Use generic arg names for functions without explicitly named arguments

Example signatures (old => new):
  foo(int) => foo(arg0: int)
  bar(Object, int) => bar(self: Object, arg0: int)

The change makes the signatures uniform for named and unnamed arguments
and it helps static analysis tools reconstruct function signatures from
docstrings.

This also tweaks the signature whitespace style to better conform to
PEP 8 for annotations and default arguments:
  " : " => ": "
  " = " => "="

example/example-arg-keywords-and-defaults.cpp

@@ -40,8 +40,13 @@ void args_kwargs_function(py::args args, py::kwargs kwargs) {
     }
 }
 
+struct KWClass {
+    void foo(int, float) {}
+};
+
 void init_ex_arg_keywords_and_defaults(py::module &m) {
-    m.def("kw_func", &kw_func, py::arg("x"), py::arg("y"));
+    m.def("kw_func0", &kw_func);
+    m.def("kw_func1", &kw_func, py::arg("x"), py::arg("y"));
     m.def("kw_func2", &kw_func, py::arg("x") = 100, py::arg("y") = 200);
     m.def("kw_func3", [](const char *) { }, py::arg("data") = std::string("Hello world!"));
 
@@ -59,4 +64,8 @@ void init_ex_arg_keywords_and_defaults(py::module &m) {
     using namespace py::literals;
     m.def("kw_func_udl", &kw_func, "x"_a, "y"_a=300);
     m.def("kw_func_udl_z", &kw_func, "x"_a, "y"_a=0);
+    
+    py::class_<KWClass>(m, "KWClass")
+        .def("foo0", &KWClass::foo)
+        .def("foo1", &KWClass::foo, "x"_a, "y"_a);
 }

example/example-arg-keywords-and-defaults.py

@@ -5,19 +5,26 @@ import pydoc
 
 sys.path.append('.')
 
-from example import kw_func, kw_func2, kw_func3, kw_func4, call_kw_func
+from example import kw_func0, kw_func1, kw_func2, kw_func3, kw_func4, call_kw_func
 from example import args_function, args_kwargs_function, kw_func_udl, kw_func_udl_z
+from example import KWClass
 
-print(pydoc.render_doc(kw_func, "Help on %s"))
+print(pydoc.render_doc(kw_func0, "Help on %s"))
+print(pydoc.render_doc(kw_func1, "Help on %s"))
 print(pydoc.render_doc(kw_func2, "Help on %s"))
 print(pydoc.render_doc(kw_func3, "Help on %s"))
 print(pydoc.render_doc(kw_func4, "Help on %s"))
 print(pydoc.render_doc(kw_func_udl, "Help on %s"))
 print(pydoc.render_doc(kw_func_udl_z, "Help on %s"))
+print(pydoc.render_doc(args_function, "Help on %s"))
+print(pydoc.render_doc(args_kwargs_function, "Help on %s"))
 
-kw_func(5, 10)
-kw_func(5, y=10)
-kw_func(y=10, x=5)
+print(KWClass.foo0.__doc__)
+print(KWClass.foo1.__doc__)
+
+kw_func1(5, 10)
+kw_func1(5, y=10)
+kw_func1(y=10, x=5)
 
 kw_func2()
 

example/example-arg-keywords-and-defaults.ref

@@ -1,32 +1,52 @@
-Help on built-in function kw_func in module example
+Help on built-in function kw_func0 in module example
 
-kkww__ffuunncc(...)
-    kw_func(x : int, y : int) -> NoneType
+kkww__ffuunncc00(...)
+    kw_func0(arg0: int, arg1: int) -> NoneType
+
+Help on built-in function kw_func1 in module example
+
+kkww__ffuunncc11(...)
+    kw_func1(x: int, y: int) -> NoneType
 
 Help on built-in function kw_func2 in module example
 
 kkww__ffuunncc22(...)
-    kw_func2(x : int = 100L, y : int = 200L) -> NoneType
+    kw_func2(x: int=100L, y: int=200L) -> NoneType
 
 Help on built-in function kw_func3 in module example
 
 kkww__ffuunncc33(...)
-    kw_func3(data : unicode = u'Hello world!') -> NoneType
+    kw_func3(data: unicode=u'Hello world!') -> NoneType
 
 Help on built-in function kw_func4 in module example
 
 kkww__ffuunncc44(...)
-    kw_func4(myList : list<int> = [13L, 17L]) -> NoneType
+    kw_func4(myList: list<int>=[13L, 17L]) -> NoneType
 
 Help on built-in function kw_func_udl in module example
 
 kkww__ffuunncc__uuddll(...)
-    kw_func_udl(x : int, y : int = 300L) -> NoneType
+    kw_func_udl(x: int, y: int=300L) -> NoneType
 
