diff --git a/ietf/api/serializer.py b/ietf/api/serializer.py
index 5982b9b41a..9d6cf1ebb8 100644
--- a/ietf/api/serializer.py
+++ b/ietf/api/serializer.py
@@ -226,7 +226,11 @@ class JsonExportMixin(object):
 #         content_type = 'application/json'
 #         return HttpResponse(serialize([ obj ], sort_keys=True, indent=3)[2:-2], content_type=content_type)
 
-    def json_view(self, request, filter={}, expand=[]):
+    def json_view(self, request, filter=None, expand=None):
+        if expand is None:
+            expand = []
+        if filter is None:
+            filter = {}
         qfilter, exclude = filter_from_queryargs(request)
         for k in list(qfilter.keys()):
             if k.startswith("_"):
diff --git a/ietf/doc/utils.py b/ietf/doc/utils.py
index 34265ffaed..be5ba44926 100644
--- a/ietf/doc/utils.py
+++ b/ietf/doc/utils.py
@@ -650,10 +650,12 @@ def nice_consensus(consensus):
         }
     return mapping[consensus]
 
-def has_same_ballot(doc, date1, date2=datetime.date.today()):
+def has_same_ballot(doc, date1, date2=None):
     """ Test if the most recent ballot created before the end of date1
         is the same as the most recent ballot created before the
         end of date 2. """
+    if date2 is None:
+        date2 = datetime.date.today()
     ballot1 = doc.latest_event(BallotDocEvent,type='created_ballot',time__lt=date1+datetime.timedelta(days=1))
     ballot2 = doc.latest_event(BallotDocEvent,type='created_ballot',time__lt=date2+datetime.timedelta(days=1))
     return ballot1==ballot2
@@ -926,7 +928,9 @@ def extract_complete_replaces_ancestor_mapping_for_docs(names):
 def make_rev_history(doc):
     # return document history data for inclusion in doc.json (used by timeline)
 
-    def get_predecessors(doc, predecessors=[]):
+    def get_predecessors(doc, predecessors=None):
+        if predecessors is None:
+            predecessors = []
         if hasattr(doc, 'relateddocument_set'):
             for alias in doc.related_that_doc('replaces'):
                 for document in alias.docs.all():
@@ -935,7 +939,9 @@ def get_predecessors(doc, predecessors=[]):
                         predecessors.extend(get_predecessors(document, predecessors))
         return predecessors
 
-    def get_ancestors(doc, ancestors = []):
+    def get_ancestors(doc, ancestors = None):
+        if ancestors is None:
+            ancestors = []
         if hasattr(doc, 'relateddocument_set'):
             for alias in doc.related_that('replaces'):
                 for document in alias.docs.all():
@@ -1328,4 +1334,4 @@ def fuzzy_find_documents(name, rev=None):
             rev = None  # found a doc by name with rev = None, so update that
 
     FoundDocuments = namedtuple('FoundDocuments', 'documents matched_name matched_rev')
-    return FoundDocuments(docs, name, rev)
\ No newline at end of file
+    return FoundDocuments(docs, name, rev)
diff --git a/ietf/nomcom/utils.py b/ietf/nomcom/utils.py
index 894a13343c..18efaf7c70 100644
--- a/ietf/nomcom/utils.py
+++ b/ietf/nomcom/utils.py
@@ -600,7 +600,9 @@ def get_eligibility_date(nomcom=None, date=None):
         else:
             return datetime.date(datetime.date.today().year,5,1)
 
-def previous_five_meetings(date = datetime.date.today()):
+def previous_five_meetings(date = None):
+    if date is None:
+        date = datetime.date.today()
     return Meeting.objects.filter(type='ietf',date__lte=date).order_by('-date')[:5]
 
 def three_of_five_eligible(previous_five, queryset=None):
diff --git a/ietf/person/templatetags/person_filters.py b/ietf/person/templatetags/person_filters.py
index f1f9e69be8..96696456f7 100644
--- a/ietf/person/templatetags/person_filters.py
+++ b/ietf/person/templatetags/person_filters.py
@@ -13,7 +13,9 @@
 
 
 @register.filter
-def is_nomcom_eligible(person, date=datetime.date.today()):
+def is_nomcom_eligible(person, date=None):
+    if date is None:
+        date = datetime.date.today()
     return is_eligible(person=person, date=date)
 
 
@@ -88,4 +90,4 @@ def email_person_link(email, **kwargs):
         "title": title,
         "class": cls,
         "with_email": with_email,
-    }
\ No newline at end of file
+    }
diff --git a/ietf/review/utils.py b/ietf/review/utils.py
index 67d5d41bb3..3fec1b84f5 100644
--- a/ietf/review/utils.py
+++ b/ietf/review/utils.py
@@ -140,10 +140,13 @@ def days_needed_to_fulfill_min_interval_for_reviewers(team):
     "request_to_assignment_days", "assignment_to_closure_days", "request_to_closure_days"])
 
 
-def extract_review_assignment_data(teams=None, reviewers=None, time_from=None, time_to=None, ordering=[]):
+def extract_review_assignment_data(teams=None, reviewers=None, time_from=None, time_to=None, ordering=None):
     """Yield data on each review assignment, sorted by (*ordering, assigned_on)
     for easy use with itertools.groupby. Valid entries in *ordering are "team" and "reviewer"."""
 
