Added exception for when tab does not exist

django/website/tabbed_page/tab_pool.py

 _pool = {}
 
 
+class MissingTabError(Exception):
+    pass
+
+
 def register_tab(name, tab):
     global _pool
     _pool[name] = tab
@@ -9,4 +13,14 @@ def register_tab(name, tab):
 def get_tab(name):
     global _pool
 
-    return _pool[name]
+    try:
+        return _pool[name]
+    except KeyError:
+        raise MissingTabError()
+
+
+def clear_tabs():
+    # Currently only used in test code
+    global _pool
+
+    _pool = {}

django/website/tabbed_page/tests/registration_tests.py

 import pytest
 
-from ..tab_pool import register_tab, get_tab
+from ..tab_pool import (
+    register_tab,
+    get_tab,
+    clear_tabs,
+    MissingTabError,
+)
+
+
+def setup_function(function):
+    clear_tabs()
 
 
 class TestTab(object):
@@ -12,7 +21,11 @@ def tab():
     return TestTab()
 
 
-@pytest.mark.django_db
 def test_tab_is_registered(tab):
     register_tab('test-tab', tab)
     assert get_tab('test-tab') == tab
+
+
+def test_exception_when_tab_not_registered(tab):
+    with pytest.raises(MissingTabError):
+        get_tab('test-tab')