Improve consistency of array and array_t with regard to other pytypes

* `array_t(const object &)` now throws on error
* `array_t::ensure()` is intended for casters —- old constructor is
  deprecated
* `array` and `array_t` get default constructors (empty array)
* `array` gets a converting constructor
* `py::isinstance<array_T<T>>()` checks the type (but not flags)

There is only one special thing which must remain: `array_t` gets
its own `type_caster` specialization which uses `ensure` instead
of a simple check.

include/pybind11/eigen.h

@@ -54,7 +54,7 @@ struct type_caster<Type, enable_if_t<is_eigen_dense<Type>::value && !is_eigen_re
     static constexpr bool isVector = Type::IsVectorAtCompileTime;
 
     bool load(handle src, bool) {
-        array_t<Scalar> buf(src, true);
+        auto buf = array_t<Scalar>::ensure(src);
         if (!buf)
             return false;
 

include/pybind11/numpy.h

@@ -305,7 +305,7 @@ private:
 
 class array : public buffer {
 public:
-    PYBIND11_OBJECT_DEFAULT(array, buffer, detail::npy_api::get().PyArray_Check_)
+    PYBIND11_OBJECT_CVT(array, buffer, detail::npy_api::get().PyArray_Check_, raw_array)
 
     enum {
         c_style = detail::npy_api::NPY_C_CONTIGUOUS_,
@@ -313,6 +313,8 @@ public:
         forcecast = detail::npy_api::NPY_ARRAY_FORCECAST_
     };
 
+    array() : array(0, static_cast<const double *>(nullptr)) {}
+
     array(const pybind11::dtype &dt, const std::vector<size_t> &shape,
           const std::vector<size_t> &strides, const void *ptr = nullptr,
           handle base = handle()) {
@@ -478,10 +480,12 @@ public:
     }
 
     /// Ensure that the argument is a NumPy array
-    static array ensure(object input, int ExtraFlags = 0) {
-        auto& api = detail::npy_api::get();
-        return reinterpret_steal<array>(api.PyArray_FromAny_(
-            input.release().ptr(), nullptr, 0, 0, detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr));
+    /// In case of an error, nullptr is returned and the Python error is cleared.
+    static array ensure(handle h, int ExtraFlags = 0) {
+        auto result = reinterpret_steal<array>(raw_array(h.ptr(), ExtraFlags));
+        if (!result)
+            PyErr_Clear();
+        return result;
     }
 
 protected:
@@ -520,8 +524,6 @@ protected:
         return strides;
     }
 
-protected:
-
     template<typename... Ix> void check_dimensions(Ix... index) const {
         check_dimensions_impl(size_t(0), shape(), size_t(index)...);
     }
@@ -536,15 +538,31 @@ protected:
         }
         check_dimensions_impl(axis + 1, shape + 1, index...);
     }
+
+    /// Create array from any object -- always returns a new reference
+    static PyObject *raw_array(PyObject *ptr, int ExtraFlags = 0) {
+        if (ptr == nullptr)
+            return nullptr;
+        return detail::npy_api::get().PyArray_FromAny_(
+            ptr, nullptr, 0, 0, detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr);
+    }
 };
 
 template <typename T, int ExtraFlags = array::forcecast> class array_t : public array {
 public:
-    array_t() : array() { }
+    array_t() : array(0, static_cast<const T *>(nullptr)) {}
+    array_t(handle h, borrowed_t) : array(h, borrowed) { }
+    array_t(handle h, stolen_t) : array(h, stolen) { }
 
-    array_t(handle h, bool is_borrowed) : array(h, is_borrowed) { m_ptr = ensure_(m_ptr); }
+    PYBIND11_DEPRECATED("Use array_t<T>::ensure() instead")
+    array_t(handle h, bool is_borrowed) : array(raw_array_t(h.ptr()), stolen) {
+        if (!m_ptr) PyErr_Clear();
+        if (!is_borrowed) Py_XDECREF(h.ptr());
+    }
 
-    array_t(const object &o) : array(o) { m_ptr = ensure_(m_ptr); }
+    array_t(const object &o) : array(raw_array_t(o.ptr()), stolen) {
+        if (!m_ptr) throw error_already_set();
+    }
 
     explicit array_t(const buffer_info& info) : array(info) { }
 
@@ -590,17 +608,30 @@ public:
         return *(static_cast<T*>(array::mutable_data()) + byte_offset(size_t(index)...) / itemsize());
     }
 
