Add py::isinstance<T>(obj) for generalized Python type checking

Allows checking the Python types before creating an object instead of
after. For example:
```c++
auto l = list(ptr, true);
if (l.check())
   // ...
```
The above is replaced with:
```c++
if (isinstance<list>(ptr)) {
    auto l = reinterpret_borrow(ptr);
    // ...
}
```

This deprecates `py::object::check()`. `py::isinstance()` covers the
same use case, but it can also check for user-defined types:
```c++
class Pet { ... };
py::class_<Pet>(...);

m.def("is_pet", [](py::object obj) {
    return py::isinstance<Pet>(obj); // works as expected
});
```

include/pybind11/cast.h

@@ -40,12 +40,8 @@ PYBIND11_NOINLINE inline internals &get_internals() {
         return *internals_ptr;
     handle builtins(PyEval_GetBuiltins());
     const char *id = PYBIND11_INTERNALS_ID;
-    capsule caps;
-    if (builtins.contains(id)) {
-        caps = builtins[id];
-    }
-    if (caps.check()) {
-        internals_ptr = caps;
+    if (builtins.contains(id) && isinstance<capsule>(builtins[id])) {
+        internals_ptr = capsule(builtins[id]);
     } else {
         internals_ptr = new internals();
         #if defined(WITH_THREAD)
@@ -111,6 +107,17 @@ PYBIND11_NOINLINE inline handle get_type_handle(const std::type_info &tp, bool t
     return handle(type_info ? ((PyObject *) type_info->type) : nullptr);
 }
 
+PYBIND11_NOINLINE inline bool isinstance_generic(handle obj, const std::type_info &tp) {
+    const auto type = detail::get_type_handle(tp, false);
+    if (!type)
+        return false;
+
+    const auto result = PyObject_IsInstance(obj.ptr(), type.ptr());
+    if (result == -1)
+        throw error_already_set();
+    return result != 0;
+}
+
 PYBIND11_NOINLINE inline std::string error_string() {
     if (!PyErr_Occurred()) {
         PyErr_SetString(PyExc_RuntimeError, "Unknown internal error occurred");
@@ -536,9 +543,8 @@ public:
         }
 
         /* Check if this is a capsule */
-        capsule c(h, true);
-        if (c.check()) {
-            value = (void *) c;
+        if (isinstance<capsule>(h)) {
+            value = capsule(h, true);
             return true;
         }
 
@@ -986,13 +992,17 @@ template <> struct handle_type_name<args> { static PYBIND11_DESCR name() { retur
 template <> struct handle_type_name<kwargs> { static PYBIND11_DESCR name() { return _("**kwargs"); } };
 
 template <typename type>
-struct type_caster<type, enable_if_t<is_pyobject<type>::value>> {
-public:
-    template <typename T = type, enable_if_t<!std::is_base_of<object, T>::value, int> = 0>
-    bool load(handle src, bool /* convert */) { value = type(src); return value.check(); }
+struct pyobject_caster {
+    template <typename T = type, enable_if_t<std::is_same<T, handle>::value, int> = 0>
+    bool load(handle src, bool /* convert */) { value = src; return static_cast<bool>(value); }
 
     template <typename T = type, enable_if_t<std::is_base_of<object, T>::value, int> = 0>
-    bool load(handle src, bool /* convert */) { value = type(src, true); return value.check(); }
+    bool load(handle src, bool /* convert */) {
+        if (!isinstance<type>(src))
+            return false;
+        value = type(src, true);
+        return true;
+    }
 
     static handle cast(const handle &src, return_value_policy /* policy */, handle /* parent */) {
         return src.inc_ref();
@@ -1000,6 +1010,9 @@ public:
     PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name());
 };
 
