From: Thierry Parmentelat <thierry.parmentelat@inria.fr>
Date: Thu, 17 Jul 2014 11:14:15 +0000 (+0200)
Subject: huge cleanup for removing mutables used as default
X-Git-Tag: sfa-3.1-9~5
X-Git-Url: http://git.onelab.eu/?p=sfa.git;a=commitdiff_plain;h=d59f8e0b663b71d8c349017d0ecb37cb6bc527a1

huge cleanup for removing mutables used as default
http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument
---

diff --git a/sfa/cortexlab/cortexlabaggregate.py b/sfa/cortexlab/cortexlabaggregate.py
index d9cddf3b..24a53106 100644
--- a/sfa/cortexlab/cortexlabaggregate.py
+++ b/sfa/cortexlab/cortexlabaggregate.py
@@ -292,7 +292,7 @@ class CortexlabAggregate:
         return rspec_node
 
 
-    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = {}):
+    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations=None):
         """Makes a geni sliver structure from all the nodes allocated
         to slivers in the sliver_allocations dictionary. Returns the states
         of the sliver.
@@ -312,6 +312,8 @@ class CortexlabAggregate:
         .. seealso:: node_to_rspec_node
 
         """
+        if sliver_allocations is None: sliver_allocations={}
+
         if rspec_node['sliver_id'] in sliver_allocations:
             # set sliver allocation and operational status
             sliver_allocation = sliver_allocations[rspec_node['sliver_id']]
@@ -555,7 +557,7 @@ class CortexlabAggregate:
 
 
 
-    def get_slivers(self, urns, options={}):
+    def get_slivers(self, urns, options=None):
         """Get slivers of the given slice urns. Slivers contains slice, node and
         user information.
 
@@ -569,7 +571,7 @@ class CortexlabAggregate:
 
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3/CommonConcepts#urns
         """
-
+        if options is None: options={}
 
         slice_ids = set()
         node_ids = []
@@ -667,7 +669,7 @@ class CortexlabAggregate:
         return slivers
 
 
-    def list_resources(self, version = None, options={}):
+    def list_resources(self, version = None, options=None):
         """
         Returns an advertisement Rspec of available resources at this
         aggregate. This Rspec contains a resource listing along with their
@@ -688,6 +690,8 @@ class CortexlabAggregate:
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3#ListResources
         """
 
+        if options is None: options={}
+
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(version.type,
@@ -722,7 +726,7 @@ class CortexlabAggregate:
         return rspec.toxml()
 
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
         """
         Retrieve a manifest RSpec describing the resources contained by the
         named entities, e.g. a single slice or a set of the slivers in a slice.
@@ -752,6 +756,7 @@ class CortexlabAggregate:
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3#Describe
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3/CommonConcepts#urns
         """
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(
diff --git a/sfa/cortexlab/cortexlabdriver.py b/sfa/cortexlab/cortexlabdriver.py
index bc674f76..c5ad4e6a 100644
--- a/sfa/cortexlab/cortexlabdriver.py
+++ b/sfa/cortexlab/cortexlabdriver.py
@@ -1081,7 +1081,7 @@ class CortexlabDriver(Driver):
 
 
 
-    def delete(self, slice_urns, options={}):
+    def delete(self, slice_urns, options=None):
         """
         Deletes the lease associated with the slice hrn and the credentials
             if the slice belongs to iotlab. Answer to DeleteSliver.
@@ -1099,6 +1099,7 @@ class CortexlabDriver(Driver):
         .. note:: creds are unused, and are not used either in the dummy driver
              delete_sliver .
         """
+        if options is None: options={}
         # collect sliver ids so we can update sliver allocation states after
         # we remove the slivers.
         aggregate = CortexlabAggregate(self)
@@ -1390,17 +1391,20 @@ class CortexlabDriver(Driver):
 
 
     # first 2 args are None in case of resource discovery
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         aggregate = CortexlabAggregate(self)
         rspec =  aggregate.list_resources(version=version, options=options)
         return rspec
 
 
-    def describe(self, urns, version, options={}):
+    def describe(self, urns, version, options=None):
+        if options is None: options={}
         aggregate = CortexlabAggregate(self)
         return aggregate.describe(urns, version=version, options=options)
 
-    def status (self, urns, options={}):
+    def status (self, urns, options=None):
+        if options is None: options={}
         aggregate = CortexlabAggregate(self)
         desc =  aggregate.describe(urns, version='GENI 3')
         status = {'geni_urn': desc['geni_urn'],
@@ -1408,7 +1412,8 @@ class CortexlabDriver(Driver):
         return status
 
 
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         xrn = Xrn(urn)
         aggregate = CortexlabAggregate(self)
 
@@ -1488,7 +1493,8 @@ class CortexlabDriver(Driver):
 
         return aggregate.describe([xrn.get_urn()], version=rspec.version)
 
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         # update users
         slices = CortexlabSlices(self)
         aggregate = CortexlabAggregate(self)
diff --git a/sfa/cortexlab/cortexlabslices.py b/sfa/cortexlab/cortexlabslices.py
index ee160d43..888d7dbe 100644
--- a/sfa/cortexlab/cortexlabslices.py
+++ b/sfa/cortexlab/cortexlabslices.py
@@ -351,7 +351,7 @@ class CortexlabSlices:
         return sfa_slice
 
 
-    def verify_persons(self, slice_hrn, slice_record, users, options={}):
+    def verify_persons(self, slice_hrn, slice_record, users, options=None):
         """Ensures the users in users list exist and are enabled in LDAP. Adds
         person if needed(AddPerson).
 
@@ -378,6 +378,7 @@ class CortexlabSlices:
 
         """
 
+        if options is None: options={}
 
         logger.debug("CortexlabSlices \tverify_persons \tslice_hrn  %s  \
                     \t slice_record %s\r\n users %s \t  "
@@ -525,10 +526,11 @@ class CortexlabSlices:
         return added_persons
 
 
-    def verify_keys(self, persons, users, peer, options={}):
+    def verify_keys(self, persons, users, peer, options=None):
         """
         .. warning:: unused
         """
+        if options is None: options={}
         # existing keys
         key_ids = []
         for person in persons:
diff --git a/sfa/dummy/dummy_testbed_api.py b/sfa/dummy/dummy_testbed_api.py
index 2673166d..f553e40e 100644
--- a/sfa/dummy/dummy_testbed_api.py
+++ b/sfa/dummy/dummy_testbed_api.py
@@ -12,7 +12,12 @@ for i in range(1,11):
 
 slices_list = []
 for i in range(1,3):
-    slice = {'slice_name': 'slice'+str(i), 'user_ids': range(i,4,2), 'slice_id': i, 'node_ids': range(i,10,2), 'enabled': True, 'expires': int(time.time())+60*60*24*30}
+    slice = {'slice_name': 'slice'+str(i), 
+             'user_ids': range(i,4,2), 
+             'slice_id': i, 
+             'node_ids': range(i,10,2),
+             'enabled': True,
+             'expires': int(time.time())+60*60*24*30}
     slices_list.append(slice)
 
 users_list = []
@@ -43,7 +48,8 @@ def FilterList(myfilter, mylist):
 def GetTestbedInfo():
     return {'name': 'dummy', 'longitude': 123456, 'latitude': 654321, 'domain':'dummy-testbed.org'}
 
-def GetNodes(filter={}):
+def GetNodes(filter=None):
+    if filter is None: filter={}
     global DB
     result = []
     result.extend(DB['nodes_list'])
@@ -55,7 +61,8 @@ def GetNodes(filter={}):
         result = FilterList(filter, result)
     return result
 
-def GetSlices(filter={}):
+def GetSlices(filter=None):
+    if filter is None: filter={}
     global DB
     result = []
     result.extend(DB['slices_list'])
@@ -69,7 +76,8 @@ def GetSlices(filter={}):
     return result
 
 
-def GetUsers(filter={}):
+def GetUsers(filter=None):
+    if filter is None: filter={}
     global DB
     result = []
     result.extend(DB['users_list'])
diff --git a/sfa/dummy/dummyaggregate.py b/sfa/dummy/dummyaggregate.py
index 576ccd58..c5b4d10c 100644
--- a/sfa/dummy/dummyaggregate.py
+++ b/sfa/dummy/dummyaggregate.py
@@ -52,12 +52,14 @@ class DummyAggregate:
 
         return (slice, slivers)
 
-    def get_nodes(self, options={}):
+    def get_nodes(self, options=None):
+        if options is None: options={}
         filter = {}
         nodes = self.driver.shell.GetNodes(filter)
         return nodes
 
-    def get_slivers(self, urns, options={}):
+    def get_slivers(self, urns, options=None):
+        if options is None: options={}
         slice_names = set()
         slice_ids = set()
         node_ids = []
@@ -122,7 +124,8 @@ class DummyAggregate:
             slivers.append(node)
         return slivers
 
