Add py::print() function

Replicates Python API including keyword arguments.

include/pybind11/pybind11.h

@@ -1233,6 +1233,33 @@ public:
     }
 };
 
+NAMESPACE_BEGIN(detail)
+PYBIND11_NOINLINE inline void print(tuple args, dict kwargs) {
+    auto strings = tuple(args.size());
+    for (size_t i = 0; i < args.size(); ++i) {
+        strings[i] = args[i].cast<object>().str();
+    }
+    auto sep = kwargs["sep"] ? kwargs["sep"] : cast(" ");
+    auto line = sep.attr("join").cast<object>()(strings);
+
+    auto file = kwargs["file"] ? kwargs["file"].cast<object>()
+                               : module::import("sys").attr("stdout");
+    auto write = file.attr("write").cast<object>();
+    write(line);
+    write(kwargs["end"] ? kwargs["end"] : cast("\n"));
+
+    if (kwargs["flush"] && kwargs["flush"].cast<bool>()) {
+        file.attr("flush").cast<object>()();
+    }
+}
+NAMESPACE_END(detail)
+
+template <return_value_policy policy = return_value_policy::automatic_reference, typename... Args>
+void print(Args &&...args) {
+    auto c = detail::collect_arguments<policy>(std::forward<Args>(args)...);
+    detail::print(c.args(), c.kwargs());
+}
+
 #if defined(WITH_THREAD)
 
 /* The functions below essentially reproduce the PyGILState_* API using a RAII

tests/conftest.py

@@ -68,18 +68,22 @@ class Capture(object):
     def __init__(self, capfd):
         self.capfd = capfd
         self.out = ""
+        self.err = ""
 
-    def _flush_stdout(self):
+    def _flush(self):
+        """Workaround for issues on Windows: to be removed after tests get py::print"""
         sys.stdout.flush()
-        os.fsync(sys.stdout.fileno())  # make sure C++ output is also read
+        os.fsync(sys.stdout.fileno())
-        return self.capfd.readouterr()[0]
+        sys.stderr.flush()
+        os.fsync(sys.stderr.fileno())
+        return self.capfd.readouterr()
 
     def __enter__(self):
-        self._flush_stdout()
+        self._flush()
         return self
 
     def __exit__(self, *_):
-        self.out = self._flush_stdout()
+        self.out, self.err = self._flush()
 
     def __eq__(self, other):
         a = Output(self.out)
@@ -100,6 +104,10 @@ class Capture(object):
     def unordered(self):
         return Unordered(self.out)
 
+    @property
+    def stderr(self):
+        return Output(self.err)
+
 
 @pytest.fixture
 def capture(capfd):

tests/test_python_types.cpp

@@ -197,4 +197,18 @@ test_initializer python_types([](py::module &m) {
         .def_readwrite_static("value", &ExamplePythonTypes::value, "Static value member")
         .def_readonly_static("value2", &ExamplePythonTypes::value2, "Static value member (readonly)")
         ;
+
+    m.def("test_print_function", []() {
+        py::print("Hello, World!");
+        py::print(1, 2.0, "three", true, std::string("-- multiple args"));
+        auto args = py::make_tuple("and", "a", "custom", "separator");
+        py::print("*args", *args, "sep"_a="-");
+        py::print("no new line here", "end"_a=" -- ");
+        py::print("next print");
+
+        auto py_stderr = py::module::import("sys").attr("stderr").cast<py::object>();
+        py::print("this goes to stderr", "file"_a=py_stderr);
+
+        py::print("flush", "flush"_a=true);
+    });
 });

tests/test_python_types.py

@@ -218,3 +218,18 @@ def test_module():
     assert ExamplePythonTypes.__module__ == "pybind11_tests"
     assert ExamplePythonTypes.get_set.__name__ == "get_set"
     assert ExamplePythonTypes.get_set.__module__ == "pybind11_tests"
+
+
+def test_print(capture):
+    from pybind11_tests import test_print_function
+
+    with capture:
+        test_print_function()
+    assert capture == """
+        Hello, World!
+        1 2.0 three True -- multiple args
+        *args-and-a-custom-separator
+        no new line here -- next print
+        flush
+    """
+    assert capture.stderr == "this goes to stderr"