GeezerVison

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I wrote a short program to understand how errors are handled and what is written.

from xml.sax.saxutils import XMLGenerator
from io import StringIO

stream = StringIO()
gen = XMLGenerator(stream, validate=True)
gen.startElement('hello', {})
gen.endElement('hello')
try:
    gen.startElement('hello2', {'': 'value'})
except ValueError:
    pass
gen.startElement('hello3', {})
gen.endElement('hello3')
print(stream.getvalue())

With this change, the output is:

<hello></hello><hello2<hello3></hello3>

Ah yes, <hello></hello> is already written.

My concern is that an unfinished <hello2 markup is written because of the validation error (ValueError).

With my suggestion, it would be possible to write:

<hello></hello><hello3></hello3>

I'm fine with not supporting this case and declare game over at the first validation error: the output should be discard in this case.

Valid XML should have the root element. If serialization fails, the close tag for the root element and other open elements is no written -- this is an invalid XML. If serialization fails before writing the root element -- this is an invalid XML.

See also #149468. This PR is modeled on PR #149469.