String literal warning.

Cython/Compiler/Errors.py

@@ -145,6 +145,20 @@ def warning(position, message, level=0):
         echo_file.write(line)
     return warn
 
+_warn_once_seen = {}
+def warn_once(position, message, level=0):
+    if level < LEVEL or message in _warn_once_seen:
+        return
+    warn = CompileWarning(position, message)
+    line = "warning: %s\n" % warn
+    if listing_file:
+        listing_file.write(line)
+    if echo_file:
+        echo_file.write(line)
+    _warn_once_seen[message] = True
+    return warn
+
+
 # These functions can be used to momentarily suppress errors. 
 
 error_stack = []

Cython/Compiler/ExprNodes.py

@@ -4,7 +4,7 @@
 
 import operator
 
-from Errors import error, warning, InternalError
+from Errors import error, warning, warn_once, InternalError
 from Errors import hold_errors, release_errors, held_errors, report_error
 from Cython.Utils import UtilityCode
 import StringEncoding
@@ -942,6 +942,7 @@ class StringNode(ConstNode):
         # Arrange for a Python version of the string to be pre-allocated
         # when coercing to a Python type.
         if dst_type.is_pyobject and not self.type.is_pyobject:
+            warn_once(self.pos, "String literals will no longer be Py3 bytes in Cython 0.12.", 1)
             node = self.as_py_string_node(env)
         else:
             node = self