diff --git a/owners.py b/owners.py
index f46fef284..7fa69e365 100644
--- a/owners.py
+++ b/owners.py
@@ -77,7 +77,7 @@ GLOBAL_STATUS = '*'
 
 
 def _assert_is_collection(obj):
-  assert not isinstance(obj, basestring)
+  assert not isinstance(obj, str)
   # Module 'collections' has no 'Iterable' member
   # pylint: disable=no-member
   if hasattr(collections, 'Iterable') and hasattr(collections, 'Sized'):
@@ -508,7 +508,7 @@ class Database(object):
       # Now that we've used `owner` and covered all their dirs, remove them
       # from consideration.
       del all_possible_owners[owner]
-      for o, dirs in all_possible_owners.items():
+      for o, dirs in list(all_possible_owners.items()):
         new_dirs = [(d, dist) for (d, dist) in dirs if d not in dirs_to_remove]
         if not new_dirs:
           del all_possible_owners[o]
@@ -606,9 +606,9 @@ class Database(object):
                                                          dirs)
     # Return the lowest cost owner. In the case of a tie, pick one randomly.
     lowest_cost = min(total_costs_by_owner.values())
-    lowest_cost_owners = filter(
-        lambda owner: total_costs_by_owner[owner] == lowest_cost,
-        total_costs_by_owner)
+    lowest_cost_owners = [
+        owner for owner, cost in total_costs_by_owner.items()
+        if cost == lowest_cost]
     return random.Random().choice(lowest_cost_owners)
 
   def owners_rooted_at_file(self, filename):
diff --git a/testing_support/filesystem_mock.py b/testing_support/filesystem_mock.py
index 36a258e2d..47c86e32a 100644
--- a/testing_support/filesystem_mock.py
+++ b/testing_support/filesystem_mock.py
@@ -6,7 +6,12 @@ import errno
 import fnmatch
 import os
 import re
-import StringIO
+import sys
+
+if sys.version_info.major == 2:
+  from StringIO import StringIO
+else:
+  from io import StringIO
 
 
 def _RaiseNotFound(path):
@@ -63,7 +68,7 @@ class MockFileSystem(object):
     # We need to use a copy of the keys here in order to avoid switching
     # to a different thread and potentially modifying the dict in
     # mid-iteration.
-    files = self.files.keys()[:]
+    files = list(self.files.keys())[:]
     return any(f.startswith(path) for f in files)
 
   def join(self, *comps):
@@ -75,7 +80,7 @@ class MockFileSystem(object):
     return fnmatch.filter(self.files.keys(), path)
 
   def open_for_reading(self, path):
-    return StringIO.StringIO(self.read_binary_file(path))
+    return StringIO(self.read_binary_file(path))
 
   def normpath(self, path):
     # This is not a complete implementation of normpath. Only covers what we
diff --git a/tests/owners_finder_test.py b/tests/owners_finder_test.py
index 5804d4f46..6ebf23517 100755
--- a/tests/owners_finder_test.py
+++ b/tests/owners_finder_test.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
diff --git a/tests/owners_unittest.py b/tests/owners_unittest.py
index b8c008639..b1f4ee4b5 100755
--- a/tests/owners_unittest.py
+++ b/tests/owners_unittest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.