Warn when SoupStrainer is used with the html5lib tree builder.

2 files changed, 11 insertions, 2 deletions

diff --git a/bs4/builder/_html5lib.py b/bs4/builder/_html5lib.py
index 26b1773..cf716df 100644
--- a/bs4/builder/_html5lib.py
+++ b/bs4/builder/_html5lib.py
@@ -2,6 +2,7 @@ __all__ = [
     'HTML5TreeBuilder',
     ]
 
+import warnings
 from bs4.builder import (
     PERMISSIVE,
     HTML,
@@ -30,6 +31,8 @@ class HTML5TreeBuilder(HTMLTreeBuilder):
 
     # These methods are defined by Beautiful Soup.
     def feed(self, markup):
+        if self.soup.parse_only is not None:
+            warnings.warn("You provided a value for parse_only, but the html5lib tree builder doesn't support parse_only. The entire document will be parsed.")
         parser = html5lib.HTMLParser(tree=self.create_treebuilder)
         doc = parser.parse(markup, encoding=self.user_specified_encoding)
 
diff --git a/bs4/tests/test_html5lib.py b/bs4/tests/test_html5lib.py
index 0828cfd..f195f7d 100644
--- a/bs4/tests/test_html5lib.py
+++ b/bs4/tests/test_html5lib.py
@@ -1,5 +1,7 @@
 """Tests to ensure that the html5lib tree builder generates good trees."""
 
+import warnings
+
 try:
     from bs4.builder import HTML5TreeBuilder
     HTML5LIB_PRESENT = True
@@ -26,11 +28,15 @@ class HTML5LibBuilderSmokeTest(SoupTest, HTML5TreeBuilderSmokeTest):
         # The html5lib tree builder does not support SoupStrainers.
         strainer = SoupStrainer("b")
         markup = "<p>A <b>bold</b> statement.</p>"
-        soup = self.soup(markup,
-                         parse_only=strainer)
+        with warnings.catch_warnings(record=True) as w:
+            soup = self.soup(markup, parse_only=strainer)
         self.assertEqual(
             soup.decode(), self.document_for(markup))
 
+        self.assertTrue(
+            "the html5lib tree builder doesn't support parse_only" in
+            str(w[0].message))
+
     def test_correctly_nested_tables(self):
         """html5lib inserts <tbody> tags where other parsers don't."""
         markup = ('<table id="1">'