array: set exception message on failure

When attempting to get a raw array pointer we return nullptr if given a
nullptr, which triggers an error_already_set(), but we haven't set an
exception message, which results in "Unknown internal error".

Callers that want explicit allowing of a nullptr here already handle it
(by clearing the exception after the call).

include/pybind11/numpy.h

@@ -702,8 +702,10 @@ protected:
 
     /// Create array from any object -- always returns a new reference
     static PyObject *raw_array(PyObject *ptr, int ExtraFlags = 0) {
-        if (ptr == nullptr)
+        if (ptr == nullptr) {
+            PyErr_SetString(PyExc_ValueError, "cannot create a pybind11::array from a nullptr");
             return nullptr;
+        }
         return detail::npy_api::get().PyArray_FromAny_(
             ptr, nullptr, 0, 0, detail::npy_api::NPY_ARRAY_ENSUREARRAY_ | ExtraFlags, nullptr);
     }
@@ -808,8 +810,10 @@ public:
 protected:
     /// Create array from any object -- always returns a new reference
     static PyObject *raw_array_t(PyObject *ptr) {
-        if (ptr == nullptr)
+        if (ptr == nullptr) {
+            PyErr_SetString(PyExc_ValueError, "cannot create a pybind11::array_t from a nullptr");
             return nullptr;
+        }
         return detail::npy_api::get().PyArray_FromAny_(
             ptr, dtype::of<T>().release().ptr(), 0, 0,
             detail::npy_api::NPY_ARRAY_ENSUREARRAY_ | ExtraFlags, nullptr);

tests/test_numpy_array.cpp

@@ -264,4 +264,8 @@ test_initializer numpy_array([](py::module &m) {
     sm.def("array_auxiliaries2", [](py::array_t<double> a) {
         return auxiliaries(a, a);
     });
+
+    // Issue #785: Uninformative "Unknown internal error" exception when constructing array from empty object:
+    sm.def("array_fail_test", []() { return py::array(py::object()); });
+    sm.def("array_t_fail_test", []() { return py::array_t<double>(py::object()); });
 });

tests/test_numpy_array.py

@@ -377,3 +377,15 @@ def test_array_unchecked_dyn_dims(msg):
 
     assert proxy_auxiliaries2_dyn(z1) == [11, 11, True, 2, 8, 2, 2, 4, 32]
     assert proxy_auxiliaries2_dyn(z1) == array_auxiliaries2(z1)
+
+
+def test_array_failure():
+    from pybind11_tests.array import array_fail_test, array_t_fail_test
+
+    with pytest.raises(ValueError) as excinfo:
+        array_fail_test()
+    assert str(excinfo.value) == 'cannot create a pybind11::array from a nullptr'
+
+    with pytest.raises(ValueError) as excinfo:
+        array_t_fail_test()
+    assert str(excinfo.value) == 'cannot create a pybind11::array_t from a nullptr'