Allow py::arg().none(false) argument attribute

This attribute lets you disable (or explicitly enable) passing None to
an argument that otherwise would allow it by accepting
a value by raw pointer or shared_ptr.

docs/advanced/functions.rst

@@ -406,6 +406,53 @@ name, i.e. by specifying ``py::arg().noconvert()``.
     need to specify a ``py::arg()`` annotation for each argument with the
     no-convert argument modified to ``py::arg().noconvert()``.
 
+Allow/Prohibiting None arguments
+================================
+
+When a C++ type registered with :class:`py::class_` is passed as an argument to
+a function taking the instance as pointer or shared holder (e.g. ``shared_ptr``
+or a custom, copyable holder as described in :ref:`smart_pointers`), pybind
+allows ``None`` to be passed from Python which results in calling the C++
+function with ``nullptr`` (or an empty holder) for the argument.
+
+To explicitly enable or disable this behaviour, using the
+``.none`` method of the :class:`py::arg` object:
+
+.. code-block:: cpp
+
+    py::class_<Dog>(m, "Dog").def(py::init<>());
+    py::class_<Cat>(m, "Cat").def(py::init<>());
+    m.def("bark", [](Dog *dog) -> std::string {
+        if (dog) return "woof!"; /* Called with a Dog instance */
+        else return "(no dog)"; /* Called with None, d == nullptr */
+    }, py::arg("dog").none(true));
+    m.def("meow", [](Cat *cat) -> std::string {
+        // Can't be called with None argument
+        return "meow";
+    }, py::arg("cat").none(false));
+
+With the above, the Python call ``bark(None)`` will return the string ``"(no
+dog)"``, while attempting to call ``meow(None)`` will throw a :exc:`TypeError`:
+
+.. code-block:: pycon
+
+    >>> from animals import Dog, Cat, bark, meow
+    >>> bark(Dog())
+    'woof!'
+    >>> meow(Cat())
+    'meow'
+    >>> bark(None)
+    '(no dog)'
+    >>> meow(None)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in <module>
+    TypeError: meow(): incompatible function arguments. The following argument types are supported:
+        1. (cat: animals.Cat) -> str
+
+    Invoked with: None
+
+The default behaviour when the tag is unspecified is to allow ``None``.
+
 Overload resolution order
 =========================
 

include/pybind11/attr.h

@@ -123,9 +123,10 @@ struct argument_record {
     const char *descr; ///< Human-readable version of the argument value
     handle value;      ///< Associated Python object
     bool convert : 1;  ///< True if the argument is allowed to convert when loading
+    bool none : 1;     ///< True if None is allowed when loading
 
-    argument_record(const char *name, const char *descr, handle value, bool convert)
-        : name(name), descr(descr), value(value), convert(convert) { }
+    argument_record(const char *name, const char *descr, handle value, bool convert, bool none)
+        : name(name), descr(descr), value(value), convert(convert), none(none) { }
 };
 
 /// Internal data structure which holds metadata about a bound function (signature, overloads, etc.)
@@ -338,8 +339,8 @@ template <> struct process_attribute<is_operator> : process_attribute_default<is
 template <> struct process_attribute<arg> : process_attribute_default<arg> {
     static void init(const arg &a, function_record *r) {
         if (r->is_method && r->args.empty())
-            r->args.emplace_back("self", nullptr, handle(), true /*convert*/);
-        r->args.emplace_back(a.name, nullptr, handle(), !a.flag_noconvert);
+            r->args.emplace_back("self", nullptr, handle(), true /*convert*/, false /*none not allowed*/);
+        r->args.emplace_back(a.name, nullptr, handle(), !a.flag_noconvert, a.flag_none);
     }
 };
 