+    if ordering is None:
+        ordering = []
+
     filters = Q()
 
     if teams:
diff --git a/ietf/stats/views.py b/ietf/stats/views.py
index ad7404dea1..90d953ce5f 100644
--- a/ietf/stats/views.py
+++ b/ietf/stats/views.py
@@ -133,7 +133,9 @@ def add_labeled_top_series_from_bins(chart_data, bins, limit):
         })
 
 def document_stats(request, stats_type=None):
-    def build_document_stats_url(stats_type_override=Ellipsis, get_overrides={}):
+    def build_document_stats_url(stats_type_override=Ellipsis, get_overrides=None):
+        if get_overrides is None:
+            get_overrides={}
         kwargs = {
             "stats_type": stats_type if stats_type_override is Ellipsis else stats_type_override,
         }
@@ -762,7 +764,9 @@ def meeting_stats(request, num=None, stats_type=None):
     if num is not None:
         meeting = get_object_or_404(Meeting, number=num, type="ietf")
 
-    def build_meeting_stats_url(number=None, stats_type_override=Ellipsis, get_overrides={}):
+    def build_meeting_stats_url(number=None, stats_type_override=Ellipsis, get_overrides=None):
+        if get_overrides is None:
+            get_overrides = {}
         kwargs = {
             "stats_type": stats_type if stats_type_override is Ellipsis else stats_type_override,
         }
@@ -1009,7 +1013,9 @@ def review_stats(request, stats_type=None, acronym=None):
     # and statistics type) are incorporated directly into the URL to
     # be a bit nicer.
 
-    def build_review_stats_url(stats_type_override=Ellipsis, acronym_override=Ellipsis, get_overrides={}):
+    def build_review_stats_url(stats_type_override=Ellipsis, acronym_override=Ellipsis, get_overrides=None):
+        if get_overrides is None:
+            get_overrides = {}
         kwargs = {
             "stats_type": stats_type if stats_type_override is Ellipsis else stats_type_override,
         }
diff --git a/ietf/utils/mail.py b/ietf/utils/mail.py
index 88f74a972c..45a145b30a 100644
--- a/ietf/utils/mail.py
+++ b/ietf/utils/mail.py
@@ -405,8 +405,12 @@ def send_mail_mime(request, to, frm, subject, msg, cc=None, extra=None, toUser=F
 
     return msg
 
-def parse_preformatted(preformatted, extra={}, override={}):
+def parse_preformatted(preformatted, extra=None, override=None):
     """Parse preformatted string containing mail with From:, To:, ...,"""
+    if extra is None:
+        extra = {}
+    if override is None:
+        override = {}
     assert isinstance(preformatted, str)
     msg = message_from_bytes(preformatted.encode('utf-8'))
     msg.set_charset('UTF-8')
@@ -458,19 +462,26 @@ def parse_preformatted(preformatted, extra={}, override={}):
         assertion('len(list(set(v))) == len(v)')
     return (msg, extra, bcc)
 
-def send_mail_preformatted(request, preformatted, extra={}, override={}):
+def send_mail_preformatted(request, preformatted, extra=None, override=None):
     """Parse preformatted string containing mail with From:, To:, ...,
     and send it through the standard IETF mail interface (inserting
     extra headers as needed)."""
 
+    if extra is None:
+        extra = {}
+    if override is None:
+        override = {}
+
     (msg, extra, bcc) = parse_preformatted(preformatted, extra, override)
     txt = msg.get_payload()
     send_mail_text(request, msg['To'], msg["From"], msg["Subject"], txt, extra=extra, bcc=bcc)
     return msg
 
-def send_mail_message(request, message, extra={}):
+def send_mail_message(request, message, extra=None):
     """Send a Message object."""
     # note that this doesn't handle MIME messages at the moment
+    if extra is None:
+        extra = {}
     assertion('isinstance(message.to, str) and isinstance(message.cc, str) and isinstance(message.bcc, str)')
 
     e = extra.copy()
diff --git a/ietf/utils/management/commands/sqldumpdump.py b/ietf/utils/management/commands/sqldumpdump.py
index 8decd22cf3..da021d4f21 100644
--- a/ietf/utils/management/commands/sqldumpdump.py
+++ b/ietf/utils/management/commands/sqldumpdump.py
@@ -157,7 +157,9 @@ def get_objects(self, app_list, pks, count_only=False):
                         yield obj
 
