Support keyword-only arguments

This adds support for a `py::args_kw_only()` annotation that can be
specified between `py::arg` annotations to indicate that any following
arguments are keyword-only.  This allows you to write:

    m.def("f", [](int a, int b) { /* ... */ },
          py::arg("a"), py::args_kw_only(), py::arg("b"));

and have it work like Python 3's:

    def f(a, *, b):
        # ...

with respect to how `a` and `b` arguments are accepted (that is, `a` can
be positional or by keyword; `b` can only be specified by keyword).

docs/advanced/functions.rst

@@ -362,6 +362,34 @@ like so:
     py::class_<MyClass>("MyClass")
         .def("myFunction", py::arg("arg") = (SomeType *) nullptr);
 
+Keyword-only arguments
+======================
+
+Python 3 introduced keyword-only arguments by specifying an unnamed ``*``
+argument in a function definition:
+
+.. code-block:: python
+
+    def f(a, *, b):  # a can be positional or via keyword; b must be via keyword
+        pass
+
+    f(a=1, b=2)  # good
+    f(b=2, a=1)  # good
+    f(1, b=2)    # good
+    f(1, 2)      # TypeError: f() takes 1 positional argument but 2 were given
+
+Pybind11 provides a ``py::args_kw_only`` object that allows you to implement
+the same behaviour by specifying the object between positional and keyword-only
+argument annotations when registering the function:
+
+.. code-block:: cpp
+
+    m.def("f", [](int a, int b) { /* ... */ },
+          py::arg("a"), py::args_kw_only(), py::arg("b"));
+
+Note that, as in Python, you cannot combine this with a ``py::args`` argument.
+This feature does *not* require Python 3 to work.
+
 .. _nonconverting_arguments:
 
 Non-converting arguments

include/pybind11/attr.h

@@ -137,7 +137,8 @@ struct argument_record {
 struct function_record {
     function_record()
         : is_constructor(false), is_new_style_constructor(false), is_stateless(false),
-          is_operator(false), has_args(false), has_kwargs(false), is_method(false) { }
+          is_operator(false), is_method(false),
+          has_args(false), has_kwargs(false), has_kw_only_args(false) { }
 
     /// Function name
     char *name = nullptr; /* why no C++ strings? They generate heavier code.. */
@@ -175,18 +176,24 @@ struct function_record {
     /// True if this is an operator (__add__), etc.
     bool is_operator : 1;
 
+    /// True if this is a method
+    bool is_method : 1;
+
     /// True if the function has a '*args' argument
     bool has_args : 1;
 
     /// True if the function has a '**kwargs' argument
     bool has_kwargs : 1;
 
-    /// True if this is a method
-    bool is_method : 1;
+    /// True once a 'py::args_kw_only' is encountered (any following args are keyword-only)
+    bool has_kw_only_args : 1;
 
     /// Number of arguments (including py::args and/or py::kwargs, if present)
     std::uint16_t nargs;
 
+    /// Number of trailing arguments (counted in `nargs`) that are keyword-only
+    std::uint16_t nargs_kwonly = 0;
+
     /// Python method object
     PyMethodDef *def = nullptr;
 
@@ -359,12 +366,20 @@ template <> struct process_attribute<is_new_style_constructor> : process_attribu
     static void init(const is_new_style_constructor &, function_record *r) { r->is_new_style_constructor = true; }
 };
 
+inline void process_kwonly_arg(const arg &a, function_record *r) {
+    if (!a.name || strlen(a.name) == 0)
+        pybind11_fail("arg(): cannot specify an unnamed argument after an args_kw_only() annotation");
+    ++r->nargs_kwonly;
+}
+
 /// Process a keyword argument attribute (*without* a default value)
 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*/, false /*none not allowed*/);
         r->args.emplace_back(a.name, nullptr, handle(), !a.flag_noconvert, a.flag_none);
+
+        if (r->has_kw_only_args) process_kwonly_arg(a, r);
     }
 };
 
