feat: py::pos_only (#2459)

* feat: py::pos_only

* fix: review points from @YannickJadoul

* fix: review points from @bstaletic

* refactor: kwonly -> kw_only

Author: henryiii

docs/advanced/functions.rst

@@ -378,17 +378,35 @@ argument in a function definition:
     f(1, b=2)    # good
     f(1, 2)      # TypeError: f() takes 1 positional argument but 2 were given
 
-Pybind11 provides a ``py::kwonly`` object that allows you to implement
+Pybind11 provides a ``py::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::kwonly(), py::arg("b"));
+          py::arg("a"), py::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.
+Note that you currently cannot combine this with a ``py::args`` argument.  This
+feature does *not* require Python 3 to work.
+
+.. versionadded:: 2.6
+
+Positional-only arguments
+=========================
+
+Python 3.8 introduced a new positional-only argument syntax, using ``/`` in the
+function definition (note that this has been a convention for CPython
+positional arguments, such as in ``pow()``, since Python 2). You can
+do the same thing in any version of Python using ``py::pos_only()``:
+
+.. code-block:: cpp
+
+   m.def("f", [](int a, int b) { /* ... */ },
+          py::arg("a"), py::pos_only(), py::arg("b"));
+
+You now cannot give argument ``a`` by keyword. This can be combined with
+keyword-only arguments, as well.
 
 .. versionadded:: 2.6
 

docs/changelog.rst

@@ -11,9 +11,11 @@ v2.6.0 (IN PROGRESS)
 
 See :ref:`upgrade-guide-2.6` for help upgrading to the new version.
 
-* Keyword only argument supported in Python 2 or 3 with ``py::kwonly()``.
+* Keyword-only argument supported in Python 2 or 3 with ``py::kw_only()``.
   `#2100 <https://github.com/pybind/pybind11/pull/2100>`_
 
+* Positional-only argument supported in Python 2 or 3 with ``py::pos_only()``.
+
 * Perfect forwarding support for methods.
   `#2048 <https://github.com/pybind/pybind11/pull/2048>`_
 

include/pybind11/attr.h

@@ -138,7 +138,7 @@ struct function_record {
     function_record()
         : is_constructor(false), is_new_style_constructor(false), is_stateless(false),
           is_operator(false), is_method(false),
-          has_args(false), has_kwargs(false), has_kwonly_args(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.. */
@@ -185,14 +185,17 @@ struct function_record {
     /// True if the function has a '**kwargs' argument
     bool has_kwargs : 1;
 
-    /// True once a 'py::kwonly' is encountered (any following args are keyword-only)
-    bool has_kwonly_args : 1;
+    /// True once a 'py::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;
+    std::uint16_t nargs_kw_only = 0;
+
+    /// Number of leading arguments (counted in `nargs`) that are positional-only
+    std::uint16_t nargs_pos_only = 0;
 
     /// Python method object
     PyMethodDef *def = nullptr;
@@ -366,10 +369,10 @@ 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) {
+inline void process_kw_only_arg(const arg &a, function_record *r) {
     if (!a.name || strlen(a.name) == 0)
-        pybind11_fail("arg(): cannot specify an unnamed argument after an kwonly() annotation");
-    ++r->nargs_kwonly;
+        pybind11_fail("arg(): cannot specify an unnamed argument after an kw_only() annotation");
+    ++r->nargs_kw_only;
 }
 
 /// Process a keyword argument attribute (*without* a default value)
@@ -379,7 +382,7 @@ template <> struct process_attribute<arg> : process_attribute_default<arg> {
             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_kwonly_args) process_kwonly_arg(a, r);
+        if (r->has_kw_only_args) process_kw_only_arg(a, r);
     }
 };
 
@@ -412,14 +415,21 @@ template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
         }
         r->args.emplace_back(a.name, a.descr, a.value.inc_ref(), !a.flag_noconvert, a.flag_none);
 
-        if (r->has_kwonly_args) process_kwonly_arg(a, r);
+        if (r->has_kw_only_args) process_kw_only_arg(a, r);
     }
 };
 
