Removed unnecessary parantheses in yield statements

The 'yield' statement is not a function.
So it must always be followed by a space when yielding a value.

Change-Id: Ie2aa1c4822d8adf721ba71d467b63209bede6fb7

HACKING.rst

@@ -67,6 +67,7 @@ Nova Specific Commandments
   generate UUID instead of uuid4().
 - [N358] Return must always be followed by a space when returning a value.
 - [N359] Check for redundant import aliases.
+- [N360] Yield must always be followed by a space when yielding a value.
 
 Creating Unit Tests
 -------------------

nova/hacking/checks.py

@@ -99,6 +99,7 @@ log_remove_context = re.compile(
 return_not_followed_by_space = re.compile(r"^\s*return(?:\(|{|\"|'|#).*$")
 uuid4_re = re.compile(r"uuid4\(\)($|[^\.]|\.hex)")
 redundant_import_alias_re = re.compile(r"import (?:.*\.)?(.+) as \1$")
+yield_not_followed_by_space = re.compile(r"^\s*yield(?:\(|{|\[|\"|').*$")
 
 
 class BaseASTChecker(ast.NodeVisitor):
@@ -817,6 +818,23 @@ def no_redundant_import_alias(logical_line):
         yield (0, "N359: Import alias should not be redundant.")
 
 
+def yield_followed_by_space(logical_line):
+    """Yield should be followed by a space.
+
+    Yield should be followed by a space to clarify that yield is
+    not a function. Adding a space may force the developer to rethink
+    if there are unnecessary parentheses in the written code.
+
+    Not correct: yield(x), yield(a, b)
+    Correct: yield x, yield (a, b), yield a, b
+
+    N360
+    """
+    if yield_not_followed_by_space.match(logical_line):
+        yield (0,
+               "N360: Yield keyword should be followed by a space.")
+
+
 def factory(register):
     register(import_no_db_in_virt)
     register(no_db_session_in_public_api)
@@ -859,3 +877,4 @@ def factory(register):
     register(check_uuid4)
     register(return_followed_by_space)
     register(no_redundant_import_alias)
+    register(yield_followed_by_space)

nova/tests/unit/test_hacking.py

@@ -802,3 +802,25 @@ class HackingTestCase(test.NoDBTestCase):
                   import ab.cd.efg as d.efg
                """
         self._assert_has_no_errors(code, checks.no_redundant_import_alias)
+
+    def test_yield_followed_by_space(self):
+        code = """
+                  yield(x, y)
+                  yield{"type": "test"}
+                  yield[a, b, c]
+                  yield"test"
+                  yield'test'
+               """
+        errors = [(x + 1, 0, 'N360') for x in range(5)]
+        self._assert_has_errors(code, checks.yield_followed_by_space,
+                                expected_errors=errors)
+        code = """
+                  yield x
+                  yield (x, y)
+                  yield {"type": "test"}
+                  yield [a, b, c]
+                  yield "test"
+                  yield 'test'
+                  yieldx_func(a, b)
+               """
+        self._assert_has_no_errors(code, checks.yield_followed_by_space)