@@ -396,6 +411,15 @@ template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
 #endif
         }
         r->args.emplace_back(a.name, a.descr, a.value.inc_ref(), !a.flag_noconvert, a.flag_none);
+
+        if (r->has_kw_only_args) process_kwonly_arg(a, r);
+    }
+};
+
+/// Process a keyword-only-arguments-follow pseudo argument
+template <> struct process_attribute<args_kw_only> : process_attribute_default<args_kw_only> {
+    static void init(const args_kw_only &, function_record *r) {
+        r->has_kw_only_args = true;
     }
 };
 

include/pybind11/cast.h

@@ -1887,6 +1887,11 @@ public:
 #endif
 };
 
+/// \ingroup annotations
+/// Annotation indicating that all following arguments are keyword-only; the is the equivalent of an
+/// unnamed '*' argument (in Python 3)
+struct args_kw_only {};
+
 template <typename T>
 arg_v arg::operator=(T &&value) const { return {std::move(*this), std::forward<T>(value)}; }
 

include/pybind11/pybind11.h

@@ -168,6 +168,14 @@ protected:
         /* Process any user-provided function attributes */
         process_attributes<Extra...>::init(extra..., rec);
 
+        {
+            constexpr bool has_kw_only_args = any_of<std::is_same<args_kw_only, Extra>...>::value,
+                           has_args = any_of<std::is_same<args, Args>...>::value,
+                           has_arg_annotations = any_of<is_keyword<Extra>...>::value;
+            static_assert(has_arg_annotations || !has_kw_only_args, "py::args_kw_only requires the use of argument annotations");
+            static_assert(!(has_args && has_kw_only_args), "py::args_kw_only cannot be combined with a py::args argument");
+        }
+
         /* Generate a readable signature describing the function's arguments and return value types */
         static constexpr auto signature = _("(") + cast_in::arg_names + _(") -> ") + cast_out::name;
         PYBIND11_DESCR_CONSTEXPR auto types = decltype(signature)::types();
@@ -483,15 +491,16 @@ protected:
                  */
 
                 const function_record &func = *it;
-                size_t pos_args = func.nargs;    // Number of positional arguments that we need
-                if (func.has_args) --pos_args;   // (but don't count py::args
-                if (func.has_kwargs) --pos_args; //  or py::kwargs)
+                size_t num_args = func.nargs;    // Number of positional arguments that we need
+                if (func.has_args) --num_args;   // (but don't count py::args
+                if (func.has_kwargs) --num_args; //  or py::kwargs)
+                size_t pos_args = num_args - func.nargs_kwonly;
 
                 if (!func.has_args && n_args_in > pos_args)
-                    continue; // Too many arguments for this overload
+                    continue; // Too many positional arguments for this overload
 
                 if (n_args_in < pos_args && func.args.size() < pos_args)
-                    continue; // Not enough arguments given, and not enough defaults to fill in the blanks
+                    continue; // Not enough positional arguments given, and not enough defaults to fill in the blanks
 
                 function_call call(func, parent);
 
@@ -535,10 +544,10 @@ protected:
                 dict kwargs = reinterpret_borrow<dict>(kwargs_in);
 
                 // 2. Check kwargs and, failing that, defaults that may help complete the list
-                if (args_copied < pos_args) {
+                if (args_copied < num_args) {
                     bool copied_kwargs = false;
 
-                    for (; args_copied < pos_args; ++args_copied) {
+                    for (; args_copied < num_args; ++args_copied) {
                         const auto &arg = func.args[args_copied];
 
                         handle value;
@@ -564,7 +573,7 @@ protected:
                             break;
                     }
 
-                    if (args_copied < pos_args)
+                    if (args_copied < num_args)
                         continue; // Not enough arguments, defaults, or kwargs to fill the positional arguments
                 }
 

tests/test_kwargs_and_defaults.cpp

@@ -94,6 +94,30 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
 //    m.def("bad_args6", [](py::args, py::args) {});
 //    m.def("bad_args7", [](py::kwargs, py::kwargs) {});
 