 /// Process a keyword-only-arguments-follow pseudo argument
-template <> struct process_attribute<kwonly> : process_attribute_default<kwonly> {
-    static void init(const kwonly &, function_record *r) {
-        r->has_kwonly_args = true;
+template <> struct process_attribute<kw_only> : process_attribute_default<kw_only> {
+    static void init(const kw_only &, function_record *r) {
+        r->has_kw_only_args = true;
+    }
+};
+
+/// Process a positional-only-argument maker
+template <> struct process_attribute<pos_only> : process_attribute_default<pos_only> {
+    static void init(const pos_only &, function_record *r) {
+        r->nargs_pos_only = static_cast<std::uint16_t>(r->args.size());
     }
 };
 

include/pybind11/cast.h

@@ -1910,7 +1910,12 @@ struct arg_v : arg {
 /// \ingroup annotations
 /// Annotation indicating that all following arguments are keyword-only; the is the equivalent of an
 /// unnamed '*' argument (in Python 3)
-struct kwonly {};
+struct kw_only {};
+
+/// \ingroup annotations
+/// Annotation indicating that all previous arguments are positional-only; the is the equivalent of an
+/// unnamed '/' argument (in Python 3.8)
+struct pos_only {};
 
 template <typename T>
 arg_v arg::operator=(T &&value) const { return {std::move(*this), std::forward<T>(value)}; }

include/pybind11/pybind11.h

@@ -187,11 +187,13 @@ class cpp_function : public function {
         process_attributes<Extra...>::init(extra..., rec);
 
         {
-            constexpr bool has_kwonly_args = any_of<std::is_same<kwonly, Extra>...>::value,
+            constexpr bool has_kw_only_args = any_of<std::is_same<kw_only, Extra>...>::value,
+                           has_pos_only_args = any_of<std::is_same<pos_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_kwonly_args, "py::kwonly requires the use of argument annotations");
-            static_assert(!(has_args && has_kwonly_args), "py::kwonly cannot be combined with a py::args argument");
+            static_assert(has_arg_annotations || !has_kw_only_args, "py::kw_only requires the use of argument annotations");
+            static_assert(has_arg_annotations || !has_pos_only_args, "py::pos_only requires the use of argument annotations (for docstrings and aligning the annotations to the argument)");
+            static_assert(!(has_args && has_kw_only_args), "py::kw_only cannot be combined with a py::args argument");
         }
 
         /* Generate a readable signature describing the function's arguments and return value types */
@@ -257,7 +259,10 @@ class cpp_function : public function {
                 // Write arg name for everything except *args and **kwargs.
                 if (*(pc + 1) == '*')
                     continue;
-
+                // Separator for keyword-only arguments, placed before the kw
+                // arguments start
+                if (rec->nargs_kw_only > 0 && arg_index + rec->nargs_kw_only == args)
+                    signature += "*, ";
                 if (arg_index < rec->args.size() && rec->args[arg_index].name) {
                     signature += rec->args[arg_index].name;
                 } else if (arg_index == 0 && rec->is_method) {
@@ -272,6 +277,10 @@ class cpp_function : public function {
                     signature += " = ";
                     signature += rec->args[arg_index].descr;
                 }
+                // Separator for positional-only arguments (placed after the
+                // argument, rather than before like *
+                if (rec->nargs_pos_only > 0 && (arg_index + 1) == rec->nargs_pos_only)
+                    signature += ", /";
                 arg_index++;
             } else if (c == '%') {
                 const std::type_info *t = types[type_index++];
@@ -297,6 +306,7 @@ class cpp_function : public function {
                 signature += c;
             }
         }
+
         if (arg_index != args || types[type_index] != nullptr)
             pybind11_fail("Internal error while parsing type signature (2)");
 
