Keyword arguments and generalized unpacking for C++ API

docs/advanced.rst

@@ -1622,24 +1622,76 @@ It is also possible to call python functions via ``operator()``.
     py::object result_py = f(1234, "hello", some_instance);
     MyClass &result = result_py.cast<MyClass>();
 
-The special ``f(*args)`` and ``f(*args, **kwargs)`` syntax is also supported to
-supply arbitrary argument and keyword lists, although these cannot be mixed
-with other parameters.
+Keyword arguments are also supported. In Python, there is the usual call syntax:
+
+.. code-block:: python
+
+    def f(number, say, to):
+        ...  # function code
+
+    f(1234, say="hello", to=some_instance)  # keyword call in Python
+
+In C++, the same call can be made using:
 
 .. code-block:: cpp
 
-    py::function f = <...>;
+    using pybind11::literals; // to bring in the `_a` literal
+    f(1234, "say"_a="hello", "to"_a=some_instance); // keyword call in C++
+
+Unpacking of ``*args`` and ``**kwargs`` is also possible and can be mixed with
+other arguments:
+
+.. code-block:: cpp
+
+    // * unpacking
+    py::tuple args = py::make_tuple(1234, "hello", some_instance);
+    f(*args);
+
+    // ** unpacking
+    py::dict kwargs = py::dict("number"_a=1234, "say"_a="hello", "to"_a=some_instance);
+    f(**kwargs);
+
+    // mixed keywords, * and ** unpacking
     py::tuple args = py::make_tuple(1234);
-    py::dict kwargs;
-    kwargs["y"] = py::cast(5678);
-    py::object result = f(*args, **kwargs);
+    py::dict kwargs = py::dict("to"_a=some_instance);
+    f(*args, "say"_a="hello", **kwargs);
+
+Generalized unpacking according to PEP448_ is also supported:
+
+.. code-block:: cpp
+
+    py::dict kwargs1 = py::dict("number"_a=1234);
+    py::dict kwargs2 = py::dict("to"_a=some_instance);
+    f(**kwargs1, "say"_a="hello", **kwargs2);
 
 .. seealso::
 
     The file :file:`tests/test_python_types.cpp` contains a complete
     example that demonstrates passing native Python types in more detail. The
-    file :file:`tests/test_kwargs_and_defaults.cpp` discusses usage
-    of ``args`` and ``kwargs``.
+    file :file:`tests/test_callbacks.cpp` presents a few examples of calling
+    Python functions from C++, including keywords arguments and unpacking.
+
+.. _PEP448: https://www.python.org/dev/peps/pep-0448/
+
+Using Python's print function in C++
+====================================
+
+The usual way to write output in C++ is using ``std::cout`` while in Python one
+would use ``print``. Since these methods use different buffers, mixing them can
+lead to output order issues. To resolve this, pybind11 modules can use the
+:func:`py::print` function which writes to Python's ``sys.stdout`` for consistency.
+
+Python's ``print`` function is replicated in the C++ API including optional
+keyword arguments ``sep``, ``end``, ``file``, ``flush``. Everything works as
+expected in Python:
+
+.. code-block:: cpp
+
+    py::print(1, 2.0, "three"); // 1 2.0 three
+    py::print(1, 2.0, "three", "sep"_a="-"); // 1-2.0-three
+
+    auto args = py::make_tuple("unpacked", true);
+    py::print("->", *args, "end"_a="<-"); // -> unpacked True <-
 
 Default arguments revisited
 ===========================

docs/changelog.rst

@@ -46,6 +46,13 @@ Breaking changes queued for v2.0.0 (Not yet released)
 * Added constructors for ``str`` and ``bytes`` from zero-terminated char pointers,
   and from char pointers and length.
 * Added ``memoryview`` wrapper type which is constructible from ``buffer_info``.
+* New syntax to call a Python function from C++ using keyword arguments and unpacking,
+  e.g. ``foo(1, 2, "z"_a=3)`` or ``bar(1, *args, "z"_a=3, **kwargs)``.
+* Added ``py::print()`` function which replicates Python's API and writes to Python's
+  ``sys.stdout`` by default (as opposed to C's ``stdout`` like ``std::cout``).
+* Added ``py::dict`` keyword constructor:``auto d = dict("number"_a=42, "name"_a="World");``
+* Added ``py::str::format()`` method and ``_s`` literal:
+  ``py::str s = "1 + 2 = {}"_s.format(3);``
 * Various minor improvements of library internals (no user-visible changes)
 
