Fixed a crash when overriding multi_valued_attributes and using the

  html5lib parser. [bug=1948488]

3 files changed, 24 insertions, 2 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 5e02c4b..6e9e66d 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -11,6 +11,9 @@ Python 2 was revision 605.
   to make it possible to treat ruby text specially in get_text() calls.
   [bug=1941980]
 
+* Fixed a crash when overriding multi_valued_attributes and using the
+  html5lib parser. [bug=1948488]
+
 = 4.10.0 (20210907)
 
 * This is the first release of Beautiful Soup to only support Python
diff --git a/bs4/builder/_html5lib.py b/bs4/builder/_html5lib.py
index 69aefd7..914b1df 100644
--- a/bs4/builder/_html5lib.py
+++ b/bs4/builder/_html5lib.py
@@ -242,8 +242,8 @@ class AttrList(object):
     def __setitem__(self, name, value):
         # If this attribute is a multi-valued attribute for this element,
         # turn its value into a list.
-        list_attr = self.element.cdata_list_attributes
-        if (name in list_attr['*']
+        list_attr = self.element.cdata_list_attributes or {}
+        if (name in list_attr.get('*')
             or (self.element.name in list_attr
                 and name in list_attr[self.element.name])):
             # A node that is being cloned may have already undergone
diff --git a/bs4/tests/__init__.py b/bs4/tests/__init__.py
index 6b70c34..6677d2e 100644
--- a/bs4/tests/__init__.py
+++ b/bs4/tests/__init__.py
@@ -252,6 +252,25 @@ class SoupTest(object):
 class TreeBuilderSmokeTest(object):
     # Tests that are common to HTML and XML tree builders.
 
+    @pytest.mark.parametrize(
+        "multi_valued_attributes",
+        [None, dict(b=['class']), {'*': ['notclass']}]
+    )
+    def test_attribute_not_multi_valued(self, multi_valued_attributes):
+        markup = '<a class="a b c">'
+        soup = self.soup(markup, multi_valued_attributes=multi_valued_attributes)
+        assert soup.a['class'] == 'a b c'
+
+    @pytest.mark.parametrize(
+        "multi_valued_attributes", [dict(a=['class']), {'*': ['class']}]
+    )
+    def test_attribute_multi_valued(self, multi_valued_attributes):
+        markup = '<a class="a b c">'
+        soup = self.soup(
+            markup, multi_valued_attributes=multi_valued_attributes
+        )
+        assert soup.a['class'] == ['a', 'b', 'c']
+        
     def test_fuzzed_input(self):
         # This test centralizes in one place the various fuzz tests
         # for Beautiful Soup created by the oss-fuzz project.