Use immutable arg rather mutable arg

Passing mutable objects as default args is a known Python pitfall.
We'd better avoid this. This commit changes mutable default args with
None.

Change-Id: I7817c92069d32d1926e185ca0d0ab14e738b8b3d

cinderclient/tests/fakes.py

@@ -22,7 +22,10 @@ places where actual behavior differs from the spec.
 from __future__ import print_function
 
 
-def assert_has_keys(dict, required=[], optional=[]):
+def assert_has_keys(dict, required=None, optional=None):
+    required = required or []
+    optional = optional or []
+
     for k in required:
         try:
             assert k in dict

cinderclient/tests/v1/test_shell.py

@@ -79,8 +79,8 @@ class ShellTest(utils.TestCase):
         # mimic the result of argparse's parse_args() method
         class Arguments:
 
-            def __init__(self, metadata=[]):
-                self.metadata = metadata
+            def __init__(self, metadata=None):
+                self.metadata = metadata or []
 
         inputs = [
             ([], {}),

cinderclient/utils.py

@@ -154,7 +154,8 @@ def _print(pt, order):
         print(strutils.safe_encode(pt.get_string(sortby=order)))
 
 
-def print_list(objs, fields, formatters={}, order_by=None):
+def print_list(objs, fields, formatters=None, order_by=None):
+    formatters = formatters or {}
     mixed_case_fields = ['serverId']
     pt = prettytable.PrettyTable([f for f in fields], caching=False)
     pt.aligns = ['l' for f in fields]