@@ -512,7 +522,7 @@ class cpp_function : public function {
                 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;
+                size_t pos_args = num_args - func.nargs_kw_only;
 
                 if (!func.has_args && n_args_in > pos_args)
                     continue; // Too many positional arguments for this overload
@@ -561,6 +571,26 @@ class cpp_function : public function {
                 // We'll need to copy this if we steal some kwargs for defaults
                 dict kwargs = reinterpret_borrow<dict>(kwargs_in);
 
+                // 1.5. Fill in any missing pos_only args from defaults if they exist
+                if (args_copied < func.nargs_pos_only) {
+                    for (; args_copied < func.nargs_pos_only; ++args_copied) {
+                        const auto &arg = func.args[args_copied];
+                        handle value;
+
+                        if (arg.value) {
+                            value = arg.value;
+                        }
+                        if (value) {
+                            call.args.push_back(value);
+                            call.args_convert.push_back(arg.convert);
+                        } else
+                            break;
+                    }
+
+                    if (args_copied < func.nargs_pos_only)
+                        continue; // Not enough defaults to fill the positional arguments
+                }
+
                 // 2. Check kwargs and, failing that, defaults that may help complete the list
                 if (args_copied < num_args) {
                     bool copied_kwargs = false;

tests/test_kwargs_and_defaults.cpp

@@ -95,28 +95,39 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
 //    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::kwonly(), 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::kwonly(), 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::kwonly(), "k"_a = 5, "z"_a);
-    m.def("kwonly_mixed", [](int i, int j) { return py::make_tuple(i, j); },
-            "i"_a, py::kwonly(), "j"_a);
-    m.def("kwonly_plus_more", [](int i, int j, int k, py::kwargs kwargs) {
+    m.def("kw_only_all", [](int i, int j) { return py::make_tuple(i, j); },
+            py::kw_only(), py::arg("i"), py::arg("j"));
+    m.def("kw_only_some", [](int i, int j, int k) { return py::make_tuple(i, j, k); },
+            py::arg(), py::kw_only(), py::arg("j"), py::arg("k"));
+    m.def("kw_only_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::kw_only(), "k"_a = 5, "z"_a);
+    m.def("kw_only_mixed", [](int i, int j) { return py::make_tuple(i, j); },
+            "i"_a, py::kw_only(), "j"_a);
+    m.def("kw_only_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::kwonly(), py::arg("k") /* kw-only */);
+            py::arg() /* positional */, py::arg("j") = -1 /* both */, py::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::kwonly(), py::arg() /* invalid unnamed argument */, "j"_a);
+    m.def("register_invalid_kw_only", [](py::module m) {
+        m.def("bad_kw_only", [](int i, int j) { return py::make_tuple(i, j); },
+                py::kw_only(), py::arg() /* invalid unnamed argument */, "j"_a);
     });
 
+    // test_positional_only_args
+    m.def("pos_only_all", [](int i, int j) { return py::make_tuple(i, j); },
+            py::arg("i"), py::arg("j"), py::pos_only());
+    m.def("pos_only_mix", [](int i, int j) { return py::make_tuple(i, j); },
+            py::arg("i"), py::pos_only(), py::arg("j"));
+    m.def("pos_kw_only_mix", [](int i, int j, int k) { return py::make_tuple(i, j, k); },
+            py::arg("i"), py::pos_only(), py::arg("j"), py::kw_only(), py::arg("k"));
+    m.def("pos_only_def_mix", [](int i, int j, int k) { return py::make_tuple(i, j, k); },
+            py::arg("i"), py::arg("j") = 2, py::pos_only(), py::arg("k") = 3);
+
+
     // These should fail to compile:
-    // argument annotations are required when using kwonly
-//    m.def("bad_kwonly1", [](int) {}, py::kwonly());
-    // can't specify both `py::kwonly` and a `py::args` argument
-//    m.def("bad_kwonly2", [](int i, py::args) {}, py::kwonly(), "i"_a);
+    // argument annotations are required when using kw_only
+//    m.def("bad_kw_only1", [](int) {}, py::kw_only());
+    // can't specify both `py::kw_only` and a `py::args` argument
+//    m.def("bad_kw_only2", [](int i, py::args) {}, py::kw_only(), "i"_a);
 
