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HTML examples seen in the wild that cause parse errors
in HTMLParser include:
<a width="100%"cellspacing=0>
 -- note lack of space between val and next attr name
<a foo=>
 -- trailing attribute has no value after =
<a href=javascript:popup('/popup/html.html')>
 -- javascript fragment with embedded quotes
My patch contains improvements to the 'attrfind' and
'locatestarttagend' regexps that allow these examples
to parse.
The existing test_htmlparser.py unit test continues to
pass, except for the one test case where it considers
<a foo=> to be an error.
I commented out that case and added new test cases to
cover the examples above.

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Base version for HTMLParser.py is 1.11.2.1; base version for
test_htmlparser.py is 1.8.8.1

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This also fixes bugs 683938 and 699079.

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This patch allows developers to override the behavior of HTMLParser
when parsing malformed HTML. Normally HTMLParser calls the function
self.error(), which raises an exception. This patch adds appropriate
return values for situations where self.error has been redefined in
subclasses to *not* raise an exception.
It does not change the default behavior of HTMLParser and so presents
no backwards compatibility issues.
The patch itself consists of an added comment and two added lines of
code that call 'return' with appropriate values after a self.error call.
Nothing wrong with 'em. I can't verify that the "junk characters" error
call will leave the parser in a good state, though, if execution returns
from error().
The library documentation could be updated to reflect the ability to 
override
error() behavior; I've written a short patch, available at
http://issola.caltech.edu/~t/transfer/HTMLParser-doc-error.patch
More problems exist with markupbase.py, upon which HTMLParser is 
based.
markupbase calls error() as well, and has some stickier situations. See
comments in bug 917188 as well.
Comments in 683938 and 699079 suggest that raising an exception is the
correct response to the parse errors. I recommend application of the
patch anyway, because it (a) doesn't change any behavior by default
and (b) may solve some problems for people.
An alternative would be to distinguish between unrecoverable errors
and recoverable errors by having two different functions, e.g. error() 
(for
recoverable errors) and _fail() (for unrecoverable errors). By default
error() would call _fail() and internal code could be changed to call
_fail() where recovery is impossible. This might alter behavior in
situations where subclasses override error() but then again that's not
legitimate to do anyway, at least not at the moment -- error() isn't
in the docs ;).
If nothing done, at least close patch 755660 and bug 736428 with a
comment saying that this behavior will not be addressed ;).

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whoops, attached to wrong patch! dangitall. sorry...

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I don't think HTMLParser should parse clearly invalid HTML without
complaining. Perhaps such behavior should be overrideable (see patch
755660 & comments therein), but this patch changes behavior to parse 
invalid HTML w/o complaint.
Patch 699079, to "fix" similar behavior, was closed by adsr and bcannon 
because such behavior is correct. Patch 683938 is also similar but is 
being kept open for some reason.
Recommend closing patch w/o applying.

I for one thank smroid for the patch. I also have hit *all* of these 
cases in the wild. This patch makes real-life a lot less frustrating. 
This patch is surely a lot more preferable than HTMLParser's tendency 
to just throw up its hands and quietly give up. And one might even 
make a case that in the horrific world of HTML "standards" case #2 and 
#3 can be considered actually valid (as much as it hurts to say so).

There are messages both for and against the patch which contains a unit test. Can we have a statement from a knowledgeable HTML person as to whether the patch should be accepted or rejected.

Attached patch includes the tests in diff.txt. On Python 3, with strict=False, the first test (adjacent attributes) passes, but the other two still fail.
See also #12629.

New changeset 3c3009f63700 by Ezio Melotti in branch '2.7':
#1745761, #755670, #13357, #12629, #1200313: improve attribute handling in HTMLParser.
http://hg.python.org/cpython/rev/3c3009f63700
New changeset 16ed15ff0d7c by Ezio Melotti in branch '3.2':
#1745761, #755670, #13357, #12629, #1200313: improve attribute handling in HTMLParser.
http://hg.python.org/cpython/rev/16ed15ff0d7c
New changeset 426f7a2b1826 by Ezio Melotti in branch 'default':
#1745761, #755670, #13357, #12629, #1200313: merge with 3.2.
http://hg.python.org/cpython/rev/426f7a2b1826 

Fixed, thanks for the report!