BUG: fix where not filling string types properly

the copyswap part of where used the input arrays descriptions to copy
into the destination so if they had a smaller size the destination was
not properly padded with zeros.
Closes gh-4778

2 files changed, 18 insertions, 4 deletions

diff --git a/numpy/core/src/multiarray/multiarraymodule.c b/numpy/core/src/multiarray/multiarraymodule.c
index c34bdf583..682705a1b 100644
--- a/numpy/core/src/multiarray/multiarraymodule.c
+++ b/numpy/core/src/multiarray/multiarraymodule.c
@@ -2790,6 +2790,9 @@ PyArray_Where(PyObject *condition, PyObject *x, PyObject *y)
 
         needs_api = NpyIter_IterationNeedsAPI(iter);
 
+        /* Get the result from the iterator object array */
+        ret = (PyObject*)NpyIter_GetOperandArray(iter)[0];
+
         NPY_BEGIN_THREADS_NDITER(iter);
 
         if (NpyIter_GetIterSize(iter) != 0) {
@@ -2836,10 +2839,10 @@ PyArray_Where(PyObject *condition, PyObject *x, PyObject *y)
                     npy_intp i;
                     for (i = 0; i < n; i++) {
                         if (*csrc) {
-                            copyswapx(dst, xsrc, axswap, ax);
+                            copyswapx(dst, xsrc, axswap, ret);
                         }
                         else {
-                            copyswapy(dst, ysrc, ayswap, ay);
+                            copyswapy(dst, ysrc, ayswap, ret);
                         }
                         dst += itemsize;
                         xsrc += xstride;
@@ -2852,8 +2855,6 @@ PyArray_Where(PyObject *condition, PyObject *x, PyObject *y)
 
         NPY_END_THREADS;
 
-        /* Get the result from the iterator object array */
-        ret = (PyObject*)NpyIter_GetOperandArray(iter)[0];
         Py_INCREF(ret);
         Py_DECREF(arr);
         Py_DECREF(ax);
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index bc3670653..2e40a2b7c 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -4354,6 +4354,19 @@ class TestWhere(TestCase):
         assert_raises(ValueError, np.where, c, a, a)
         assert_raises(ValueError, np.where, c[0], a, b)
 
+    def test_string(self):
+        # gh-4778 check strings are properly filled with nulls
+        a = np.array("abc")
+        b = np.array("x" * 753)
+        assert_equal(np.where(True, a, b), "abc")
+        assert_equal(np.where(False, b, a), "abc")
+
+        # check native datatype sized strings
+        a = np.array("abcd")
+        b = np.array("x" * 8)
+        assert_equal(np.where(True, a, b), "abcd")
+        assert_equal(np.where(False, b, a), "abcd")
+
 
 if __name__ == "__main__":
     run_module_suite()