG: core: Fix *_like strides for str and bytes dtype.

When a user passed a noncontiguous array to empty_like (or any other _like
function) with dtype=str or dtype=bytes, the returned array would have
strides that were all 0, even though the data type was 'U1' or 'S1'.

The problem was in the C code in the function PyArray_NewLikeArrayWithShape.
Because the default order is "keep", the branch with the comment
/* KEEPORDER needs some analysis of the strides */ was taken. That branch
uses the itemsize (i.e. dtype->elsize) of the requested dtype, but for str
or bytes, that itemsize is 0, so the computed strides end up being 0. It
is in the subsequent call to PyArray_NewFromDescr_int that the itemsize is
updated to be nonzero.

The fix is to use the same check for str or bytes that is used in
PyArray_NewFromDescr_int in PyArray_NewLikeArrayWithShape to figure out
the itemsize when computing the strides in the KEEPORDER  branch.

Closes gh-19860.

2 files changed, 26 insertions, 0 deletions

diff --git a/numpy/core/src/multiarray/ctors.c b/numpy/core/src/multiarray/ctors.c
index 73d864a4d..04be45f00 100644
--- a/numpy/core/src/multiarray/ctors.c
+++ b/numpy/core/src/multiarray/ctors.c
@@ -1020,6 +1020,17 @@ PyArray_NewLikeArrayWithShape(PyArrayObject *prototype, NPY_ORDER order,
 
         /* Build the new strides */
         stride = dtype->elsize;
+        if (stride == 0 && PyDataType_ISSTRING(dtype)) {
+            /* Special case for dtype=str or dtype=bytes. */
+            if (dtype->type_num == NPY_STRING) {
+                /* dtype is bytes */
+                stride = 1;
+            }
+            else {
+                /* dtype is str (type_num is NPY_UNICODE) */
+                stride = 4;
+            }
+        }
         for (idim = ndim-1; idim >= 0; --idim) {
             npy_intp i_perm = strideperm[idim].perm;
             strides[i_perm] = stride;
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
index 19de0a8aa..4510333a1 100644
--- a/numpy/core/tests/test_numeric.py
+++ b/numpy/core/tests/test_numeric.py
@@ -2893,6 +2893,21 @@ class TestLikeFuncs:
         self.check_like_function(np.full_like, 123.456, True)
         self.check_like_function(np.full_like, np.inf, True)
 
+    @pytest.mark.parametrize('likefunc', [np.empty_like, np.full_like,
+                                          np.zeros_like, np.ones_like])
+    @pytest.mark.parametrize('dtype', [str, bytes])
+    def test_dtype_str_bytes(self, likefunc, dtype):
+        # Regression test for gh-19860
+        a = np.arange(16).reshape(2, 8)
+        b = a[:, ::2]  # Ensure b is not contiguous.
+        kwargs = {'fill_value': ''} if likefunc == np.full_like else {}
+        result = likefunc(b, dtype=dtype, **kwargs)
+        if dtype == str:
+            assert result.strides == (16, 4)
+        else:
+            # dtype is bytes
+            assert result.strides == (4, 1)
+
 
 class TestCorrelate:
     def _setup(self, dt):