Hi Christof:

Thanks for the additional analysis.

Ok, here is the joke. In client.py the line:

        # see if we need to re-parse the environment for the form (eg Zope)
        if form is None:

form is not None, it is:

  FieldStorage(None, None, b'test')

but form.list is None because b'test' isn't a list.

if I move:

            # In some case (e.g. content-type application/xml), cgi
            # will not parse anything. Fake a list property in this case
            if self.form.list is None:
                self.form.list = []

after the 'if form is None:' so it applies in both cases of the if statement,
things work as expected.

This looked safe to me because the form should always be some type of cgi.FieldStorage.

However it broke a bunch of poorly written (by me) tests. I have fixed the tests.
I have also added a 'try: [...] except AttributeError [...]' block around it and
log a request to post to roundup-users when self.form.list generates an AttributeError.

I have checked it in on changeset:   8268:05d8806b25ad.
It has passed CI on github.

Christof can you run the current repository tip and verify that it fixes your issue.

Thanks.