-    def node_to_rspec_node(self, node, options={}):
+    def node_to_rspec_node(self, node, options=None):
+        if options is None: options={}
         rspec_node = NodeElement()
         site=self.driver.testbedInfo
         rspec_node['component_id'] = hostname_to_urn(self.driver.hrn, site['name'], node['hostname'])
@@ -163,7 +166,8 @@ class DummyAggregate:
                       })
         return rspec_node
 
-    def get_slice_nodes(self, slice, options={}):
+    def get_slice_nodes(self, slice, options=None):
+        if options is None: options={}
         nodes_dict = {}
         filter = {}
         if slice and slice.get('node_ids'):
@@ -176,7 +180,8 @@ class DummyAggregate:
             nodes_dict[node['node_id']] = node
         return nodes_dict
 
-    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = {}):
+    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = None):
+        if sliver_allocations is None: sliver_allocations={}
         if rspec_node['sliver_id'] in sliver_allocations:
             # set sliver allocation and operational status
             sliver_allocation = sliver_allocations[rspec_node['sliver_id']]
@@ -202,7 +207,8 @@ class DummyAggregate:
                        }
         return geni_sliver
 
-    def list_resources(self, version = None, options={}):
+    def list_resources(self, version = None, options=None):
+        if options is None: options={}
 
         version_manager = VersionManager()
         version = version_manager.get_version(version)
@@ -224,7 +230,8 @@ class DummyAggregate:
 
         return rspec.toxml()
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(version.type, version.version, 'manifest')
diff --git a/sfa/dummy/dummydriver.py b/sfa/dummy/dummydriver.py
index 4d7f7a38..a69662e1 100644
--- a/sfa/dummy/dummydriver.py
+++ b/sfa/dummy/dummydriver.py
@@ -412,16 +412,19 @@ class DummyDriver (Driver):
     def aggregate_version (self):
         return {}
 
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         aggregate = DummyAggregate(self)
         rspec =  aggregate.list_resources(version=version, options=options)
         return rspec
 
-    def describe(self, urns, version, options={}):
+    def describe(self, urns, version, options=None):
+        if options is None: options={}
         aggregate = DummyAggregate(self)
         return aggregate.describe(urns, version=version, options=options)
     
-    def status (self, urns, options={}):
+    def status (self, urns, options=None):
+        if options is None: options={}
         aggregate = DummyAggregate(self)
         desc =  aggregate.describe(urns, version='GENI 3')
         status = {'geni_urn': desc['geni_urn'],
@@ -429,7 +432,8 @@ class DummyDriver (Driver):
         return status
 
         
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         xrn = Xrn(urn)
         aggregate = DummyAggregate(self)
         slices = DummySlices(self)
@@ -453,7 +457,8 @@ class DummyDriver (Driver):
 
         return aggregate.describe([xrn.get_urn()], version=rspec.version)
 
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         # update users
         slices = DummySlices(self)
         aggregate = DummyAggregate(self)
@@ -469,7 +474,8 @@ class DummyDriver (Driver):
         rspec_version = version_manager.get_version(options['geni_rspec_version'])
         return self.describe(urns, rspec_version, options=options)
 
-    def delete(self, urns, options={}):
+    def delete(self, urns, options=None):
+        if options is None: options={}
         # collect sliver ids so we can update sliver allocation states after
         # we remove the slivers.
         aggregate = DummyAggregate(self)
@@ -504,7 +510,8 @@ class DummyDriver (Driver):
                  'geni_expires': datetime_to_string(utcparse(sliver['expires']))})  
         return geni_slivers
 
-    def renew (self, urns, expiration_time, options={}):
+    def renew (self, urns, expiration_time, options=None):
+        if options is None: options={}
         aggregate = DummyAggregate(self)
         slivers = aggregate.get_slivers(urns)
         if not slivers:
@@ -516,7 +523,8 @@ class DummyDriver (Driver):
         description = self.describe(urns, 'GENI 3', options)
         return description['geni_slivers']
 
-    def perform_operational_action (self, urns, action, options={}):
+    def perform_operational_action (self, urns, action, options=None):
+        if options is None: options={}
         # Dummy doesn't support operational actions. Lets pretend like it
         # supports start, but reject everything else.
         action = action.lower()
@@ -535,7 +543,8 @@ class DummyDriver (Driver):
         geni_slivers = self.describe(urns, 'GENI 3', options)['geni_slivers']
         return geni_slivers
 
-    def shutdown (self, xrn, options={}):
+    def shutdown (self, xrn, options=None):
+        if options is None: options={}
         xrn = DummyXrn(xrn=xrn, type='slice')
         slicename = xrn.pl_slicename()
         slices = self.shell.GetSlices({'name': slicename}, ['slice_id'])
diff --git a/sfa/dummy/dummyslices.py b/sfa/dummy/dummyslices.py
index cf5a6da9..7ab94ba2 100644
--- a/sfa/dummy/dummyslices.py
+++ b/sfa/dummy/dummyslices.py
@@ -110,7 +110,8 @@ class DummySlices:
         return resulting_nodes
         
 
-    def verify_slice(self, slice_hrn, slice_record, expiration, options={}):
+    def verify_slice(self, slice_hrn, slice_record, expiration, options=None):
+        if options is None: options={}
         slicename = hrn_to_dummy_slicename(slice_hrn)
         parts = slicename.split("_")
         login_base = parts[0]
@@ -130,7 +131,8 @@ class DummySlices:
        
         return slice
 
-    def verify_users(self, slice_hrn, slice_record, users, options={}):
+    def verify_users(self, slice_hrn, slice_record, users, options=None):
+        if options is None: options={}
         slice_name = hrn_to_dummy_slicename(slice_hrn)
         users_by_email = {}
         for user in users:
@@ -162,7 +164,8 @@ class DummySlices:
             pass
             
 
-    def verify_keys(self, old_users, new_users, options={}):
+    def verify_keys(self, old_users, new_users, options=None):
+        if options is None: options={}
         # existing keys 
         existing_keys = []
         for user in old_users:
diff --git a/sfa/iotlab/iotlabaggregate.py b/sfa/iotlab/iotlabaggregate.py
index 34eb38fa..56e40e4f 100644
--- a/sfa/iotlab/iotlabaggregate.py
+++ b/sfa/iotlab/iotlabaggregate.py
@@ -306,7 +306,7 @@ class IotlabAggregate:
         return rspec_node
 
 
-    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = {}):
+    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = None):
         """Makes a geni sliver structure from all the nodes allocated
         to slivers in the sliver_allocations dictionary. Returns the states
         of the sliver.
@@ -326,6 +326,7 @@ class IotlabAggregate:
         .. seealso:: node_to_rspec_node
 
         """
+        if sliver_allocations is None: sliver_allocations={}
         if rspec_node['sliver_id'] in sliver_allocations:
             # set sliver allocation and operational status
             sliver_allocation = sliver_allocations[rspec_node['sliver_id']]
@@ -400,7 +401,8 @@ class IotlabAggregate:
         return rspec_node
 
 
-    def get_leases(self, slice=None, options={}):
+    def get_leases(self, slice=None, options=None):
+        if options is None: options={}
         filter={}
         if slice:
            filter.update({'name':slice['slice_name']})
@@ -598,7 +600,7 @@ class IotlabAggregate:
                        FINAL RSPEC %s \r\n" % (rspec.toxml()))
         return rspec.toxml()
 
-    def get_slivers(self, urns, options={}):
+    def get_slivers(self, urns, options=None):
         """Get slivers of the given slice urns. Slivers contains slice, node and
         user information.
 
@@ -613,7 +615,7 @@ class IotlabAggregate:
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3/CommonConcepts#urns
         """
 
-
+        if options is None: options={}
         slice_ids = set()
         node_ids = []
         for urn in urns:
@@ -709,7 +711,7 @@ class IotlabAggregate:
                 slivers.append(node)
         return slivers
 
-    def list_resources(self, version = None, options={}):
+    def list_resources(self, version = None, options=None):
         """
         Returns an advertisement Rspec of available resources at this
         aggregate. This Rspec contains a resource listing along with their
@@ -730,6 +732,7 @@ class IotlabAggregate:
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3#ListResources
         """
 
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(version.type,
@@ -764,7 +767,7 @@ class IotlabAggregate:
         return rspec.toxml()
 
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
         """
         Retrieve a manifest RSpec describing the resources contained by the
         named entities, e.g. a single slice or a set of the slivers in a slice.
@@ -794,6 +797,7 @@ class IotlabAggregate:
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3#Describe
         .. seealso:: http://groups.geni.net/geni/wiki/GAPI_AM_API_V3/CommonConcepts#urns
         """
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(
diff --git a/sfa/iotlab/iotlabdriver.py b/sfa/iotlab/iotlabdriver.py
index eb055d93..797f156a 100644
--- a/sfa/iotlab/iotlabdriver.py
+++ b/sfa/iotlab/iotlabdriver.py
@@ -1088,7 +1088,7 @@ class IotlabDriver(Driver):
 
 
 