+    // test_keyword_only_args
+    m.def("kwonly_all", [](int i, int j) { return py::make_tuple(i, j); },
+            py::args_kw_only(), py::arg("i"), py::arg("j"));
+    m.def("kwonly_some", [](int i, int j, int k) { return py::make_tuple(i, j, k); },
+            py::arg(), py::args_kw_only(), py::arg("j"), py::arg("k"));
+    m.def("kwonly_with_defaults", [](int i, int j, int k, int z) { return py::make_tuple(i, j, k, z); },
+            py::arg() = 3, "j"_a = 4, py::args_kw_only(), "k"_a = 5, "z"_a);
+    m.def("kwonly_mixed", [](int i, int j) { return py::make_tuple(i, j); },
+            "i"_a, py::args_kw_only(), "j"_a);
+    m.def("kwonly_plus_more", [](int i, int j, int k, py::kwargs kwargs) {
+            return py::make_tuple(i, j, k, kwargs); },
+            py::arg() /* positional */, py::arg("j") = -1 /* both */, py::args_kw_only(), py::arg("k") /* kw-only */);
+
+    m.def("register_invalid_kwonly", [](py::module m) {
+        m.def("bad_kwonly", [](int i, int j) { return py::make_tuple(i, j); },
+                py::args_kw_only(), py::arg() /* invalid unnamed argument */, "j"_a);
+    });
+
+    // These should fail to compile:
+    // argument annotations are required when using args_kw_only
+//    m.def("bad_kwonly1", [](int) {}, py::args_kw_only());
+    // can't specify both `py::args_kw_only` and a `py::args` argument
+//    m.def("bad_kwonly2", [](int i, py::args) {}, py::args_kw_only(), "i"_a);
+
     // test_function_signatures (along with most of the above)
     struct KWClass { void foo(int, float) {} };
     py::class_<KWClass>(m, "KWClass")

tests/test_kwargs_and_defaults.py

@@ -107,6 +107,44 @@ def test_mixed_args_and_kwargs(msg):
     """  # noqa: E501 line too long
 
 
+def test_keyword_only_args(msg):
+    assert m.kwonly_all(i=1, j=2) == (1, 2)
+    assert m.kwonly_all(j=1, i=2) == (2, 1)
+
+    with pytest.raises(TypeError) as excinfo:
+        assert m.kwonly_all(i=1) == (1,)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    with pytest.raises(TypeError) as excinfo:
+        assert m.kwonly_all(1, 2) == (1, 2)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    assert m.kwonly_some(1, k=3, j=2) == (1, 2, 3)
+
+    assert m.kwonly_with_defaults(z=8) == (3, 4, 5, 8)
+    assert m.kwonly_with_defaults(2, z=8) == (2, 4, 5, 8)
+    assert m.kwonly_with_defaults(2, j=7, k=8, z=9) == (2, 7, 8, 9)
+    assert m.kwonly_with_defaults(2, 7, z=9, k=8) == (2, 7, 8, 9)
+
+    assert m.kwonly_mixed(1, j=2) == (1, 2)
+    assert m.kwonly_mixed(j=2, i=3) == (3, 2)
+    assert m.kwonly_mixed(i=2, j=3) == (2, 3)
+
+    assert m.kwonly_plus_more(4, 5, k=6, extra=7) == (4, 5, 6, {'extra': 7})
+    assert m.kwonly_plus_more(3, k=5, j=4, extra=6) == (3, 4, 5, {'extra': 6})
+    assert m.kwonly_plus_more(2, k=3, extra=4) == (2, -1, 3, {'extra': 4})
+
+    with pytest.raises(TypeError) as excinfo:
+        assert m.kwonly_mixed(i=1) == (1,)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        m.register_invalid_kwonly(m)
+    assert msg(excinfo.value) == """
+        arg(): cannot specify an unnamed argument after an args_kw_only() annotation
+    """
+
+
 def test_args_refcount():
     """Issue/PR #1216 - py::args elements get double-inc_ref()ed when combined with regular
     arguments"""