Closes #13761: add a "flush" keyword argument to print().

Doc/library/functions.rst

@@ -946,7 +946,7 @@ are always available.  They are listed here in alphabetical order.
    must be of integer types, and *y* must be non-negative.
 
 
-.. function:: print([object, ...], *, sep=' ', end='\\n', file=sys.stdout)
+.. function:: print([object, ...], *, sep=' ', end='\\n', file=sys.stdout, flush=False)
 
    Print *object*\(s) to the stream *file*, separated by *sep* and followed by
    *end*.  *sep*, *end* and *file*, if present, must be given as keyword
@@ -959,9 +959,12 @@ are always available.  They are listed here in alphabetical order.
    *end*.
 
    The *file* argument must be an object with a ``write(string)`` method; if it
-   is not present or ``None``, :data:`sys.stdout` will be used. Output buffering
-   is determined by *file*. Use ``file.flush()`` to ensure, for instance,
-   immediate appearance on a screen.
+   is not present or ``None``, :data:`sys.stdout` will be used.  Whether output
+   is buffered is usually determined by *file*, but if the  *flush* keyword
+   argument is true, the stream is forcibly flushed.
+
+   .. versionchanged:: 3.3
+      Added the *flush* keyword argument.
 
 
 .. function:: property(fget=None, fset=None, fdel=None, doc=None)

Lib/test/test_print.py

@@ -111,6 +111,32 @@ class TestPrint(unittest.TestCase):
         self.assertRaises(TypeError, print, '', end=3)
         self.assertRaises(AttributeError, print, '', file='')
 
+    def test_print_flush(self):
+        # operation of the flush flag
+        class filelike():
+            def __init__(self):
+                self.written = ''
+                self.flushed = 0
+            def write(self, str):
+                self.written += str
+            def flush(self):
+                self.flushed += 1
+
+        f = filelike()
+        print(1, file=f, end='', flush=True)
+        print(2, file=f, end='', flush=True)
+        print(3, file=f, flush=False)
+        self.assertEqual(f.written, '123\n')
+        self.assertEqual(f.flushed, 2)
+
+        # ensure exceptions from flush are passed through
+        class noflush():
+            def write(self, str):
+                pass
+            def flush(self):
+                raise RuntimeError
+        self.assertRaises(RuntimeError, print, 1, file=noflush(), flush=True)
+
 def test_main():
     support.run_unittest(TestPrint)
 

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #13761: Add a "flush" keyword argument to the print() function,
+  used to ensure flushing the output stream.
+
 - Issue #13645: pyc files now contain the size of the corresponding source
   code, to avoid timestamp collisions (especially on filesystems with a low
   timestamp resolution) when checking for freshness of the bytecode.

Python/bltinmodule.c

@@ -1484,15 +1484,15 @@ equivalent to (x**y) % z, but may be more efficient (e.g. for longs).");
 static PyObject *
 builtin_print(PyObject *self, PyObject *args, PyObject *kwds)
 {
-    static char *kwlist[] = {"sep", "end", "file", 0};
+    static char *kwlist[] = {"sep", "end", "file", "flush", 0};
     static PyObject *dummy_args;
-    PyObject *sep = NULL, *end = NULL, *file = NULL;
+    PyObject *sep = NULL, *end = NULL, *file = NULL, *flush = NULL;
     int i, err;
 
     if (dummy_args == NULL && !(dummy_args = PyTuple_New(0)))
-            return NULL;
-    if (!PyArg_ParseTupleAndKeywords(dummy_args, kwds, "|OOO:print",
-                                     kwlist, &sep, &end, &file))
+        return NULL;
+    if (!PyArg_ParseTupleAndKeywords(dummy_args, kwds, "|OOOO:print",
+                                     kwlist, &sep, &end, &file, &flush))
         return NULL;
     if (file == NULL || file == Py_None) {
         file = PySys_GetObject("stdout");
@@ -1543,6 +1543,20 @@ builtin_print(PyObject *self, PyObject *args, PyObject *kwds)
     if (err)
         return NULL;
 
+    if (flush != NULL) {
+        PyObject *tmp;
+        int do_flush = PyObject_IsTrue(flush);
+        if (do_flush == -1)
+            return NULL;
+        else if (do_flush) {
+            tmp = PyObject_CallMethod(file, "flush", "");
+            if (tmp == NULL)
+                return NULL;
+            else
+                Py_DECREF(tmp);
+        }
+    }
+
     Py_RETURN_NONE;
 }