-    def delete(self, slice_urns, options={}):
+    def delete(self, slice_urns, options=None):
         """
         Deletes the lease associated with the slice hrn and the credentials
             if the slice belongs to iotlab. Answer to DeleteSliver.
@@ -1106,6 +1106,7 @@ class IotlabDriver(Driver):
         .. note:: creds are unused, and are not used either in the dummy driver
              delete_sliver .
         """
+        if options is None: options={}
         # collect sliver ids so we can update sliver allocation states after
         # we remove the slivers.
         aggregate = IotlabAggregate(self)
@@ -1398,16 +1399,19 @@ class IotlabDriver(Driver):
             'geni_ad_rspec_versions': ad_rspec_versions}
 
     # first 2 args are None in case of resource discovery
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         aggregate = IotlabAggregate(self)
         rspec =  aggregate.list_resources(version=version, options=options)
         return rspec
 
-    def describe(self, urns, version, options={}):
+    def describe(self, urns, version, options=None):
+        if options is None: options={}
         aggregate = IotlabAggregate(self)
         return aggregate.describe(urns, version=version, options=options)
 
-    def status (self, urns, options={}):
+    def status (self, urns, options=None):
+        if options is None: options={}
         aggregate = IotlabAggregate(self)
         desc =  aggregate.describe(urns, version='GENI 3')
         status = {'geni_urn': desc['geni_urn'],
@@ -1415,7 +1419,8 @@ class IotlabDriver(Driver):
         return status
 
 
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         xrn = Xrn(urn)
         aggregate = IotlabAggregate(self)
 
@@ -1529,7 +1534,8 @@ class IotlabDriver(Driver):
 
         return aggregate.describe([xrn.get_urn()], version=rspec.version)
 
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         # update users
         slices = IotlabSlices(self)
         aggregate = IotlabAggregate(self)
diff --git a/sfa/iotlab/iotlabslices.py b/sfa/iotlab/iotlabslices.py
index e4866247..966a26b6 100644
--- a/sfa/iotlab/iotlabslices.py
+++ b/sfa/iotlab/iotlabslices.py
@@ -356,7 +356,7 @@ class IotlabSlices:
         return sfa_slice
 
 
-    def verify_persons(self, slice_hrn, slice_record, users, options={}):
+    def verify_persons(self, slice_hrn, slice_record, users, options=None):
         """Ensures the users in users list exist and are enabled in LDAP. Adds
         person if needed (AddPerson).
 
@@ -382,7 +382,7 @@ class IotlabSlices:
 
 
         """
-
+        if options is None: options={}
         logger.debug("IOTLABSLICES \tverify_persons \tslice_hrn  %s  \
                     \t slice_record %s\r\n users %s \t  "
                      % (slice_hrn, slice_record, users))
@@ -528,10 +528,11 @@ class IotlabSlices:
         return added_persons
 
 
-    def verify_keys(self, persons, users, peer, options={}):
+    def verify_keys(self, persons, users, peer, options=None):
         """
         .. warning:: unused
         """
+        if options is None: options={}
         # existing keys
         key_ids = []
         for person in persons:
diff --git a/sfa/managers/aggregate_manager.py b/sfa/managers/aggregate_manager.py
index 15211035..8b736444 100644
--- a/sfa/managers/aggregate_manager.py
+++ b/sfa/managers/aggregate_manager.py
@@ -29,7 +29,8 @@ class AggregateManager:
             'geni_ad_rspec_versions': ad_rspec_versions,
             }
 
-    def get_rspec_version_string(self, rspec_version, options={}):
+    def get_rspec_version_string(self, rspec_version, options=None):
+        if options is None: options={}
         version_string = "rspec_%s" % (rspec_version)
 
         #panos adding the info option to the caching key (can be improved)
@@ -149,12 +150,14 @@ class AggregateManager:
 
         return api.driver.renew(xrns, expiration_time, options)
 
-    def PerformOperationalAction(self, api, xrns, creds, action, options={}):
+    def PerformOperationalAction(self, api, xrns, creds, action, options=None):
+        if options is None: options={}
         call_id = options.get('call_id')
         if Callids().already_handled(call_id): return True
         return api.driver.perform_operational_action(xrns, action, options) 
 
-    def Shutdown(self, api, xrn, creds, options={}):
+    def Shutdown(self, api, xrn, creds, options=None):
+        if options is None: options={}
         call_id = options.get('call_id')
         if Callids().already_handled(call_id): return True
         return api.driver.shutdown(xrn, options) 
diff --git a/sfa/managers/driver.py b/sfa/managers/driver.py
index 0e8b71dc..1985e0ee 100644
--- a/sfa/managers/driver.py
+++ b/sfa/managers/driver.py
@@ -78,7 +78,8 @@ class Driver:
 
     # answer to ListResources
     # returns : advertisment rspec (xml string)
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         return "dummy Driver.list_resources needs to be redefined"
 
     # the answer to Describe on a slice or a set of the slivers in a slice
@@ -97,40 +98,48 @@ class Driver:
     #              ...
     #                ]
     #}
-    def describe (self, urns, version, options={}):
+    def describe (self, urns, version, options=None):
+        if options is None: options={}
         return "dummy Driver.describe needs to be redefined"
 
     # the answer to Allocate on a given slicei or a set of the slivers in a slice
     # returns: same struct as for describe.
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         return "dummy Driver.allocate needs to be redefined"
 
     # the answer to Provision on a given slice or a set of the slivers in a slice
     # returns: same struct as for describe.
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         return "dummy Driver.provision needs to be redefined"
 
     # the answer to PerformOperationalAction on a given slice or a set of the slivers in a slice
     # returns: struct containing "geni_slivers" list of the struct returned by describe.
-    def perform_operational_action (self, urns, action, options={}):
+    def perform_operational_action (self, urns, action, options=None):
+        if options is None: options={}
         return "dummy Driver.perform_operational_action needs to be redefined"
 
     # the answer to Status on a given slice or a set of the slivers in a slice
     # returns: struct containing "geni_urn" and "geni_slivers" list of the struct returned by describe.
-    def status (self, urns, options={}): 
+    def status (self, urns, options=None): 
+        if options is None: options={}
         return "dummy Driver.status needs to be redefined"
 
     # the answer to Renew on a given slice or a set of the slivers in a slice
     # returns: struct containing "geni_slivers" list of the struct returned by describe.
-    def renew (self, urns, expiration_time, options={}):
+    def renew (self, urns, expiration_time, options=None):
+        if options is None: options={}
         return "dummy Driver.renew needs to be redefined"
 
     # the answer to Delete on a given slice
     # returns: struct containing "geni_slivers" list of the struct returned by describe.
-    def delete(self, urns, options={}):
+    def delete(self, urns, options=None):
+        if options is None: options={}
         return "dummy Driver.delete needs to be redefined"
 
     # the answer to Shutdown on a given slice
     # returns: boolean
-    def shutdown (self, xrn, options={}):
+    def shutdown (self, xrn, options=None):
+        if options is None: options={}
         return False
diff --git a/sfa/managers/registry_manager.py b/sfa/managers/registry_manager.py
index 7306380d..5a49d479 100644
--- a/sfa/managers/registry_manager.py
+++ b/sfa/managers/registry_manager.py
@@ -237,7 +237,8 @@ class RegistryManager:
     
         return records
     
-    def List (self, api, xrn, origin_hrn=None, options={}):
+    def List (self, api, xrn, origin_hrn=None, options=None):
+        if options is None: options={}
         dbsession=api.dbsession()
         # load all know registry names into a prefix tree and attempt to find
         # the longest matching prefix
diff --git a/sfa/managers/slice_manager.py b/sfa/managers/slice_manager.py
index 05b0f1ee..2a99b6f4 100644
--- a/sfa/managers/slice_manager.py
+++ b/sfa/managers/slice_manager.py
@@ -508,7 +508,8 @@ class SliceManager:
         multiclient.get_results()    
         return 1
      
-    def Shutdown(self, api, xrn, creds, options={}):
+    def Shutdown(self, api, xrn, creds, options=None):
+        if options is None: options={}
         xrn = Xrn(xrn)  
         # get the callers hrn
         valid_cred = api.auth.checkCredentials(creds, 'stopslice', xrn.hrn)[0]
diff --git a/sfa/methods/GetVersion.py b/sfa/methods/GetVersion.py
index cb682e44..f043992e 100644
--- a/sfa/methods/GetVersion.py
+++ b/sfa/methods/GetVersion.py
@@ -15,6 +15,7 @@ class GetVersion(Method):
     returns = Parameter(dict, "Version information")
 
     # API v2 specifies options is optional, so..
-    def call(self, options={}):
+    def call(self, options=None):
+        if options is None: options={}
         self.api.logger.info("interface: %s\tmethod-name: %s" % (self.api.interface, self.name))
         return self.api.manager.GetVersion(self.api, options)
diff --git a/sfa/methods/List.py b/sfa/methods/List.py
index d53a0a5e..83d7a6e0 100644
--- a/sfa/methods/List.py
+++ b/sfa/methods/List.py
@@ -25,7 +25,8 @@ class List(Method):
     # xxx used to be [SfaRecord]
     returns = [Parameter(dict, "registry record")]
     