 Help on built-in function kw_func_udl_z in module example
 
 kkww__ffuunncc__uuddll__zz(...)
-    kw_func_udl_z(x : int, y : int = 0L) -> NoneType
+    kw_func_udl_z(x: int, y: int=0L) -> NoneType
+
+Help on built-in function args_function in module example
+
+aarrggss__ffuunnccttiioonn(...)
+    args_function(*args) -> NoneType
+
+Help on built-in function args_kwargs_function in module example
+
+aarrggss__kkwwaarrggss__ffuunnccttiioonn(...)
+    args_kwargs_function(*args, **kwargs) -> NoneType
+
+
+foo0(self: KWClass, arg0: int, arg1: float) -> NoneType
+foo1(self: KWClass, x: int, y: float) -> NoneType
+
 
 kw_func(x=5, y=10)
 kw_func(x=5, y=10)
@@ -38,7 +58,7 @@ kw_func(x=100, y=10)
 kw_func(x=5, y=10)
 kw_func(x=5, y=10)
 Caught expected exception: Incompatible function arguments. The following argument types are supported:
-    1. (x : int = 100L, y : int = 200L) -> NoneType
+    1. (x: int=100L, y: int=200L) -> NoneType
     Invoked with:
 kw_func4: 13 17 
 kw_func4: 1 2 3 

example/example-callbacks.ref

@@ -6,7 +6,7 @@ Molly is a dog
 Molly is a dog
 Woof!
 The following error is expected: Incompatible function arguments. The following argument types are supported:
-    1. (example.Dog) -> NoneType
+    1. (arg0: example.Dog) -> NoneType
     Invoked with: <example.Pet object at 0>
 Callback function 1 called!
 False

example/issues.ref

@@ -6,10 +6,10 @@ Yay..
 [3, 5, 7, 9, 11, 13, 15]
 0==0, 1==1, 2==2, 3==3, 4==4, 5==5, 6==6, 7==7, 8==8, 9==9, 
 Failed as expected: Incompatible function arguments. The following argument types are supported:
-    1. (example.issues.ElementA) -> NoneType
+    1. (arg0: example.issues.ElementA) -> NoneType
     Invoked with: None
 Failed as expected: Incompatible function arguments. The following argument types are supported:
-    1. (int) -> int
+    1. (arg0: int) -> int
     Invoked with: 5.2
 12.0
 C++ version
@@ -21,6 +21,6 @@ In python f()
 StrIssue.__str__ called
 StrIssue[3]
 Failed as expected: Incompatible constructor arguments. The following argument types are supported:
-    1. example.issues.StrIssue(int)
+    1. example.issues.StrIssue(arg0: int)
     2. example.issues.StrIssue()
     Invoked with: no, such, constructor

include/pybind11/pybind11.h

@@ -157,7 +157,7 @@ protected:
 
     /// Register a function call with Python (generic non-templated code goes here)
     void initialize_generic(detail::function_record *rec, const char *text,
-                            const std::type_info *const *types, int args) {
+                            const std::type_info *const *types, size_t args) {
 
         /* Create copies of all referenced C-style strings */
         rec->name = strdup(rec->name ? rec->name : "");
@@ -182,16 +182,23 @@ protected:
                 break;
 
             if (c == '{') {
-                if (type_depth == 1 && arg_index < rec->args.size()) {
-                    signature += rec->args[arg_index].name;
-                    signature += " : ";
+                // Write arg name for everything except *args, **kwargs and return type.
+                if (type_depth == 1 && text[char_index] != '*' && arg_index < args) {
+                    if (!rec->args.empty()) {
+                        signature += rec->args[arg_index].name;
+                    } else if (arg_index == 0 && rec->class_) {
+                        signature += "self";
+                    } else {
+                        signature += "arg" + std::to_string(arg_index - (rec->class_ ? 1 : 0));
+                    }
+                    signature += ": ";
                 }
                 ++type_depth;
             } else if (c == '}') {
                 --type_depth;
-                if (type_depth == 1 && arg_index < rec->args.size()) {
-                    if (rec->args[arg_index].descr) {
-                        signature += " = ";
+                if (type_depth == 1) {
+                    if (arg_index < rec->args.size() && rec->args[arg_index].descr) {
+                        signature += "=";
                         signature += rec->args[arg_index].descr;
                     }
                     arg_index++;
@@ -453,9 +460,9 @@ protected:
 
                 bool wrote_sig = false;
                 if (overloads->is_constructor) {
-                    // For a constructor, rewrite `(Object, arg0, ...) -> NoneType` as `Object(arg0, ...)`
+                    // For a constructor, rewrite `(self: Object, arg0, ...) -> NoneType` as `Object(arg0, ...)`
                     std::string sig = it2->signature;
-                    size_t start = sig.find('(') + 1;
+                    size_t start = sig.find('(') + 7; // skip "(self: "
                     if (start < sig.size()) {
                         // End at the , for the next argument
                         size_t end = sig.find(", "), next = end + 2;