+template <typename T>
+class type_caster<T, enable_if_t<is_pyobject<T>::value>> : public pyobject_caster<T> { };
+
 // Our conditions for enabling moving are quite restrictive:
 // At compile time:
 // - T needs to be a non-const, non-pointer, non-reference type

include/pybind11/eigen.h

@@ -55,7 +55,7 @@ struct type_caster<Type, enable_if_t<is_eigen_dense<Type>::value && !is_eigen_re
 
     bool load(handle src, bool) {
         array_t<Scalar> buf(src, true);
-        if (!buf.check())
+        if (!buf)
             return false;
 
         if (buf.ndim() == 1) {
@@ -201,7 +201,7 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
         auto shape = pybind11::tuple((pybind11::object) obj.attr("shape"));
         auto nnz = obj.attr("nnz").cast<Index>();
 
-        if (!values.check() || !innerIndices.check() || !outerIndices.check())
+        if (!values || !innerIndices || !outerIndices)
             return false;
 
         value = Eigen::MappedSparseMatrix<Scalar, Type::Flags, StorageIndex>(

include/pybind11/numpy.h

@@ -324,10 +324,9 @@ public:
 
         int flags = 0;
         if (base && ptr) {
-            array base_array(base, true);
-            if (base_array.check())
+            if (isinstance<array>(base))
                 /* Copy flags from base (except baseship bit) */
-                flags = base_array.flags() & ~detail::npy_api::NPY_ARRAY_OWNDATA_;
+                flags = array(base, true).flags() & ~detail::npy_api::NPY_ARRAY_OWNDATA_;
             else
                 /* Writable by default, easy to downgrade later on if needed */
                 flags = detail::npy_api::NPY_ARRAY_WRITEABLE_;
@@ -627,6 +626,21 @@ struct format_descriptor<T, detail::enable_if_t<std::is_enum<T>::value>> {
 };
 
 NAMESPACE_BEGIN(detail)
+template <typename T, int ExtraFlags>
+struct pyobject_caster<array_t<T, ExtraFlags>> {
+    using type = array_t<T, ExtraFlags>;
+
+    bool load(handle src, bool /* convert */) {
+        value = type(src, true);
+        return static_cast<bool>(value);
+    }
+
+    static handle cast(const handle &src, return_value_policy /* policy */, handle /* parent */) {
+        return src.inc_ref();
+    }
+    PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name());
+};
+
 template <typename T> struct is_std_array : std::false_type { };
 template <typename T, size_t N> struct is_std_array<std::array<T, N>> : std::true_type { };
 

include/pybind11/pytypes.h

@@ -22,6 +22,7 @@ struct arg; struct arg_v;
 
 NAMESPACE_BEGIN(detail)
 class args_proxy;
+inline bool isinstance_generic(handle obj, const std::type_info &tp);
 
 // Accessor forward declarations
 template <typename Policy> class accessor;
@@ -91,6 +92,7 @@ public:
     explicit operator bool() const { return m_ptr != nullptr; }
     bool operator==(const handle &h) const { return m_ptr == h.m_ptr; }
     bool operator!=(const handle &h) const { return m_ptr != h.m_ptr; }
+    PYBIND11_DEPRECATED("Use handle::operator bool() instead")
     bool check() const { return m_ptr != nullptr; }
 protected:
     PyObject *m_ptr = nullptr;
@@ -135,6 +137,16 @@ public:
     template <typename T> T cast() &&;
 };
 
+/// Check if `obj` is an instance of type `T`
+template <typename T, detail::enable_if_t<std::is_base_of<object, T>::value, int> = 0>
+bool isinstance(handle obj) { return T::_check(obj); }
+
+template <typename T, detail::enable_if_t<!std::is_base_of<object, T>::value, int> = 0>
+bool isinstance(handle obj) { return detail::isinstance_generic(obj, typeid(T)); }
+
+template <> inline bool isinstance<handle>(handle obj) = delete;
+template <> inline bool isinstance<object>(handle obj) { return obj.ptr() != nullptr; }
+
 inline bool hasattr(handle obj, handle name) {
     return PyObject_HasAttr(obj.ptr(), name.ptr()) == 1;
 }
@@ -386,7 +398,9 @@ NAMESPACE_END(detail)
         Name(object&& o) noexcept : Parent(std::move(o)) { CvtStmt; } \
         Name& operator=(object&& o) noexcept { (void) object::operator=(std::move(o)); CvtStmt; return *this; } \
         Name& operator=(const object& o) { return static_cast<Name&>(object::operator=(o)); CvtStmt; } \
-        bool check() const { return m_ptr != nullptr && (bool) CheckFun(m_ptr); }
+        PYBIND11_DEPRECATED("Use py::isinstance<py::python_type>(obj) instead") \
+        bool check() const { return m_ptr != nullptr && (bool) CheckFun(m_ptr); } \
+        static bool _check(handle h) { return h.ptr() != nullptr && CheckFun(h.ptr()); }
 