-    def call(self, xrn, creds, options={}):
+    def call(self, xrn, creds, options=None):
+        if options is None: options={}
         hrn, type = urn_to_hrn(xrn)
         valid_creds = self.api.auth.checkCredentials(creds, 'list')
 
diff --git a/sfa/methods/Resolve.py b/sfa/methods/Resolve.py
index f3a6e671..dc34f759 100644
--- a/sfa/methods/Resolve.py
+++ b/sfa/methods/Resolve.py
@@ -30,7 +30,8 @@ class Resolve(Method):
     # xxx used to be [SfaRecord]
     returns = [Parameter(dict, "registry record")]
     
-    def call(self, xrns, creds, options={}):
+    def call(self, xrns, creds, options=None):
+        if options is None: options={}
         # use details=False by default, only when explicitly specified do we want 
         # to mess with the testbed details
         if 'details' in options: details=options['details']
diff --git a/sfa/nitos/nitosaggregate.py b/sfa/nitos/nitosaggregate.py
index bb7c56d6..832a2c7c 100644
--- a/sfa/nitos/nitosaggregate.py
+++ b/sfa/nitos/nitosaggregate.py
@@ -68,7 +68,9 @@ class NitosAggregate:
        
 
 
-    def get_nodes(self, slice_xrn, slice=None,slivers={}, options={}):
+    def get_nodes(self, slice_xrn, slice=None,slivers=None, options=None):
+        if slivers is None: slivers={}
+        if options is None: options={}
         # if we are dealing with a slice that has no node just return 
         # and empty list    
         if slice_xrn:
@@ -126,8 +128,9 @@ class NitosAggregate:
             rspec_nodes.append(rspec_node)
         return rspec_nodes 
 
-    def get_leases_and_channels(self, slice=None, slice_xrn=None,  options={}):
-        
+    def get_leases_and_channels(self, slice=None, slice_xrn=None,  options=None):
+
+        if options is None: options={}
         slices = self.driver.shell.getSlices({}, [])
         nodes = self.driver.shell.getNodes({}, [])
         leases = self.driver.shell.getReservedNodes({}, [])
@@ -216,8 +219,9 @@ class NitosAggregate:
         return (rspec_leases, rspec_channels)
 
 
-    def get_channels(self, slice=None, options={}):
- 
+    def get_channels(self, slice=None, options=None):
+        if options is None: options={}
+
         all_channels = self.driver.shell.getChannels({}, [])
         channels = []
         if slice:
@@ -245,7 +249,8 @@ class NitosAggregate:
 
 
     
-    def get_rspec(self, slice_xrn=None, version = None, options={}):
+    def get_rspec(self, slice_xrn=None, version = None, options=None):
+        if options is None: options={}
 
         version_manager = VersionManager()
         version = version_manager.get_version(version)
diff --git a/sfa/nitos/nitosslices.py b/sfa/nitos/nitosslices.py
index 3eac8aa3..875a5a90 100644
--- a/sfa/nitos/nitosslices.py
+++ b/sfa/nitos/nitosslices.py
@@ -153,7 +153,8 @@ class NitosSlices:
 
                         
         
-    def verify_slice(self, slice_hrn, slice_record, sfa_peer, options={}):
+    def verify_slice(self, slice_hrn, slice_record, sfa_peer, options=None):
+        if options is None: options={}
         slicename = hrn_to_nitos_slicename(slice_hrn)
         slices = self.driver.shell.getSlices({}, []) 
         slices = self.driver.filter_nitos_results(slices, {'slice_name': slicename})
@@ -168,7 +169,8 @@ class NitosSlices:
        
         return slice
 
-    def verify_users(self, slice_hrn, slice_record, users, sfa_peer, options={}):
+    def verify_users(self, slice_hrn, slice_record, users, sfa_peer, options=None):
+        if options is None: options={}
         # get slice info
         slicename = hrn_to_nitos_slicename(slice_hrn)
         slices = self.driver.shell.getSlices({}, [])
@@ -204,7 +206,8 @@ class NitosSlices:
         return added_users
 
 
-    def verify_keys(self, persons, users, options={}):
+    def verify_keys(self, persons, users, options=None):
+        if options is None: options={}
         # existing keys 
         key_ids = []
         for person in persons:
diff --git a/sfa/openstack/image.py b/sfa/openstack/image.py
index 54aaf502..555b1b9b 100644
--- a/sfa/openstack/image.py
+++ b/sfa/openstack/image.py
@@ -4,7 +4,8 @@ from sfa.rspecs.elements.disk_image import DiskImage
 
 class Image:
     
-    def __init__(self, image={}):
+    def __init__(self, image=None):
+        if image is None: image={}
         self.id = None
         self.container_format = None
         self.kernel_id = None
diff --git a/sfa/openstack/nova_driver.py b/sfa/openstack/nova_driver.py
index e36946e6..39ea2f91 100644
--- a/sfa/openstack/nova_driver.py
+++ b/sfa/openstack/nova_driver.py
@@ -355,23 +355,27 @@ class NovaDriver(Driver):
         return {}
 
     # first 2 args are None in case of resource discovery
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         aggregate = OSAggregate(self)
         rspec =  aggregate.list_resources(version=version, options=options)
         return rspec
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
+        if options is None: options={}
         aggregate = OSAggregate(self)
         return aggregate.describe(urns, version=version, options=options)
     
-    def status (self, urns, options={}):
+    def status (self, urns, options=None):
+        if options is None: options={}
         aggregate = OSAggregate(self)
         desc =  aggregate.describe(urns)
         status = {'geni_urn': desc['geni_urn'],
                   'geni_slivers': desc['geni_slivers']}
         return status
 
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         xrn = Xrn(urn) 
         aggregate = OSAggregate(self)
 
@@ -401,7 +405,8 @@ class NovaDriver(Driver):
    
         return aggregate.describe(urns=[urn], version=rspec.version)
 
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         # update sliver allocation states and set them to geni_provisioned
         aggregate = OSAggregate(self)
         instances = aggregate.get_instances(urns)
@@ -415,7 +420,8 @@ class NovaDriver(Driver):
         rspec_version = version_manager.get_version(options['geni_rspec_version'])
         return self.describe(urns, rspec_version, options=options) 
 
-    def delete (self, urns, options={}):
+    def delete (self, urns, options=None):
+        if options is None: options={}
         # collect sliver ids so we can update sliver allocation states after
         # we remove the slivers.
         aggregate = OSAggregate(self)
@@ -441,11 +447,13 @@ class NovaDriver(Driver):
                  'geni_expires': None})        
         return geni_slivers
 
-    def renew (self, urns, expiration_time, options={}):
+    def renew (self, urns, expiration_time, options=None):
+        if options is None: options={}
         description = self.describe(urns, None, options)
         return description['geni_slivers']
 
-    def perform_operational_action  (self, urns, action, options={}):
+    def perform_operational_action  (self, urns, action, options=None):
+        if options is None: options={}
         aggregate = OSAggregate(self)
         action = action.lower() 
         if action == 'geni_start':
@@ -474,7 +482,8 @@ class NovaDriver(Driver):
         geni_slivers = self.describe(urns, None, options)['geni_slivers']
         return geni_slivers
 
-    def shutdown(self, xrn, options={}):
+    def shutdown(self, xrn, options=None):
+        if options is None: options={}
         xrn = OSXrn(xrn=xrn, type='slice')
         tenant_name = xrn.get_tenant_name()
         name = xrn.get_slicename()
diff --git a/sfa/openstack/osaggregate.py b/sfa/openstack/osaggregate.py
index f31891b8..2b653997 100644
--- a/sfa/openstack/osaggregate.py
+++ b/sfa/openstack/osaggregate.py
@@ -58,7 +58,8 @@ class OSAggregate:
             zones = [zone.name for zone in zones]
         return zones
 
-    def list_resources(self, version=None, options={}):
+    def list_resources(self, version=None, options=None):
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(version.type, version.version, 'ad')
@@ -67,7 +68,8 @@ class OSAggregate:
         rspec.version.add_nodes(nodes)
         return rspec.toxml()
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
+        if options is None: options={}
         # update nova connection
         tenant_name = OSXrn(xrn=urns[0], type='slice').get_tenant_name()
         self.driver.shell.nova_manager.connect(tenant=tenant_name)
@@ -211,7 +213,8 @@ class OSAggregate:
                          'storage':  str(instance.disk)})
         return sliver   
 
-    def instance_to_geni_sliver(self, instance, sliver_allocations = {}):
+    def instance_to_geni_sliver(self, instance, sliver_allocations=None):
+        if sliver_allocations is None: sliver_allocations={}
         sliver_hrn = '%s.%s' % (self.driver.hrn, instance.id)
         sliver_id = Xrn(sliver_hrn, type='sliver').urn
  
