Demonstrate about the generator issue for pre-3.3 version of Python.

Signed-off-by: HE, Tao <sighingnow@gmail.com>

src/parsec/__init__.py

@@ -364,6 +364,11 @@ def desc(p, description):
 # These parsers are applied successively and their results are sent back to the
 # generator using `.send()` protocol. The generator should return the result or
 # another parser, which is equivalent to applying it and returning its result.
+#
+# Note that `return` with arguments inside generator is not supported in Python 2.
+# Instead, we can raise a `StopIteration` to return the result in Python 2.
+#
+# See #15 and `test_generate_raise` in tests/test_parsec.py
 ##########################################################################
 
 

tests/test_parsec.py

@@ -326,5 +326,27 @@ def xy():
         # should not finish executing xy()
         self.assertEqual(parser.parse('z'), 'z')
 
+    def test_generate_raise(self):
+
+        # `return` with argument inside generator is not supported in Python 2.
+        # Instead, we can raise a `StopIteration` directly with the intended
+        # result in generator for Python 2.
+        #
+        # Before Python 3.3, the `StopIteration` didn't have the `value` attribute,
+        # we need to assign the attribute manually.
+        #
+        # See #15.
+
+        @generate
+        def xy():
+            yield string('x')
+            yield string('y')
+            r = StopIteration('success')
+            r.value = 'success'  # for pre-3.3 Python
+            raise r
+
+        parser = xy
+        self.assertEqual(parser.parse('xy'), 'success')
+
 if __name__ == '__main__':
     unittest.main()