@@ -347,7 +348,7 @@ template <> struct process_attribute<arg> : process_attribute_default<arg> {
 template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
     static void init(const arg_v &a, function_record *r) {
         if (r->is_method && r->args.empty())
-            r->args.emplace_back("self", nullptr /*descr*/, handle() /*parent*/, true /*convert*/);
+            r->args.emplace_back("self", nullptr /*descr*/, handle() /*parent*/, true /*convert*/, false /*none not allowed*/);
 
         if (!a.value) {
 #if !defined(NDEBUG)
@@ -370,7 +371,7 @@ template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
                           "Compile in debug mode for more information.");
 #endif
         }
-        r->args.emplace_back(a.name, a.descr, a.value.inc_ref(), !a.flag_noconvert);
+        r->args.emplace_back(a.name, a.descr, a.value.inc_ref(), !a.flag_noconvert, a.flag_none);
     }
 };
 

include/pybind11/cast.h

@@ -1381,14 +1381,17 @@ template <return_value_policy policy = return_value_policy::automatic_reference,
 /// Annotation for arguments
 struct arg {
     /// Constructs an argument with the name of the argument; if null or omitted, this is a positional argument.
-    constexpr explicit arg(const char *name = nullptr) : name(name), flag_noconvert(false) { }
+    constexpr explicit arg(const char *name = nullptr) : name(name), flag_noconvert(false), flag_none(true) { }
     /// Assign a value to this argument
     template <typename T> arg_v operator=(T &&value) const;
     /// Indicate that the type should not be converted in the type caster
     arg &noconvert(bool flag = true) { flag_noconvert = flag; return *this; }
+    /// Indicates that the argument should/shouldn't allow None (e.g. for nullable pointer args)
+    arg &none(bool flag = true) { flag_none = flag; return *this; }
 
     const char *name; ///< If non-null, this is a named kwargs argument
     bool flag_noconvert : 1; ///< If set, do not allow conversion (requires a supporting type caster!)
+    bool flag_none : 1; ///< If set (the default), allow None to be passed to this argument
 };
 
 /// \ingroup annotations
@@ -1421,6 +1424,9 @@ public:
     /// Same as `arg::noconvert()`, but returns *this as arg_v&, not arg&
     arg_v &noconvert(bool flag = true) { arg::noconvert(flag); return *this; }
 
+    /// Same as `arg::nonone()`, but returns *this as arg_v&, not arg&
+    arg_v &none(bool flag = true) { arg::none(flag); return *this; }
+
     /// The default value
     object value;
     /// The (optional) description of the default value

include/pybind11/pybind11.h

@@ -466,18 +466,23 @@ protected:
                 size_t args_copied = 0;
 
                 // 1. Copy any position arguments given.
-                bool bad_kwarg = false;
+                bool bad_arg = false;
                 for (; args_copied < args_to_copy; ++args_copied) {
-                    if (kwargs_in && args_copied < func.args.size() && func.args[args_copied].name
-                            && PyDict_GetItemString(kwargs_in, func.args[args_copied].name)) {
-                        bad_kwarg = true;
+                    argument_record *arg_rec = args_copied < func.args.size() ? &func.args[args_copied] : nullptr;
+                    if (kwargs_in && arg_rec && arg_rec->name && PyDict_GetItemString(kwargs_in, arg_rec->name)) {
+                        bad_arg = true;
                         break;
                     }
 
-                    call.args.push_back(PyTuple_GET_ITEM(args_in, args_copied));
-                    call.args_convert.push_back(args_copied < func.args.size() ? func.args[args_copied].convert : true);
+                    handle arg(PyTuple_GET_ITEM(args_in, args_copied));
+                    if (arg_rec && !arg_rec->none && arg.is_none()) {
+                        bad_arg = true;
+                        break;
+                    }
+                    call.args.push_back(arg);
+                    call.args_convert.push_back(arg_rec ? arg_rec->convert : true);
                 }
-                if (bad_kwarg)
+                if (bad_arg)
                     continue; // Maybe it was meant for another overload (issue #688)
 