@@ -302,7 +305,8 @@ class OSAggregate:
         return key_name       
         
 
-    def create_security_group(self, slicename, fw_rules=[]):
+    def create_security_group(self, slicename, fw_rules=None):
+        if fw_rules is None: fw_rules=[]
         # use default group by default
         group_name = 'default' 
         if isinstance(fw_rules, list) and fw_rules:
diff --git a/sfa/planetlab/plaggregate.py b/sfa/planetlab/plaggregate.py
index cae6495b..28766948 100644
--- a/sfa/planetlab/plaggregate.py
+++ b/sfa/planetlab/plaggregate.py
@@ -31,7 +31,8 @@ class PlAggregate:
     def __init__(self, driver):
         self.driver = driver
 
-    def get_nodes(self, options={}):
+    def get_nodes(self, options=None):
+        if options is None: options={}
         filter = {'peer_id': None}
         geni_available = options.get('geni_available')    
         if geni_available == True:
@@ -40,13 +41,15 @@ class PlAggregate:
        
         return nodes  
  
-    def get_sites(self, filter={}):
+    def get_sites(self, filter=None):
+        if filter is None: filter={}
         sites = {}
         for site in self.driver.shell.GetSites(filter):
             sites[site['site_id']] = site
         return sites
 
-    def get_interfaces(self, filter={}):
+    def get_interfaces(self, filter=None):
+        if filter is None: filter={}
         interfaces = {}
         for interface in self.driver.shell.GetInterfaces(filter):
             iface = Interface()
@@ -98,20 +101,23 @@ class PlAggregate:
 
         return links
 
-    def get_node_tags(self, filter={}):
+    def get_node_tags(self, filter=None):
+        if filter is None: filter={}
         node_tags = {}
         for node_tag in self.driver.shell.GetNodeTags(filter):
             node_tags[node_tag['node_tag_id']] = node_tag
         return node_tags
 
-    def get_pl_initscripts(self, filter={}):
+    def get_pl_initscripts(self, filter=None):
+        if filter is None: filter={}
         pl_initscripts = {}
         filter.update({'enabled': True})
         for initscript in self.driver.shell.GetInitScripts(filter):
             pl_initscripts[initscript['initscript_id']] = initscript
         return pl_initscripts
 
-    def get_slivers(self, urns, options={}):
+    def get_slivers(self, urns, options=None):
+        if options is None: options={}
         names = set()
         slice_ids = set()
         node_ids = []
@@ -193,7 +199,9 @@ class PlAggregate:
             slivers.append(node)
         return slivers
 
-    def node_to_rspec_node(self, node, sites, interfaces, node_tags, pl_initscripts=[], grain=None, options={}):
+    def node_to_rspec_node(self, node, sites, interfaces, node_tags, pl_initscripts=None, grain=None, options=None):
+        if pl_initscripts is None: pl_initscripts=[]
+        if options is None: options={}
         rspec_node = NodeElement()
         # xxx how to retrieve site['login_base']
         site=sites[node['site_id']]
@@ -286,7 +294,8 @@ class PlAggregate:
             tags_dict[tag['node_id']] = tag
         return tags_dict
 
-    def get_slice_nodes(self, slice, options={}):
+    def get_slice_nodes(self, slice, options=None):
+        if options is None: options={}
         nodes_dict = {}
         filter = {'peer_id': None}
         tags_filter = {}
@@ -304,7 +313,8 @@ class PlAggregate:
             nodes_dict[node['node_id']] = node
         return nodes_dict
 
-    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations = {}):
+    def rspec_node_to_geni_sliver(self, rspec_node, sliver_allocations=None):
+        if sliver_allocations is None: sliver_allocations={}
         if rspec_node['sliver_id'] in sliver_allocations:
             # set sliver allocation and operational status
             sliver_allocation = sliver_allocations[rspec_node['sliver_id']]
@@ -333,7 +343,8 @@ class PlAggregate:
                        }
         return geni_sliver        
 
-    def get_leases(self, slice=None, options={}):
+    def get_leases(self, slice=None, options=None):
+        if options is None: options={}
         
         now = int(time.time())
         filter={}
@@ -370,7 +381,8 @@ class PlAggregate:
         return rspec_leases
 
     
-    def list_resources(self, version = None, options={}):
+    def list_resources(self, version = None, options=None):
+        if options is None: options={}
 
         version_manager = VersionManager()
         version = version_manager.get_version(version)
@@ -410,7 +422,8 @@ class PlAggregate:
 
         return rspec.toxml()
 
-    def describe(self, urns, version=None, options={}):
+    def describe(self, urns, version=None, options=None):
+        if options is None: options={}
         version_manager = VersionManager()
         version = version_manager.get_version(version)
         rspec_version = version_manager._get_version(version.type, version.version, 'manifest')
diff --git a/sfa/planetlab/pldriver.py b/sfa/planetlab/pldriver.py
index 1c59fc9e..53f12563 100644
--- a/sfa/planetlab/pldriver.py
+++ b/sfa/planetlab/pldriver.py
@@ -627,23 +627,27 @@ class PlDriver (Driver):
         return {}
 
     # first 2 args are None in case of resource discovery
-    def list_resources (self, version=None, options={}):
+    def list_resources (self, version=None, options=None):
+        if options is None: options={}
         aggregate = PlAggregate(self)
         rspec =  aggregate.list_resources(version=version, options=options)
         return rspec
 
-    def describe(self, urns, version, options={}):
+    def describe(self, urns, version, options=None):
+        if options is None: options={}
         aggregate = PlAggregate(self)
         return aggregate.describe(urns, version=version, options=options)
     
-    def status (self, urns, options={}):
+    def status (self, urns, options=None):
+        if options is None: options={}
         aggregate = PlAggregate(self)
         desc =  aggregate.describe(urns, version='GENI 3')
         status = {'geni_urn': desc['geni_urn'],
                   'geni_slivers': desc['geni_slivers']}
         return status
 
-    def allocate (self, urn, rspec_string, expiration, options={}):
+    def allocate (self, urn, rspec_string, expiration, options=None):
+        if options is None: options={}
         xrn = Xrn(urn)
         aggregate = PlAggregate(self)
         slices = PlSlices(self)
@@ -680,7 +684,8 @@ class PlDriver (Driver):
 
         return aggregate.describe([xrn.get_urn()], version=rspec.version)
 
-    def provision(self, urns, options={}):
+    def provision(self, urns, options=None):
+        if options is None: options={}
         # update users
         slices = PlSlices(self)
         aggregate = PlAggregate(self)
@@ -715,7 +720,8 @@ class PlDriver (Driver):
         rspec_version = version_manager.get_version(options['geni_rspec_version']) 
         return self.describe(urns, rspec_version, options=options)
 
-    def delete(self, urns, options={}):
+    def delete(self, urns, options=None):
+        if options is None: options={}
         # collect sliver ids so we can update sliver allocation states after
         # we remove the slivers.
         aggregate = PlAggregate(self)
@@ -754,7 +760,8 @@ class PlDriver (Driver):
                  'geni_expires': datetime_to_string(utcparse(sliver['expires']))})  
         return geni_slivers
 
-    def renew (self, urns, expiration_time, options={}):
+    def renew (self, urns, expiration_time, options=None):
+        if options is None: options={}
         aggregate = PlAggregate(self)
         slivers = aggregate.get_slivers(urns)
         if not slivers:
@@ -767,7 +774,8 @@ class PlDriver (Driver):
         return description['geni_slivers']
             
 
-    def perform_operational_action (self, urns, action, options={}):
+    def perform_operational_action (self, urns, action, options=None):
+        if options is None: options={}
         # MyPLC doesn't support operational actions. Lets pretend like it
         # supports start, but reject everything else.
         action = action.lower()
@@ -787,7 +795,8 @@ class PlDriver (Driver):
         return geni_slivers
 
     # set the 'enabled' tag to 0
-    def shutdown (self, xrn, options={}):
+    def shutdown (self, xrn, options=None):
+        if options is None: options={}
         hrn, _ = urn_to_hrn(xrn)
         top_auth_hrn = top_auth(hrn)
         site_hrn = '.'.join(hrn.split('.')[:-1])
diff --git a/sfa/planetlab/plslices.py b/sfa/planetlab/plslices.py
index b6cec880..1226f12f 100644
--- a/sfa/planetlab/plslices.py
+++ b/sfa/planetlab/plslices.py
@@ -322,7 +322,9 @@ class PlSlices:
                         
         
 
-    def verify_site(self, slice_xrn, slice_record={}, sfa_peer=None, options={}):
+    def verify_site(self, slice_xrn, slice_record=None, sfa_peer=None, options=None):
+        if slice_record is None: slice_record={}
+        if options is None: options={}
         (slice_hrn, type) = urn_to_hrn(slice_xrn)
         top_auth_hrn = top_auth(slice_hrn)
         site_hrn = '.'.join(slice_hrn.split('.')[:-1])
@@ -367,7 +369,8 @@ class PlSlices:
         return site
 