-    static PyObject *ensure_(PyObject *ptr) {
-        if (ptr == nullptr)
-            return nullptr;
-        auto& api = detail::npy_api::get();
-        PyObject *result = api.PyArray_FromAny_(ptr, pybind11::dtype::of<T>().release().ptr(), 0, 0,
-                                                detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr);
+    /// Ensure that the argument is a NumPy array of the correct dtype.
+    /// In case of an error, nullptr is returned and the Python error is cleared.
+    static array_t ensure(handle h) {
+        auto result = reinterpret_steal<array_t>(raw_array_t(h.ptr()));
         if (!result)
             PyErr_Clear();
-        Py_DECREF(ptr);
         return result;
     }
+
+    static bool _check(handle h) {
+        const auto &api = detail::npy_api::get();
+        return api.PyArray_Check_(h.ptr())
+               && api.PyArray_EquivTypes_(PyArray_GET_(h.ptr(), descr), dtype::of<T>().ptr());
+    }
+
+protected:
+    /// Create array from any object -- always returns a new reference
+    static PyObject *raw_array_t(PyObject *ptr) {
+        if (ptr == nullptr)
+            return nullptr;
+        return detail::npy_api::get().PyArray_FromAny_(
+            ptr, dtype::of<T>().release().ptr(), 0, 0,
+            detail::npy_api::NPY_ENSURE_ARRAY_ | ExtraFlags, nullptr);
+    }
 };
 
 template <typename T>
@@ -631,7 +662,7 @@ struct pyobject_caster<array_t<T, ExtraFlags>> {
     using type = array_t<T, ExtraFlags>;
 
     bool load(handle src, bool /* convert */) {
-        value = type(src, true);
+        value = type::ensure(src);
         return static_cast<bool>(value);
     }
 

tests/test_numpy_array.cpp

@@ -126,4 +126,28 @@ test_initializer numpy_array([](py::module &m) {
     );
 
     sm.def("function_taking_uint64", [](uint64_t) { });
+
+    sm.def("isinstance_untyped", [](py::object yes, py::object no) {
+        return py::isinstance<py::array>(yes) && !py::isinstance<py::array>(no);
+    });
+
+    sm.def("isinstance_typed", [](py::object o) {
+        return py::isinstance<py::array_t<double>>(o) && !py::isinstance<py::array_t<int>>(o);
+    });
+
+    sm.def("default_constructors", []() {
+        return py::dict(
+            "array"_a=py::array(),
+            "array_t<int32>"_a=py::array_t<std::int32_t>(),
+            "array_t<double>"_a=py::array_t<double>()
+        );
+    });
+
+    sm.def("converting_constructors", [](py::object o) {
+        return py::dict(
+            "array"_a=py::array(o),
+            "array_t<int32>"_a=py::array_t<std::int32_t>(o),
+            "array_t<double>"_a=py::array_t<double>(o)
+        );
+    });
 });

tests/test_numpy_array.py

@@ -245,3 +245,30 @@ def test_cast_numpy_int64_to_uint64():
     from pybind11_tests.array import function_taking_uint64
     function_taking_uint64(123)
     function_taking_uint64(np.uint64(123))
+
+
+@pytest.requires_numpy
+def test_isinstance():
+    from pybind11_tests.array import isinstance_untyped, isinstance_typed
+
+    assert isinstance_untyped(np.array([1, 2, 3]), "not an array")
+    assert isinstance_typed(np.array([1.0, 2.0, 3.0]))
+
+
+@pytest.requires_numpy
+def test_constructors():
+    from pybind11_tests.array import default_constructors, converting_constructors
+
+    defaults = default_constructors()
+    for a in defaults.values():
+        assert a.size == 0
+    assert defaults["array"].dtype == np.array([]).dtype
+    assert defaults["array_t<int32>"].dtype == np.int32
+    assert defaults["array_t<double>"].dtype == np.float64
+
+    results = converting_constructors([1, 2, 3])
+    for a in results.values():
+        np.testing.assert_array_equal(a, [1, 2, 3])
+    assert results["array"].dtype == np.int_
+    assert results["array_t<int32>"].dtype == np.int32
+    assert results["array_t<double>"].dtype == np.float64