                 // We'll need to copy this if we steal some kwargs for defaults

tests/test_methods_and_attributes.cpp

@@ -162,6 +162,14 @@ public:
 /// Issue/PR #648: bad arg default debugging output
 class NotRegistered {};
 
+// Test None-allowed py::arg argument policy
+class NoneTester { public: int answer = 42; };
+int none1(const NoneTester &obj) { return obj.answer; }
+int none2(NoneTester *obj) { return obj ? obj->answer : -1; }
+int none3(std::shared_ptr<NoneTester> &obj) { return obj ? obj->answer : -1; }
+int none4(std::shared_ptr<NoneTester> *obj) { return obj && *obj ? (*obj)->answer : -1; }
+int none5(std::shared_ptr<NoneTester> obj) { return obj ? obj->answer : -1; }
+
 test_initializer methods_and_attributes([](py::module &m) {
     py::class_<ExampleMandA> emna(m, "ExampleMandA");
     emna.def(py::init<>())
@@ -322,4 +330,18 @@ test_initializer methods_and_attributes([](py::module &m) {
         auto m = py::module::import("pybind11_tests");
         m.def("should_fail", [](int, NotRegistered) {}, py::arg(), py::arg() = NotRegistered());
     });
+
+    py::class_<NoneTester, std::shared_ptr<NoneTester>>(m, "NoneTester")
+        .def(py::init<>());
+    m.def("no_none1", &none1, py::arg().none(false));
+    m.def("no_none2", &none2, py::arg().none(false));
+    m.def("no_none3", &none3, py::arg().none(false));
+    m.def("no_none4", &none4, py::arg().none(false));
+    m.def("no_none5", &none5, py::arg().none(false));
+    m.def("ok_none1", &none1);
+    m.def("ok_none2", &none2, py::arg().none(true));
+    m.def("ok_none3", &none3);
+    m.def("ok_none4", &none4, py::arg().none(true));
+    m.def("ok_none5", &none5);
+
 });

tests/test_methods_and_attributes.py

@@ -369,3 +369,47 @@ def test_bad_arg_default(msg):
         "arg(): could not convert default argument into a Python object (type not registered "
         "yet?). Compile in debug mode for more information."
     )
+
+
+def test_accepts_none():
+    from pybind11_tests import (NoneTester,
+                                no_none1, no_none2, no_none3, no_none4, no_none5,
+                                ok_none1, ok_none2, ok_none3, ok_none4, ok_none5)
+
+    a = NoneTester()
+    assert no_none1(a) == 42
+    assert no_none2(a) == 42
+    assert no_none3(a) == 42
+    assert no_none4(a) == 42
+    assert no_none5(a) == 42
+    assert ok_none1(a) == 42
+    assert ok_none2(a) == 42
+    assert ok_none3(a) == 42
+    assert ok_none4(a) == 42
+    assert ok_none5(a) == 42
+
+    with pytest.raises(TypeError) as excinfo:
+        no_none1(None)
+    assert "incompatible function arguments" in str(excinfo.value)
+    with pytest.raises(TypeError) as excinfo:
+        no_none2(None)
+    assert "incompatible function arguments" in str(excinfo.value)
+    with pytest.raises(TypeError) as excinfo:
+        no_none3(None)
+    assert "incompatible function arguments" in str(excinfo.value)
+    with pytest.raises(TypeError) as excinfo:
+        no_none4(None)
+    assert "incompatible function arguments" in str(excinfo.value)
+    with pytest.raises(TypeError) as excinfo:
+        no_none5(None)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    # The first one still raises because you can't pass None as a lvalue reference arg:
+    with pytest.raises(TypeError) as excinfo:
+        assert ok_none1(None) == -1
+    assert "incompatible function arguments" in str(excinfo.value)
+    # The rest take the argument as pointer or holder, and accept None:
+    assert ok_none2(None) == -1
+    assert ok_none3(None) == -1
+    assert ok_none4(None) == -1
+    assert ok_none5(None) == -1