 
-    def verify_slice(self, slice_hrn, slice_record, sfa_peer, expiration, options={}):
+    def verify_slice(self, slice_hrn, slice_record, sfa_peer, expiration, options=None):
+        if options is None: options={}
         top_auth_hrn = top_auth(slice_hrn)
         site_hrn = '.'.join(slice_hrn.split('.')[:-1])
         slice_part = slice_hrn.split('.')[-1]
@@ -416,7 +419,8 @@ class PlSlices:
         return self.driver.shell.GetSlices(int(slice['slice_id']))[0]
 
 
-    def verify_persons(self, slice_hrn, slice_record, users, sfa_peer, options={}):
+    def verify_persons(self, slice_hrn, slice_record, users, sfa_peer, options=None):
+        if options is None: options={}
         top_auth_hrn = top_auth(slice_hrn)
         site_hrn = '.'.join(slice_hrn.split('.')[:-1])
         slice_part = slice_hrn.split('.')[-1]
@@ -504,7 +508,8 @@ class PlSlices:
         return persons_to_add
 
 
-    def verify_keys(self, persons_to_verify_keys, options={}):
+    def verify_keys(self, persons_to_verify_keys, options=None):
+        if options is None: options={}
         # we only add keys that comes from sfa to persons in PL
         for person_id in persons_to_verify_keys:
              person_sfa_keys = persons_to_verify_keys[person_id].get('keys', [])
@@ -518,7 +523,8 @@ class PlSlices:
                   self.driver.shell.AddPersonKey(int(person_id), key)
 
 
-    def verify_slice_attributes(self, slice, requested_slice_attributes, options={}, admin=False):
+    def verify_slice_attributes(self, slice, requested_slice_attributes, options=None, admin=False):
+        if options is None: options={}
         append = options.get('append', True)
         # get list of attributes users ar able to manage
         filter = {'category': '*slice*'}
diff --git a/sfa/rspecs/elements/element.py b/sfa/rspecs/elements/element.py
index 7f79e810..36ad12f7 100644
--- a/sfa/rspecs/elements/element.py
+++ b/sfa/rspecs/elements/element.py
@@ -2,7 +2,8 @@ class Element(dict):
 
     fields = {}
 
-    def __init__(self, fields={}, element=None, keys=None):
+    def __init__(self, fields=None, element=None, keys=None):
+        if fields is None: fields={}
         self.element = element
         dict.__init__(self, dict.fromkeys(self.fields))
         if not keys:
diff --git a/sfa/rspecs/elements/versions/iotlabv1Lease.py b/sfa/rspecs/elements/versions/iotlabv1Lease.py
index eebb1ae2..bfc503aa 100644
--- a/sfa/rspecs/elements/versions/iotlabv1Lease.py
+++ b/sfa/rspecs/elements/versions/iotlabv1Lease.py
@@ -29,7 +29,8 @@ class Iotlabv1Lease:
 
 
     @staticmethod
-    def get_leases(xml, filter={}):
+    def get_leases(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//lease%s | //default:lease%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         lease_elems = xml.xpath(xpath)
         return Iotlabv1Lease.get_lease_objs(lease_elems)
diff --git a/sfa/rspecs/elements/versions/iotlabv1Node.py b/sfa/rspecs/elements/versions/iotlabv1Node.py
index af6fe2a4..49f7cfd2 100644
--- a/sfa/rspecs/elements/versions/iotlabv1Node.py
+++ b/sfa/rspecs/elements/versions/iotlabv1Node.py
@@ -160,14 +160,16 @@ class Iotlabv1Node:
         return node_elems
 
     @staticmethod
-    def get_nodes(xml, filter={}):
+    def get_nodes(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//node%s | //default:node%s' % (XpathFilter.xpath(filter), \
                                                     XpathFilter.xpath(filter))
         node_elems = xml.xpath(xpath)
         return Iotlabv1Node.get_node_objs(node_elems)
 
     @staticmethod
-    def get_nodes_with_slivers(xml, sliver_filter={}):
+    def get_nodes_with_slivers(xml, sliver_filter=None):
+        if sliver_filter is None: sliver_filter={}
 
         xpath = '//node[count(sliver)>0] | \
                                 //default:node[count(default:sliver) > 0]'
diff --git a/sfa/rspecs/elements/versions/iotlabv1Sliver.py b/sfa/rspecs/elements/versions/iotlabv1Sliver.py
index f26ace6b..0f9fb014 100644
--- a/sfa/rspecs/elements/versions/iotlabv1Sliver.py
+++ b/sfa/rspecs/elements/versions/iotlabv1Sliver.py
@@ -35,7 +35,8 @@ class Iotlabv1Sliver:
                     for (key, value) in attrib_dict.items():
                         attrib_elem.set(key, value)
     @staticmethod
-    def get_slivers(xml, filter={}):
+    def get_slivers(xml, filter=None):
+        if filter is None: filter={}
         xpath = './default:sliver | ./sliver'
 
         sliver_elems = xml.xpath(xpath)
@@ -54,5 +55,6 @@ class Iotlabv1Sliver:
         return slivers
 
     @staticmethod
-    def get_sliver_attributes(xml, filter={}):
-        return []
\ No newline at end of file
+    def get_sliver_attributes(xml, filter=None):
+        if filter is None: filter={}
+        return []
diff --git a/sfa/rspecs/elements/versions/nitosv1Channel.py b/sfa/rspecs/elements/versions/nitosv1Channel.py
index 60582e35..cf4a5f61 100644
--- a/sfa/rspecs/elements/versions/nitosv1Channel.py
+++ b/sfa/rspecs/elements/versions/nitosv1Channel.py
@@ -54,7 +54,8 @@ class NITOSv1Channel:
 
 
     @staticmethod
-    def get_channels(xml, filter={}):
+    def get_channels(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//channel%s | //default:channel%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         channel_elems = xml.xpath(xpath)
         return NITOSv1Channel.get_channel_objs(channel_elems)
diff --git a/sfa/rspecs/elements/versions/nitosv1Lease.py b/sfa/rspecs/elements/versions/nitosv1Lease.py
index 99e815a5..dd3041c5 100644
--- a/sfa/rspecs/elements/versions/nitosv1Lease.py
+++ b/sfa/rspecs/elements/versions/nitosv1Lease.py
@@ -73,7 +73,8 @@ class NITOSv1Lease:
             
 
     @staticmethod
-    def get_leases(xml, filter={}):
+    def get_leases(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//lease%s | //default:lease%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         lease_elems = xml.xpath(xpath)
         return NITOSv1Lease.get_lease_objs(lease_elems)
diff --git a/sfa/rspecs/elements/versions/nitosv1Node.py b/sfa/rspecs/elements/versions/nitosv1Node.py
index 44b9b523..ea59b3d5 100644
--- a/sfa/rspecs/elements/versions/nitosv1Node.py
+++ b/sfa/rspecs/elements/versions/nitosv1Node.py
@@ -135,7 +135,8 @@ class NITOSv1Node:
                     node.element.remove(sliver.element)
         
     @staticmethod
-    def get_nodes(xml, filter={}):
+    def get_nodes(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//node%s | //default:node%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         node_elems = xml.xpath(xpath)
         return NITOSv1Node.get_node_objs(node_elems)
diff --git a/sfa/rspecs/elements/versions/nitosv1PLTag.py b/sfa/rspecs/elements/versions/nitosv1PLTag.py
index 7d03fe06..ea34ff4e 100644
--- a/sfa/rspecs/elements/versions/nitosv1PLTag.py
+++ b/sfa/rspecs/elements/versions/nitosv1PLTag.py
@@ -9,7 +9,8 @@ class NITOSv1PLTag:
             pl_tag_elem.set_text(value)
               
     @staticmethod
-    def get_pl_tags(xml, ignore=[]):
+    def get_pl_tags(xml, ignore=None):
+        if ignore is None: ignore=[]
         pl_tags = []
         for elem in xml.iterchildren():
             if elem.tag not in ignore:
diff --git a/sfa/rspecs/elements/versions/nitosv1Sliver.py b/sfa/rspecs/elements/versions/nitosv1Sliver.py
index 0f40211d..feac8879 100644
--- a/sfa/rspecs/elements/versions/nitosv1Sliver.py
+++ b/sfa/rspecs/elements/versions/nitosv1Sliver.py
@@ -43,7 +43,8 @@ class NITOSv1Sliver:
         return attribs 
                 
     @staticmethod
-    def get_slivers(xml, filter={}):
+    def get_slivers(xml, filter=None):
+        if filter is None: filter={}
         xpath = './default:sliver | ./sliver'
         sliver_elems = xml.xpath(xpath)
         slivers = []
diff --git a/sfa/rspecs/elements/versions/ofeliav1Port.py b/sfa/rspecs/elements/versions/ofeliav1Port.py
index 07520ef3..f4cf74dc 100644
--- a/sfa/rspecs/elements/versions/ofeliav1Port.py
+++ b/sfa/rspecs/elements/versions/ofeliav1Port.py
@@ -39,7 +39,8 @@ class Ofeliav1Port:
         return attribs 
                 
