Method's default argument shouldn't be mutable

http://docs.python-guide.org/en/latest/writing/gotchas/

Change-Id: I5e22e66de245da0bac1f35f81460ea9cfdc00772

HACKING.rst

@@ -10,7 +10,8 @@ Senlin Specific Commandments
 
 - [S318] Change assertEqual(A, None) or assertEqual(None, A) by optimal assert
   like assertIsNone(A)
-- [N319] Ensure that jsonutils.%(fun)s must be used instead of json.%(fun)s
+- [S319] Ensure that jsonutils.%(fun)s must be used instead of json.%(fun)s
+- [S320] Method's default argument shouldn't be mutable!
 
 Working on APIs
 ---------------

senlin/hacking/checks.py

@@ -14,6 +14,7 @@ import re
 
 asse_equal_end_with_none_re = re.compile(r"assertEqual\(.*?,\s+None\)$")
 asse_equal_start_with_none_re = re.compile(r"assertEqual\(None,")
+mutable_default_args = re.compile(r"^\s*def .+\((.+=\{\}|.+=\[\])")
 
 
 def assert_equal_none(logical_line):
@@ -39,6 +40,13 @@ def use_jsonutils(logical_line, filename):
                 yield (pos, msg % {'fun': f[:-1]})
 
 
+def no_mutable_default_args(logical_line):
+    msg = "S320: Method's default argument shouldn't be mutable!"
+    if mutable_default_args.match(logical_line):
+        yield (0, msg)
+
+
 def factory(register):
     register(assert_equal_none)
     register(use_jsonutils)
+    register(no_mutable_default_args)

senlin/tests/functional/api.py

@@ -16,7 +16,9 @@ import requests
 
 def create_cluster(client, name, profile_id, desired_capacity,
                    min_size=0, max_size=-1, parent=None,
-                   metadata={}, timeout=120):
+                   metadata=None, timeout=120):
+    if not metadata:
+        metadata = {}
     rel_url = 'clusters'
     status = [202]
     data = {
@@ -126,7 +128,9 @@ def delete_node(client, node_id):
     return
 
 
-def create_profile(client, name, spec, metadata={}):
+def create_profile(client, name, spec, metadata=None):
+    if not metadata:
+        metadata = {}
     rel_url = 'profiles'
     status = [201]
     data = {

senlin/tests/unit/test_hacking.py

@@ -38,3 +38,13 @@ class HackingTestCase(base.SenlinTestCase):
                 "jsonx.%s(" % method, "./senlin/engine/cluster.py"))))
         self.assertEqual(0, len(list(checks.use_jsonutils(
             "json.dumb", "./senlin/engine/cluster.py"))))
+
+    def test_no_mutable_default_args(self):
+        self.assertEqual(1, len(list(checks.no_mutable_default_args(
+            "def create_cluster(mapping={}, **params)"))))
+
+        self.assertEqual(0, len(list(checks.no_mutable_default_args(
+            "defined = []"))))
+
+        self.assertEqual(0, len(list(checks.no_mutable_default_args(
+            "defined, undefined = [], {}"))))