[3.6] bpo-31232: Backport custom print rshift message (GH-3155)

bpo-30721 added a "Did you mean ...?" suggestion to rshift
TypeError messages that triggers when the LHS is a Python
C function called "print".

Since this custom error message is expected to be triggered
primarily by attempts to use Python 2 print redirection syntax
in Python 3, and is incredibly unlikely to be encountered
otherwise, it is also being backported to the next 3.6
maintenance release.

Initial patch by Sanyam Khurana.

3 files changed, 50 insertions, 0 deletions

diff --git a/Lib/test/test_print.py b/Lib/test/test_print.py
index 03f13b4..e6434fe 100644
--- a/Lib/test/test_print.py
+++ b/Lib/test/test_print.py
@@ -1,4 +1,5 @@
 import unittest
+import sys
 from io import StringIO
 
 from test import support
@@ -155,6 +156,38 @@ class TestPy2MigrationHint(unittest.TestCase):
 
         self.assertIn('print("Hello World", end=" ")', str(context.exception))
 
+    def test_stream_redirection_hint_for_py2_migration(self):
+        # Test correct hint produced for Py2 redirection syntax
+        with self.assertRaises(TypeError) as context:
+            print >> sys.stderr, "message"
+        self.assertIn('Did you mean "print(<message>, '
+                'file=<output_stream>)"?', str(context.exception))
+
+        # Test correct hint is produced in the case where RHS implements
+        # __rrshift__ but returns NotImplemented
+        with self.assertRaises(TypeError) as context:
+            print >> 42
+        self.assertIn('Did you mean "print(<message>, '
+                'file=<output_stream>)"?', str(context.exception))
+
+        # Test stream redirection hint is specific to print
+        with self.assertRaises(TypeError) as context:
+            max >> sys.stderr
+        self.assertNotIn('Did you mean ', str(context.exception))
+
+        # Test stream redirection hint is specific to rshift
+        with self.assertRaises(TypeError) as context:
+            print << sys.stderr
+        self.assertNotIn('Did you mean', str(context.exception))
+
+        # Ensure right operand implementing rrshift still works
+        class OverrideRRShift:
+            def __rrshift__(self, lhs):
+                return 42 # Force result independent of LHS
+
+        self.assertEqual(print >> OverrideRRShift(), 42)
+
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-08-18-15-15-20.bpo-30721.Hmc56z.rst b/Misc/NEWS.d/next/Core and Builtins/2017-08-18-15-15-20.bpo-30721.Hmc56z.rst
new file mode 100644
index 0000000..da553d6
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-08-18-15-15-20.bpo-30721.Hmc56z.rst
@@ -0,0 +1,2 @@
+``print`` now shows correct usage hint for using Python 2 redirection
+syntax.  Patch by Sanyam Khurana.
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 38de774..6c6c86c 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -809,6 +809,21 @@ binary_op(PyObject *v, PyObject *w, const int op_slot, const char *op_name)
     PyObject *result = binary_op1(v, w, op_slot);
     if (result == Py_NotImplemented) {
         Py_DECREF(result);
+
+        if (op_slot == NB_SLOT(nb_rshift) &&
+            PyCFunction_Check(v) &&
+            strcmp(((PyCFunctionObject *)v)->m_ml->ml_name, "print") == 0)
+        {
+            PyErr_Format(PyExc_TypeError,
+                "unsupported operand type(s) for %.100s: "
+                "'%.100s' and '%.100s'. Did you mean \"print(<message>, "
+                "file=<output_stream>)\"?",
+                op_name,
+                v->ob_type->tp_name,
+                w->ob_type->tp_name);
+            return NULL;
+        }
+
         return binop_type_error(v, w, op_name);
     }
     return result;