     @staticmethod
-    def get_ports(xml, filter={}):
+    def get_ports(xml, filter=None):
+        if filter is None: filter={}
         xpath = './openflow:port | ./port'
         port_elems = xml.xpath(xpath)
         ports = []
diff --git a/sfa/rspecs/elements/versions/ofeliav1datapath.py b/sfa/rspecs/elements/versions/ofeliav1datapath.py
index 86a38002..a1849735 100644
--- a/sfa/rspecs/elements/versions/ofeliav1datapath.py
+++ b/sfa/rspecs/elements/versions/ofeliav1datapath.py
@@ -21,7 +21,8 @@ from sfa.rspecs.elements.versions.ofeliav1Port import Ofeliav1Port
 class Ofeliav1Datapath:
 
     @staticmethod
-    def get_datapaths(xml, filter={}):
+    def get_datapaths(xml, filter=None):
+        if filter is None: filter={}
         #xpath = '//datapath%s | //default:datapath%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         xpath = '//datapath%s | //openflow:datapath%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         datapath_elems = xml.xpath(xpath)
@@ -144,7 +145,8 @@ class Ofeliav1Datapath:
 #                    node.element.remove(sliver.element)
 #        
 #    @staticmethod
-#    def get_nodes(xml, filter={}):
+#    def get_nodes(xml, filter=None):
+#        if filter is None: filter={}
 #        xpath = '//node%s | //default:node%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
 #        node_elems = xml.xpath(xpath)
 #        return SFAv1Node.get_node_objs(node_elems)
diff --git a/sfa/rspecs/elements/versions/ofeliav1link.py b/sfa/rspecs/elements/versions/ofeliav1link.py
index 3fc2eb24..83a20969 100644
--- a/sfa/rspecs/elements/versions/ofeliav1link.py
+++ b/sfa/rspecs/elements/versions/ofeliav1link.py
@@ -8,7 +8,8 @@ from sfa.rspecs.elements.link import Link
 class Ofeliav1Link:
 
     @staticmethod
-    def get_links(xml, filter={}):
+    def get_links(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//link%s | //openflow:link%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         link_elems = xml.xpath(xpath)
         return Ofeliav1Link.get_link_objs(link_elems)
diff --git a/sfa/rspecs/elements/versions/pgv2DiskImage.py b/sfa/rspecs/elements/versions/pgv2DiskImage.py
index 51363de3..4a6df82e 100644
--- a/sfa/rspecs/elements/versions/pgv2DiskImage.py
+++ b/sfa/rspecs/elements/versions/pgv2DiskImage.py
@@ -13,7 +13,8 @@ class PGv2DiskImage:
             xml.add_instance('disk_image', image, DiskImage.fields)
     
     @staticmethod
-    def get_images(xml, filter={}):
+    def get_images(xml, filter=None):
+        if filter is None: filter={}
         xpath = './default:disk_image | ./disk_image'
         image_elems = xml.xpath(xpath)
         images = []
diff --git a/sfa/rspecs/elements/versions/pgv2Lease.py b/sfa/rspecs/elements/versions/pgv2Lease.py
index 68c44f0c..b04f7dc2 100644
--- a/sfa/rspecs/elements/versions/pgv2Lease.py
+++ b/sfa/rspecs/elements/versions/pgv2Lease.py
@@ -51,7 +51,8 @@ class PGv2Lease:
 
 
     @staticmethod
-    def get_leases(xml, filter={}):
+    def get_leases(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//lease%s | //default:lease%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         lease_elems = xml.xpath(xpath)
         return PGv2Lease.get_lease_objs(lease_elems)
diff --git a/sfa/rspecs/elements/versions/pgv2Node.py b/sfa/rspecs/elements/versions/pgv2Node.py
index d553d219..60447b03 100644
--- a/sfa/rspecs/elements/versions/pgv2Node.py
+++ b/sfa/rspecs/elements/versions/pgv2Node.py
@@ -82,13 +82,15 @@ class PGv2Node:
 
 
     @staticmethod
-    def get_nodes(xml, filter={}):
+    def get_nodes(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//node%s | //default:node%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         node_elems = xml.xpath(xpath)
         return PGv2Node.get_node_objs(node_elems)
 
     @staticmethod
-    def get_nodes_with_slivers(xml, filter={}):
+    def get_nodes_with_slivers(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//node[count(sliver_type)>0] | //default:node[count(default:sliver_type) > 0]' 
         node_elems = xml.xpath(xpath)        
         return PGv2Node.get_node_objs(node_elems)
diff --git a/sfa/rspecs/elements/versions/pgv2SliverType.py b/sfa/rspecs/elements/versions/pgv2SliverType.py
index 1f3ec0c7..3ad687f8 100644
--- a/sfa/rspecs/elements/versions/pgv2SliverType.py
+++ b/sfa/rspecs/elements/versions/pgv2SliverType.py
@@ -40,7 +40,8 @@ class PGv2SliverType:
                     for (key, value) in attrib_dict.items():
                         attrib_elem.set(key, value)                
     @staticmethod
-    def get_slivers(xml, filter={}):
+    def get_slivers(xml, filter=None):
+        if filter is None: filter={}
         xpath = './default:sliver_type | ./sliver_type'
         sliver_elems = xml.xpath(xpath)
         slivers = []
@@ -56,5 +57,6 @@ class PGv2SliverType:
         return slivers
 
     @staticmethod
-    def get_sliver_attributes(xml, filter={}):
+    def get_sliver_attributes(xml, filter=None):
+        if filter is None: filter={}
         return []             
diff --git a/sfa/rspecs/elements/versions/sfav1Lease.py b/sfa/rspecs/elements/versions/sfav1Lease.py
index 039d2d13..0c7cb26b 100644
--- a/sfa/rspecs/elements/versions/sfav1Lease.py
+++ b/sfa/rspecs/elements/versions/sfav1Lease.py
@@ -72,7 +72,8 @@ class SFAv1Lease:
 
 
     @staticmethod
-    def get_leases(xml, filter={}):
+    def get_leases(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//lease%s | //default:lease%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         lease_elems = xml.xpath(xpath)
         return SFAv1Lease.get_lease_objs(lease_elems)
diff --git a/sfa/rspecs/elements/versions/sfav1Node.py b/sfa/rspecs/elements/versions/sfav1Node.py
index 51076985..1931a584 100644
--- a/sfa/rspecs/elements/versions/sfav1Node.py
+++ b/sfa/rspecs/elements/versions/sfav1Node.py
@@ -125,7 +125,8 @@ class SFAv1Node:
                     node.element.remove(sliver.element)
         
     @staticmethod
-    def get_nodes(xml, filter={}):
+    def get_nodes(xml, filter=None):
+        if filter is None: filter={}
         xpath = '//node%s | //default:node%s' % (XpathFilter.xpath(filter), XpathFilter.xpath(filter))
         node_elems = xml.xpath(xpath)
         return SFAv1Node.get_node_objs(node_elems)
diff --git a/sfa/rspecs/elements/versions/sfav1PLTag.py b/sfa/rspecs/elements/versions/sfav1PLTag.py
index b523124f..907c962a 100644
--- a/sfa/rspecs/elements/versions/sfav1PLTag.py
+++ b/sfa/rspecs/elements/versions/sfav1PLTag.py
@@ -9,7 +9,8 @@ class SFAv1PLTag:
             pl_tag_elem.set_text(value)
               
     @staticmethod
-    def get_pl_tags(xml, ignore=[]):
+    def get_pl_tags(xml, ignore=None):
+        if ignore is None: ignore=[]
         pl_tags = []
         for elem in xml.iterchildren():
             if elem.tag not in ignore:
diff --git a/sfa/rspecs/elements/versions/sfav1Sliver.py b/sfa/rspecs/elements/versions/sfav1Sliver.py
index a2b07a13..7e9282f2 100644
--- a/sfa/rspecs/elements/versions/sfav1Sliver.py
+++ b/sfa/rspecs/elements/versions/sfav1Sliver.py
@@ -39,7 +39,8 @@ class SFAv1Sliver:
         return attribs 
                 
     @staticmethod
-    def get_slivers(xml, filter={}):
+    def get_slivers(xml, filter=None):
+        if filter is None: filter={}
         xpath = './default:sliver | ./sliver'
         sliver_elems = xml.xpath(xpath)
         slivers = []
diff --git a/sfa/rspecs/rspec.py b/sfa/rspecs/rspec.py
index 8a66ff35..0f61e905 100755
--- a/sfa/rspecs/rspec.py
+++ b/sfa/rspecs/rspec.py
@@ -10,7 +10,8 @@ from sfa.rspecs.version_manager import VersionManager
 
 class RSpec:
  
-    def __init__(self, rspec="", version=None, user_options={}):
+    def __init__(self, rspec="", version=None, user_options=None):
+        if user_options is None: user_options={}
         self.header = '<?xml version="1.0"?>\n'
         self.template = """<RSpec></RSpec>"""
         self.version = None
@@ -72,15 +73,17 @@ class RSpec:
             raise InvalidRSpecElement(element_type, extra=msg)
         return self.elements[element_type]
 