 
-    def handle(self, filenames=[], **options):
+    def handle(self, filenames=None, **options):
+        if filenames is None:
+            filenames = []
         self.verbosity = int(options.get('verbosity'))
         format = options['format']
         indent = options['indent']
diff --git a/ietf/utils/test_runner.py b/ietf/utils/test_runner.py
index 2592216bd2..7438d1ae4e 100644
--- a/ietf/utils/test_runner.py
+++ b/ietf/utils/test_runner.py
@@ -1036,11 +1036,14 @@ def get_test_paths(self, test_labels):
         test_paths = [ os.path.join(*app.split('.')) for app in test_apps ]
         return test_apps, test_paths
 
-    def run_tests(self, test_labels, extra_tests=[], **kwargs):
+    def run_tests(self, test_labels, extra_tests=None, **kwargs):
         global old_destroy, old_create, test_database_name, template_coverage_collection, code_coverage_collection, url_coverage_collection
         from django.db import connection
         from ietf.doc.tests import TemplateTagTest
 
+        if extra_tests is None:
+            extra_tests=[]
+
         # Tests that involve switching back and forth between the real
         # database and the test database are way too dangerous to run
         # against the production database
diff --git a/ietf/utils/test_smtpserver.py b/ietf/utils/test_smtpserver.py
index e8e6727a50..66675aa0b1 100644
--- a/ietf/utils/test_smtpserver.py
+++ b/ietf/utils/test_smtpserver.py
@@ -67,7 +67,7 @@ def handle_accept(self):
             #channel = SMTPTestChannel(self, conn, addr)
             SMTPTestChannel(self, conn, addr)
 
-    def process_message(self, peer, mailfrom, rcpttos, data, mail_options=[], rcpt_options=[]):
+    def process_message(self, peer, mailfrom, rcpttos, data, mail_options=None, rcpt_options=None):
         self.inbox.append(data)
 
 
diff --git a/pyzmail/generate.py b/pyzmail/generate.py
index 76fadc76d9..2ad1f62246 100644
--- a/pyzmail/generate.py
+++ b/pyzmail/generate.py
@@ -110,7 +110,7 @@ def format_addresses(addresses, header_name=None, charset=None):
     return header
 
 
-def build_mail(text, html=None, attachments=[], embeddeds=[]):
+def build_mail(text, html=None, attachments=None, embeddeds=None):
     """
     Generate the core of the email message regarding the parameters.
     The structure of the MIME email may vary, but the general one is as follow::
@@ -184,6 +184,11 @@ def build_mail(text, html=None, attachments=[], embeddeds=[]):
     --===limit1==--
     """
 
+    if attachments is None:
+        attachments = []
+    if embeddeds is None:
+        embeddeds = []
+
     main=text_part=html_part=None
     if text:
         content, charset=text
@@ -234,7 +239,7 @@ def build_mail(text, html=None, attachments=[], embeddeds=[]):
         
     return main
 
-def complete_mail(message, sender, recipients, subject, default_charset, cc=[], bcc=[], message_id_string=None, date=None, headers=[]):
+def complete_mail(message, sender, recipients, subject, default_charset, cc=None, bcc=None, message_id_string=None, date=None, headers=None):
     """
     Fill in the From, To, Cc, Subject, Date and Message-Id I{headers} of 
     one existing message regarding the parameters.
@@ -312,6 +317,13 @@ def getaddr(address):
         else:
             return address
 
+    if cc is None:
+        cc=[]
+    if bcc is None:
+        bcc=[]
+    if headers is None:
+        headers=[]
+
     mail_from=getaddr(sender[1])
     rcpt_to=list(map(getaddr, recipients))
     rcpt_to.extend(list(map(getaddr, cc)))
@@ -341,7 +353,7 @@ def getaddr(address):
     
     return payload, mail_from, rcpt_to, msg_id
 
-def compose_mail(sender, recipients, subject, default_charset, text, html=None, attachments=[], embeddeds=[], cc=[], bcc=[], message_id_string=None, date=None, headers=[]):
+def compose_mail(sender, recipients, subject, default_charset, text, html=None, attachments=None, embeddeds=None, cc=None, bcc=None, message_id_string=None, date=None, headers=None):
     """
     Compose an email regarding the arguments. Call L{build_mail()} and 
     L{complete_mail()} at once.
@@ -356,6 +368,17 @@ def compose_mail(sender, recipients, subject, default_charset, text, html=None,
 
     >>> payload, mail_from, rcpt_to, msg_id=compose_mail((u'Me', 'me@foo.com'), [(u'Him', 'him@bar.com')], u'the subject', 'iso-8859-1', ('Hello world', 'us-ascii'), attachments=[('attached', 'text', 'plain', 'text.txt', 'us-ascii')])
     """
+    if attachments is None:
+        attachments=[]
+    if embeddeds is None:
+        embeddeds=[]
+    if cc is None:
+        cc=[]
+    if bcc is None:
+        bcc = []
+    if headers is None:
+        headers=[]
+
     message=build_mail(text, html, attachments, embeddeds)
     return complete_mail(message, sender, recipients, subject, default_charset, cc, bcc, message_id_string, date, headers)