 1.8.1 (July 12, 2016)

include/pybind11/attr.h

@@ -14,35 +14,6 @@
 
 NAMESPACE_BEGIN(pybind11)
 
-template <typename T> struct arg_t;
-
-/// Annotation for keyword arguments
-struct arg {
-    constexpr explicit arg(const char *name) : name(name) { }
-
-    template <typename T>
-    constexpr arg_t<T> operator=(const T &value) const { return {name, value}; }
-    template <typename T, size_t N>
-    constexpr arg_t<const T *> operator=(T const (&value)[N]) const {
-        return operator=((const T *) value);
-    }
-
-    const char *name;
-};
-
-/// Annotation for keyword arguments with default values
-template <typename T> struct arg_t : public arg {
-    constexpr arg_t(const char *name, const T &value, const char *descr = nullptr)
-        : arg(name), value(value), descr(descr) { }
-    T value;
-    const char *descr;
-};
-
-inline namespace literals {
-/// String literal version of arg
-constexpr arg operator"" _a(const char *name, size_t) { return arg(name); }
-}
-
 /// Annotation for methods
 struct is_method { handle class_; is_method(const handle &c) : class_(c) { } };
 
@@ -238,21 +209,14 @@ template <> struct process_attribute<arg> : process_attribute_default<arg> {
 };
 
 /// Process a keyword argument attribute (*with* a default value)
-template <typename T>
-struct process_attribute<arg_t<T>> : process_attribute_default<arg_t<T>> {
-    static void init(const arg_t<T> &a, function_record *r) {
+template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
+    static void init(const arg_v &a, function_record *r) {
         if (r->class_ && r->args.empty())
             r->args.emplace_back("self", nullptr, handle());
 
-        /* Convert keyword value into a Python object */
-        object o = object(detail::type_caster<typename detail::intrinsic_type<T>::type>::cast(
-                a.value, return_value_policy::automatic, handle()), false);
-
-        if (!o) {
+        if (!a.value) {
 #if !defined(NDEBUG)
-            std::string descr(typeid(T).name());
-            detail::clean_type_id(descr);
-            descr = "'" + std::string(a.name) + ": " + descr + "'";
+            auto descr = "'" + std::string(a.name) + ": " + a.type + "'";
             if (r->class_) {
                 if (r->name)
                     descr += " in method '" + (std::string) r->class_.str() + "." + (std::string) r->name + "'";
@@ -269,7 +233,7 @@ struct process_attribute<arg_t<T>> : process_attribute_default<arg_t<T>> {
                           "Compile in debug mode for more information.");
 #endif
         }
-        r->args.emplace_back(a.name, a.descr, o.release());
+        r->args.emplace_back(a.name, a.descr, a.value.inc_ref());
     }
 };
 
@@ -301,9 +265,6 @@ template <int Nurse, int Patient> struct process_attribute<keep_alive<Nurse, Pat
     static void postcall(handle args, handle ret) { keep_alive_impl(Nurse, Patient, args, ret); }
 };
 
-/// Ignore that a variable is unused in compiler warnings
-inline void ignore_unused(const int *) { }
-
 /// Recursively iterate over variadic template arguments
 template <typename... Args> struct process_attributes {
     static void init(const Args&... args, function_record *r) {
@@ -324,11 +285,6 @@ template <typename... Args> struct process_attributes {
     }
 };
 
-/// Compile-time integer sum
-constexpr size_t constexpr_sum() { return 0; }
-template <typename T, typename... Ts>
-constexpr size_t constexpr_sum(T n, Ts... ns) { return n + constexpr_sum(ns...); }
-
 /// Check the number of named arguments at compile time
 template <typename... Extra,
           size_t named = constexpr_sum(std::is_base_of<arg, Extra>::value...),