-    def get(self, element_type, filter={}, depth=0):
+    def get(self, element_type, filter=None, depth=0):
+        if filter is None: filter={}
         elements = self.get_elements(element_type, filter)
         elements = [self.xml.get_element_attributes(elem, depth=depth) for elem in elements]
         return elements
 
-    def get_elements(self, element_type, filter={}):
+    def get_elements(self, element_type, filter=None):
         """
         search for a registered element
         """
+        if filter is None: filter={}
         if element_type not in self.elements:
             msg = "Unable to search for element %s in rspec, expath expression not found." % \
                    element_type
diff --git a/sfa/rspecs/versions/iotlabv1.py b/sfa/rspecs/versions/iotlabv1.py
index 9d8045f7..ad491571 100644
--- a/sfa/rspecs/versions/iotlabv1.py
+++ b/sfa/rspecs/versions/iotlabv1.py
@@ -139,7 +139,8 @@ class Iotlabv1(RSpecVersion):
     def add_default_sliver_attribute(self, name, value, network=None):
         pass
 
-    def add_slivers(self, hostnames, attributes=[], sliver_urn=None, append=False):
+    def add_slivers(self, hostnames, attributes=None, sliver_urn=None, append=False):
+        if attributes is None: attributes=[]
         # all nodes hould already be present in the rspec. Remove all
         # nodes that done have slivers
         print>>sys.stderr, "\r\n \r\n \r\n \t\t\t Iotlabv1.PY add_slivers  ----->get_node "
diff --git a/sfa/rspecs/versions/nitosv1.py b/sfa/rspecs/versions/nitosv1.py
index af60d8ed..3288b48b 100644
--- a/sfa/rspecs/versions/nitosv1.py
+++ b/sfa/rspecs/versions/nitosv1.py
@@ -60,7 +60,8 @@ class NITOSv1(RSpecVersion):
 
     # Slivers
    
-    def add_slivers(self, hostnames, attributes=[], sliver_urn=None, append=False):
+    def add_slivers(self, hostnames, attributes=None, sliver_urn=None, append=False):
+        if attributes is None: attributes=[]
         # add slice name to network tag
         network_tags = self.xml.xpath('//network')
         if network_tags:
diff --git a/sfa/rspecs/versions/ofeliav1.py b/sfa/rspecs/versions/ofeliav1.py
index cd206ffc..d074694b 100755
--- a/sfa/rspecs/versions/ofeliav1.py
+++ b/sfa/rspecs/versions/ofeliav1.py
@@ -86,7 +86,8 @@ class Ofelia(RSpecVersion):
 
     # Slivers
    
-    def add_slivers(self, hostnames, attributes=[], sliver_urn=None, append=False):
+    def add_slivers(self, hostnames, attributes=None, sliver_urn=None, append=False):
+        if attributes is None: attributes=[]
         # add slice name to network tag
         network_tags = self.xml.xpath('//network')
         if network_tags:
diff --git a/sfa/rspecs/versions/pgv2.py b/sfa/rspecs/versions/pgv2.py
index 80febc90..21183bf7 100644
--- a/sfa/rspecs/versions/pgv2.py
+++ b/sfa/rspecs/versions/pgv2.py
@@ -103,7 +103,8 @@ class PGv2(RSpecVersion):
     def add_default_sliver_attribute(self, name, value, network=None):
         pass
 
-    def add_slivers(self, hostnames, attributes=[], sliver_urn=None, append=False):
+    def add_slivers(self, hostnames, attributes=None, sliver_urn=None, append=False):
+        if attributes is None: attributes=[]
         # all nodes hould already be present in the rspec. Remove all
         # nodes that done have slivers
         for hostname in hostnames:
diff --git a/sfa/rspecs/versions/sfav1.py b/sfa/rspecs/versions/sfav1.py
index 4ee8a4c8..6e973e7e 100644
--- a/sfa/rspecs/versions/sfav1.py
+++ b/sfa/rspecs/versions/sfav1.py
@@ -59,7 +59,8 @@ class SFAv1(RSpecVersion):
 
     # Slivers
    
-    def add_slivers(self, hostnames, attributes=[], sliver_urn=None, append=False):
+    def add_slivers(self, hostnames, attributes=None, sliver_urn=None, append=False):
+        if attributes is None: attributes=[]
         # add slice name to network tag
         network_tags = self.xml.xpath('//network')
         if network_tags:
diff --git a/sfa/storage/model.py b/sfa/storage/model.py
index d50d3b6d..7c749773 100644
--- a/sfa/storage/model.py
+++ b/sfa/storage/model.py
@@ -410,7 +410,8 @@ def drop_tables(engine):
 
 ##############################
 # create a record of the right type from either a dict or an xml string
-def make_record (dict={}, xml=""):
+def make_record (dict=None, xml=""):
+    if dict is None: dict={}
     if dict:    return make_record_dict (dict)
     elif xml:   return make_record_xml (xml)
     else:       raise Exception("make_record has no input")
diff --git a/sfa/storage/record.py b/sfa/storage/record.py
index 812efdeb..9fcab19f 100644
--- a/sfa/storage/record.py
+++ b/sfa/storage/record.py
@@ -35,7 +35,8 @@ class Record:
     
     # it may be important to exclude relationships, which fortunately
     # 
-    def todict (self, exclude_types=[]):
+    def todict (self, exclude_types=None):
+        if exclude_types is None: exclude_types=[]
         d=self.__dict__
         def exclude (k,v):
             if k.startswith('_'): return True
diff --git a/sfa/util/config.py b/sfa/util/config.py
index c25ec442..797bed73 100644
--- a/sfa/util/config.py
+++ b/sfa/util/config.py
@@ -194,7 +194,8 @@ DO NOT EDIT. This file was automatically generated at
             return False
 
 
-    def dump(self, sections = []):
+    def dump(self, sections=None):
+        if sections is None: sections=[]
         sys.stdout.write(output_python())
 
     def output_python(self, encoding = "utf-8"):
diff --git a/sfa/util/storage.py b/sfa/util/storage.py
index 793a38eb..89a25099 100644
--- a/sfa/util/storage.py
+++ b/sfa/util/storage.py
@@ -9,8 +9,8 @@ class SimpleStorage(dict):
     db_filename = None
     type = 'dict'
     
-    def __init__(self, db_filename, db = {}):
-
+    def __init__(self, db_filename, db = None):
+        if db is None: db={}
         dict.__init__(self, db)
         self.db_filename = db_filename
     
diff --git a/sfa/util/version.py.in b/sfa/util/version.py.in
index 46b31d39..97ae6c40 100644
--- a/sfa/util/version.py.in
+++ b/sfa/util/version.py.in
@@ -3,7 +3,8 @@ version_tag="@VERSIONTAG@"
 scm_url="@SCMURL@"
 import socket
  
-def version_core (more={}):
+def version_core (more=None):
+    if more is None: more={}
     core = { 'code_tag' : version_tag,
              'code_url' : scm_url,
              'hostname' : socket.gethostname(),
diff --git a/sfa/util/xml.py b/sfa/util/xml.py
index d6734e63..ba324c7b 100755
--- a/sfa/util/xml.py
+++ b/sfa/util/xml.py
@@ -20,7 +20,8 @@ class XpathFilter:
         return xpath
 
     @staticmethod
-    def xpath(filter={}):
+    def xpath(filter=None):
+        if filter is None: filter={}
         xpath = ""
         if filter:
             filter_list = []
@@ -78,11 +79,12 @@ class XmlElement:
     def getparent(self):
         return XmlElement(self.element.getparent(), self.namespaces)
 
-    def get_instance(self, instance_class=None, fields=[]):
+    def get_instance(self, instance_class=None, fields=None):
         """
         Returns an instance (dict) of this xml element. The instance
         holds a reference to this xml element.   
         """
+        if fields is None: fields=[]
         if not instance_class:
             instance_class = Element
         if not fields and hasattr(instance_class, 'fields'):
@@ -97,11 +99,12 @@ class XmlElement:
                    instance[field] = self.attrib[field]  
         return instance             
 
-    def add_instance(self, name, instance, fields=[]):
+    def add_instance(self, name, instance, fields=None):
         """
         Adds the specifed instance(s) as a child element of this xml 
         element. 
         """
+        if fields is None: fields=[]
         if not fields and hasattr(instance, 'keys'):
             fields = instance.keys()
         elem = self.add_element(name)
diff --git a/sfa/util/xrn.py b/sfa/util/xrn.py
index b0db4c1d..b16ea511 100644
--- a/sfa/util/xrn.py
+++ b/sfa/util/xrn.py
@@ -110,7 +110,8 @@ class Xrn:
         return Xrn.urn_meaningful(urn).split('+')
 
     @staticmethod
-    def filter_type(urns=[], type=None):
+    def filter_type(urns=None, type=None):
+        if urns is None: urns=[]
         urn_list = []
         if not type:
             return urns