implement functions to work with file descriptors

* add function to convert os.File to PyObject

* helpers to set stdin, stdout, stderr

Author: dennwc

file.go

@@ -0,0 +1,42 @@
+package python
+
+/*
+#include <stdio.h>
+#include "go-python.h"
+
+PyObject*
+_gopy_PyFile_FromFile(int fd, char *name, char *mode) {
+    FILE *f = fdopen(fd, mode);
+    PyObject *py = PyFile_FromFile(f, name, mode, NULL);
+    PyFile_SetBufSize(py, 0);
+    return py;
+}
+
+*/
+import "C"
+
+import (
+	"os"
+)
+
+// FromFile converts a Go file to Python file object.
+// Calling close from Python will not close a file descriptor.
+func FromFile(f *os.File, mode string) *PyObject {
+	p := C._gopy_PyFile_FromFile(C.int(f.Fd()), C.CString(f.Name()), C.CString(mode))
+	return togo(p)
+}
+
+// SetStdin sets a sys.stdin to a specified file descriptor.
+func SetStdin(f *os.File) error {
+	return PySys_SetObject("stdin", FromFile(f, "r"))
+}
+
+// SetStdout sets a sys.stdout to a specified file descriptor.
+func SetStdout(f *os.File) error {
+	return PySys_SetObject("stdout", FromFile(f, "w"))
+}
+
+// SetStderr sets a sys.stderr to a specified file descriptor.
+func SetStderr(f *os.File) error {
+	return PySys_SetObject("stderr", FromFile(f, "w"))
+}