Print a warning on HTMLParseErrors to let people know they should install an external parser.

3 files changed, 16 insertions, 4 deletions

diff --git a/NEWS.txt b/NEWS.txt
index 61d975f..13b49fc 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -4,6 +4,9 @@
   definitions ending with two question marks instead of
   one. [bug=984258]
 
+* Print a warning on HTMLParseErrors to let people know they should
+  install a better parser library.
+
 = 4.0.4 (20120416) =
 
 * Fixed a bug that sometimes created disconnected trees.
diff --git a/bs4/builder/_htmlparser.py b/bs4/builder/_htmlparser.py
index 3dee51b..d5d8681 100644
--- a/bs4/builder/_htmlparser.py
+++ b/bs4/builder/_htmlparser.py
@@ -4,8 +4,12 @@ __all__ = [
     'HTMLParserTreeBuilder',
     ]
 
-from HTMLParser import HTMLParser
+from HTMLParser import (
+    HTMLParser,
+    HTMLParseError,
+    )
 import sys
+import warnings
 
 # Starting in Python 3.2, the HTMLParser constructor takes a 'strict'
 # argument, which we'd like to set to False. Unfortunately,
@@ -138,8 +142,12 @@ class HTMLParserTreeBuilder(HTMLTreeBuilder):
         args, kwargs = self.parser_args
         parser = BeautifulSoupHTMLParser(*args, **kwargs)
         parser.soup = self.soup
-        parser.feed(markup)
-
+        try:
+            parser.feed(markup)
+        except HTMLParseError, e:
+            warnings.warn(RuntimeWarning(
+                "Python's built-in HTMLParser cannot parse this document. This is not a bug in Beautiful Soup. The best solution is to install an external parser (lxml or html5lib), and use Beautiful Soup with that parser. See http://www.crummy.com/software/BeautifulSoup/bs4/doc/#installing-a-parser for help."))
+            raise e
 
 # Patch 3.2 versions of HTMLParser earlier than 3.2.3 to use some
 # 3.2.3 code. This ensures they don't treat markup like <p></p> as a
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 5aab90e..734851d 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -2548,7 +2548,8 @@ Other parser problems
   parsers`_ for why this matters, and fix the problem by mentioning a
   specific parser library in the ``BeautifulSoup`` constructor.
 
-* ``HTMLParser.HTMLParseError: malformed start tag`` - Caused by
+* ``HTMLParser.HTMLParseError: malformed start tag`` or
+  ``HTMLParser.HTMLParseError: bad end tag`` - Caused by
   giving Python's built-in HTML parser a document it can't handle. Any
   other ``HTMLParseError`` is probably the same problem. Solution:
   :ref:`Install lxml or html5lib. <parser-installation>`