BUG: Fix CastToType to handle string->string casts (ticket #1748)

2 files changed, 38 insertions, 26 deletions

diff --git a/numpy/core/src/multiarray/convert_datatype.c b/numpy/core/src/multiarray/convert_datatype.c
index b24869f89..3359a5573 100644
--- a/numpy/core/src/multiarray/convert_datatype.c
+++ b/numpy/core/src/multiarray/convert_datatype.c
@@ -23,54 +23,56 @@
  * Cast an array using typecode structure.
  * steals reference to at --- cannot be NULL
  *
- * This function always makes a copy of mp, even if the dtype
+ * This function always makes a copy of arr, even if the dtype
  * doesn't change.
  */
 NPY_NO_EXPORT PyObject *
-PyArray_CastToType(PyArrayObject *mp, PyArray_Descr *at, int fortran)
+PyArray_CastToType(PyArrayObject *arr, PyArray_Descr *dtype, int fortran)
 {
     PyObject *out;
-    int ret;
-    PyArray_Descr *mpd;
+    PyArray_Descr *arr_dtype;
 
-    mpd = mp->descr;
+    arr_dtype = PyArray_DESCR(arr);
 
-    if (at->elsize == 0) {
-        PyArray_DESCR_REPLACE(at);
-        if (at == NULL) {
+    if (dtype->elsize == 0) {
+        PyArray_DESCR_REPLACE(dtype);
+        if (dtype == NULL) {
             return NULL;
         }
-        if (mpd->type_num == PyArray_STRING &&
-            at->type_num == PyArray_UNICODE) {
-            at->elsize = mpd->elsize << 2;
+
+        if (arr_dtype->type_num == dtype->type_num) {
+            dtype->elsize = arr_dtype->elsize;
+        }
+        else if (arr_dtype->type_num == NPY_STRING &&
+                                dtype->type_num == NPY_UNICODE) {
+            dtype->elsize = arr_dtype->elsize * 4;
         }
-        if (mpd->type_num == PyArray_UNICODE &&
-            at->type_num == PyArray_STRING) {
-            at->elsize = mpd->elsize >> 2;
+        else if (arr_dtype->type_num == NPY_UNICODE &&
+                                dtype->type_num == NPY_STRING) {
+            dtype->elsize = arr_dtype->elsize / 4;
         }
-        if (at->type_num == PyArray_VOID) {
-            at->elsize = mpd->elsize;
+        else if (dtype->type_num == NPY_VOID) {
+            dtype->elsize = arr_dtype->elsize;
         }
     }
 
-    out = PyArray_NewFromDescr(Py_TYPE(mp), at,
-                               mp->nd,
-                               mp->dimensions,
+    out = PyArray_NewFromDescr(Py_TYPE(arr), dtype,
+                               arr->nd,
+                               arr->dimensions,
                                NULL, NULL,
                                fortran,
-                               (PyObject *)mp);
+                               (PyObject *)arr);
 
     if (out == NULL) {
         return NULL;
     }
-    ret = PyArray_CopyInto((PyArrayObject *)out, mp);
-    if (ret != -1) {
-        return out;
-    }
 
-    Py_DECREF(out);
-    return NULL;
+    if (PyArray_CopyInto((PyArrayObject *)out, arr) < 0) {
+        Py_DECREF(out);
+        return NULL;
+    }
 
+    return out;
 }
 
 /*NUMPY_API
diff --git a/numpy/core/tests/test_regression.py b/numpy/core/tests/test_regression.py
index 3fbbb71df..48a745bfe 100644
--- a/numpy/core/tests/test_regression.py
+++ b/numpy/core/tests/test_regression.py
@@ -1535,5 +1535,15 @@ class TestRegression(TestCase):
         a[()] = np.array(4)
         assert_equal(a, np.array(4))
 
+    def test_string_astype(self):
+        "Ticket #1748"
+        s1 = asbytes('black')
+        s2 = asbytes('white')
+        s3 = asbytes('other')
+        a = np.array([[s1],[s2],[s3]])
+        assert_equal(a.dtype, np.dtype('S5'))
+        b = a.astype('str')
+        assert_equal(b.dtype, np.dtype('S5'))
+
 if __name__ == "__main__":
     run_module_suite()