 #define PYBIND11_OBJECT(Name, Parent, CheckFun) \
     PYBIND11_OBJECT_CVT(Name, Parent, CheckFun, )

include/pybind11/stl.h

@@ -45,9 +45,9 @@ template <typename Type, typename Key> struct set_caster {
     using key_conv = make_caster<Key>;
 
     bool load(handle src, bool convert) {
-        pybind11::set s(src, true);
-        if (!s.check())
+        if (!isinstance<pybind11::set>(src))
             return false;
+        pybind11::set s(src, true);
         value.clear();
         key_conv conv;
         for (auto entry : s) {
@@ -77,9 +77,9 @@ template <typename Type, typename Key, typename Value> struct map_caster {
     using value_conv = make_caster<Value>;
 
     bool load(handle src, bool convert) {
-        dict d(src, true);
-        if (!d.check())
+        if (!isinstance<dict>(src))
             return false;
+        dict d(src, true);
         key_conv kconv;
         value_conv vconv;
         value.clear();
@@ -112,9 +112,9 @@ template <typename Type, typename Value> struct list_caster {
     using value_conv = make_caster<Value>;
 
     bool load(handle src, bool convert) {
-        sequence s(src, true);
-        if (!s.check())
+        if (!isinstance<sequence>(src))
             return false;
+        sequence s(src, true);
         value_conv conv;
         value.clear();
         reserve_maybe(s, &value);
@@ -157,9 +157,9 @@ template <typename Type, size_t Size> struct type_caster<std::array<Type, Size>>
     using value_conv = make_caster<Type>;
 
     bool load(handle src, bool convert) {
-        list l(src, true);
-        if (!l.check())
+        if (!isinstance<list>(src))
             return false;
+        list l(src, true);
         if (l.size() != Size)
             return false;
         value_conv conv;

tests/test_inheritance.cpp

@@ -83,4 +83,18 @@ test_initializer inheritance([](py::module &m) {
         return new BaseClass();
     });
     m.def("return_none", []() -> BaseClass* { return nullptr; });
+
+    m.def("test_isinstance", [](py::list l) {
+        struct Unregistered { }; // checks missing type_info code path
+
+        return py::make_tuple(
+            py::isinstance<py::tuple>(l[0]),
+            py::isinstance<py::dict>(l[1]),
+            py::isinstance<Pet>(l[2]),
+            py::isinstance<Pet>(l[3]),
+            py::isinstance<Dog>(l[4]),
+            py::isinstance<Rabbit>(l[5]),
+            py::isinstance<Unregistered>(l[6])
+        );
+    });
 });

tests/test_inheritance.py

@@ -45,3 +45,11 @@ def test_automatic_upcasting():
     assert type(return_class_n(2)).__name__ == "DerivedClass2"
     assert type(return_class_n(0)).__name__ == "BaseClass"
     assert type(return_class_n(1)).__name__ == "DerivedClass1"
+
+
+def test_isinstance():
+    from pybind11_tests import test_isinstance, Pet, Dog
+
+    objects = [tuple(), dict(), Pet("Polly", "parrot")] + [Dog("Molly")] * 4
+    expected = (True, True, True, True, True, False, False)
+    assert test_isinstance(objects) == expected