Removes the use of mutables as default args

Passing mutable objects as default args is a known Python pitfall.
We'd better avoid this. This commit changes mutable default args with
None, then use 'arg = arg or {}', 'arg = arg or []'. For unit code which
doesn't use the args , just set with None. This commit also adds hacking
check.

Change-Id: Ib4f195c9c00ca2c49579f9d62648bff7c8109fcf
Closes-Bug: #1327473

HACKING.rst

@@ -16,6 +16,7 @@ Octavia Specific Commandments
 - [O322] Don't use author tags
 - [O323] Change assertEqual(True, A) or assertEqual(False, A) to the more
   specific assertTrue(A) or assertFalse(A)
+- [O324] Method's default argument shouldn't be mutable
 - [O338] Change assertEqual(A in B, True), assertEqual(True, A in B),
   assertEqual(A in B, False) or assertEqual(False, A in B) to the more
   specific assertIn/NotIn(A, B)

octavia/common/data_models.py

@@ -211,13 +211,13 @@ class SNI(BaseDataModel):
 class TLSContainer(BaseDataModel):
 
     def __init__(self, id=None, primary_cn=None, certificate=None,
-                 private_key=None, passphrase=None, intermediates=[]):
+                 private_key=None, passphrase=None, intermediates=None):
         self.id = id
         self.primary_cn = primary_cn
         self.certificate = certificate
         self.private_key = private_key
         self.passphrase = passphrase
-        self.intermediates = intermediates
+        self.intermediates = intermediates or []
 
 
 class Amphora(BaseDataModel):

octavia/hacking/checks.py

@@ -66,6 +66,7 @@ assert_equal_with_true_re = re.compile(
     r"assertEqual\(True,")
 assert_equal_with_false_re = re.compile(
     r"assertEqual\(False,")
+mutable_default_args = re.compile(r"^\s*def .+\((.+=\{\}|.+=\[\])")
 
 
 def _directory_to_check_translation(filename):
@@ -155,6 +156,12 @@ def assert_equal_true_or_false(logical_line):
                "sentences not allowed")
 
 
+def no_mutable_default_args(logical_line):
+    msg = "O324: Method's default argument shouldn't be mutable!"
+    if mutable_default_args.match(logical_line):
+        yield (0, msg)
+
+
 def assert_equal_in(logical_line):
     """Check for assertEqual(A in B, True), assertEqual(True, A in B),
 
@@ -177,4 +184,5 @@ def factory(register):
     register(use_jsonutils)
     register(no_author_tags)
     register(assert_equal_true_or_false)
+    register(no_mutable_default_args)
     register(assert_equal_in)

octavia/tests/unit/common/sample_configs/sample_configs.py

@@ -205,13 +205,13 @@ def sample_tls_sni_containers_tuple(tls_container=None):
 
 
 def sample_tls_container_tuple(id='cont_id_1', certificate=None,
-                               private_key=None, intermediates=[],
+                               private_key=None, intermediates=None,
                                primary_cn=None):
     sc = collections.namedtuple(
         'tls_container',
         'id, certificate, private_key, intermediates, primary_cn')
     return sc(id=id, certificate=certificate, private_key=private_key,
-              intermediates=intermediates, primary_cn=primary_cn)
+              intermediates=intermediates or [], primary_cn=primary_cn)
 
 
 def sample_pool_tuple(proto=None, monitor=True, persistence=True,