     // test_function_signatures (along with most of the above)
     struct KWClass { void foo(int, float) {} };

tests/test_kwargs_and_defaults.py

@@ -112,43 +112,91 @@ def test_mixed_args_and_kwargs(msg):
 
 
 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)
+    assert m.kw_only_all(i=1, j=2) == (1, 2)
+    assert m.kw_only_all(j=1, i=2) == (2, 1)
 
     with pytest.raises(TypeError) as excinfo:
-        assert m.kwonly_all(i=1) == (1,)
+        assert m.kw_only_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 m.kw_only_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.kw_only_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.kw_only_with_defaults(z=8) == (3, 4, 5, 8)
+    assert m.kw_only_with_defaults(2, z=8) == (2, 4, 5, 8)
+    assert m.kw_only_with_defaults(2, j=7, k=8, z=9) == (2, 7, 8, 9)
+    assert m.kw_only_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.kw_only_mixed(1, j=2) == (1, 2)
+    assert m.kw_only_mixed(j=2, i=3) == (3, 2)
+    assert m.kw_only_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})
+    assert m.kw_only_plus_more(4, 5, k=6, extra=7) == (4, 5, 6, {'extra': 7})
+    assert m.kw_only_plus_more(3, k=5, j=4, extra=6) == (3, 4, 5, {'extra': 6})
+    assert m.kw_only_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 m.kw_only_mixed(i=1) == (1,)
     assert "incompatible function arguments" in str(excinfo.value)
 
     with pytest.raises(RuntimeError) as excinfo:
-        m.register_invalid_kwonly(m)
+        m.register_invalid_kw_only(m)
     assert msg(excinfo.value) == """
-        arg(): cannot specify an unnamed argument after an kwonly() annotation
+        arg(): cannot specify an unnamed argument after an kw_only() annotation
     """
 
 
+def test_positional_only_args(msg):
+    assert m.pos_only_all(1, 2) == (1, 2)
+    assert m.pos_only_all(2, 1) == (2, 1)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_only_all(i=1, j=2)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    assert m.pos_only_mix(1, 2) == (1, 2)
+    assert m.pos_only_mix(2, j=1) == (2, 1)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_only_mix(i=1, j=2)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    assert m.pos_kw_only_mix(1, 2, k=3) == (1, 2, 3)
+    assert m.pos_kw_only_mix(1, j=2, k=3) == (1, 2, 3)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_kw_only_mix(i=1, j=2, k=3)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_kw_only_mix(1, 2, 3)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_only_def_mix()
+    assert "incompatible function arguments" in str(excinfo.value)
+
+    assert m.pos_only_def_mix(1) == (1, 2, 3)
+    assert m.pos_only_def_mix(1, 4) == (1, 4, 3)
+    assert m.pos_only_def_mix(1, 4, 7) == (1, 4, 7)
+    assert m.pos_only_def_mix(1, 4, k=7) == (1, 4, 7)
+
+    with pytest.raises(TypeError) as excinfo:
+        m.pos_only_def_mix(1, j=4)
+    assert "incompatible function arguments" in str(excinfo.value)
+
+
+def test_signatures():
+    assert "kw_only_all(*, i: int, j: int) -> tuple\n" == m.kw_only_all.__doc__
+    assert "kw_only_mixed(i: int, *, j: int) -> tuple\n" == m.kw_only_mixed.__doc__
+    assert "pos_only_all(i: int, j: int, /) -> tuple\n" == m.pos_only_all.__doc__
+    assert "pos_only_mix(i: int, /, j: int) -> tuple\n" == m.pos_only_mix.__doc__
+    assert "pos_kw_only_mix(i: int, /, j: int, *, k: int) -> tuple\n" == m.pos_kw_only_mix.__doc__
+
+
 @pytest.mark.xfail("env.PYPY and env.PY2", reason="PyPy2 doesn't double count")
 def test_args_refcount():
     """Issue/PR #1216 - py::args elements get double-inc_ref()ed when combined with regular