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The HTMLParser.error method raises HTMLParseError,
terminating the parse upon detection of a parse error.
This patch is to allow HTMLParser to continue parsing
if the error() method is overridden to not throw an
exception.
Doc impact is on the error() method. The existing
test_htmlparser.py unit test is unaffected by the patch.
The base file is HTMLParser.py, revision 1.11.2.1

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this fixes bug #736428 (submitted by me earlier)

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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 ;).

Tests still pass with updated patch, but new tests (and docs!) for this
feature are needed if Titus' positive review stands.

I'm using Python 3.1.1 and the patch (patch.txt, provided by smroid)
works very well. It's usefull, and I really need it, thanks :)
Without this patch, I can't parse: http://ftp.vim.org/pub/vim/ (due to a
fake tag, like "<user@mail.com>"), and many others websites.
I hope this patch will be merged in Python 3.2 :)

Site: http://ftp.vim.org/pub/vim/unstable/patches/
Outuput without error customized function:
[...]
 File "./takeit.py", line 54, in inspect
 parser.feed(data.read().decode())
 File "/home/frafra/Scrivania/takeit/html/parser.py", line 107, in feed
 self.goahead(0)
 File "/home/frafra/Scrivania/takeit/html/parser.py", line 163, in goahead
 k = self.parse_declaration(i)
 File "/usr/local/lib/python3.1/_markupbase.py", line 97, in
parse_declaration
 decltype, j = self._scan_name(j, i)
 File "/usr/local/lib/python3.1/_markupbase.py", line 387, in _scan_name
 % rawdata[declstartpos:declstartpos+20])
 File "/home/frafra/Scrivania/takeit/html/parser.py", line 122, in error
 raise HTMLParseError(message, self.getpos())
html.parser.HTMLParseError: expected name token at '<! gives an error
me', at line 153, column 48
Output with error customized function:
[...]
 File "./takeit.py", line 55, in inspect
 parser.feed(data.read().decode())
 File "/home/frafra/Scrivania/takeit/html/parser.py", line 107, in feed
 self.goahead(0)
 File "/home/frafra/Scrivania/takeit/html/parser.py", line 163, in goahead
 k = self.parse_declaration(i)
 File "/usr/local/lib/python3.1/_markupbase.py", line 97, in
parse_declaration
 decltype, j = self._scan_name(j, i)
TypeError: 'NoneType' object is not iterable

Attached a patch for py3k where the file name has changed. Doc changes could be based on the comment added to the error method in the patch. I don't think a unit test is needed but could easily be persuaded otherwise.

HTMLParser should now be able to parse invalid HTML too, so this patch is not necessary anymore.