allow el.set("attr") in HTML trees without having to specify an explicit None value

3 files changed, 24 insertions, 10 deletions

diff --git a/doc/lxmlhtml.txt b/doc/lxmlhtml.txt
index 6d9f63f4..93bb0c27 100644
--- a/doc/lxmlhtml.txt
+++ b/doc/lxmlhtml.txt
@@ -149,12 +149,13 @@ also include some extra methods:
     Returns a set-like object that allows accessing and modifying the
     names in the 'class' attribute of the element.  (New in lxml 3.5).
 
-``.set('attribute', None)``:
-    Creates a boolean attribute, like ``<form novalidate></form>``
-    or ``<div custom-attribute></div>``.  In XML attributes must
+``.set(key, value=None)``:
+    Sets an HTML attribute.  If no value is given, or if the value is
+    ``None``, it creates a boolean attribute like ``<form novalidate></form>``
+    or ``<div custom-attribute></div>``.  In XML, attributes must
     have at least the empty string as their value like ``<form
-    novalidate=""></form>``, but HTML boolean attributes can be either
-    present or absent from an element.
+    novalidate=""></form>``, but HTML boolean attributes can also be
+    just present or absent from an element without having a value.
 
 Running HTML doctests
 =====================
diff --git a/src/lxml/html/__init__.py b/src/lxml/html/__init__.py
index ba36272d..9d5ae953 100644
--- a/src/lxml/html/__init__.py
+++ b/src/lxml/html/__init__.py
@@ -239,6 +239,15 @@ class Classes(MutableSet):
 
 class HtmlMixin(object):
 
+    def set(self, key, value=None):
+        """set(self, key, value=None)
+
+        Sets an element attribute.  If no value is provided, or if the value is None,
+        creates a 'boolean' attribute without value, e.g. "<form novalidate></form>"
+        for ``form.set('novalidate')``.
+        """
+        super(HtmlElement, self).set(key, value)
+
     @property
     def classes(self):
         """
@@ -682,8 +691,9 @@ class HtmlComment(etree.CommentBase, HtmlMixin):
 
 
 class HtmlElement(etree.ElementBase, HtmlMixin):
-    # Override etree.ElementBase.cssselect, despite the MRO
+    # Override etree.ElementBase.cssselect() and set(), despite the MRO (FIXME: change base order?)
     cssselect = HtmlMixin.cssselect
+    set = HtmlMixin.set
 
 
 class HtmlProcessingInstruction(etree.PIBase, HtmlMixin):
diff --git a/src/lxml/tests/test_htmlparser.py b/src/lxml/tests/test_htmlparser.py
index 6a436a6f..386c63f3 100644
--- a/src/lxml/tests/test_htmlparser.py
+++ b/src/lxml/tests/test_htmlparser.py
@@ -596,20 +596,23 @@ class HtmlParserTestCase(HelperTestCase):
         # ability to serialize boolean attribute by setting value to None
         form = html.Element('form')
         form.set('novalidate', None)
-        self.assertEqual(html.tostring(form), 
-                _bytes('<form novalidate></form>'))
+        self.assertEqual(html.tostring(form),
+                         _bytes('<form novalidate></form>'))
+        form.set('custom')
+        self.assertEqual(html.tostring(form),
+                         _bytes('<form novalidate custom></form>'))
 
     def test_boolean_attribute_round_trip(self):
         # ability to pass boolean attributes unmodified
         fragment = '<tag attribute></tag>'
         self.assertEqual(html.tostring(html.fragment_fromstring(fragment)),
-                _bytes(fragment))
+                         _bytes(fragment))
 
     def test_boolean_attribute_xml_adds_empty_string(self):
         # html serialized as xml converts boolean attributes to empty strings
         fragment = '<tag attribute></tag>'
         self.assertEqual(self.etree.tostring(html.fragment_fromstring(fragment)),
-                _bytes('<tag attribute=""/>'))
+                         _bytes('<tag attribute=""/>'))
 
 
 def test_suite():