It appears that cgi.parse() in Python 3.7.6 [GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] fatally chokes on POST requests with multipart/form-data due to some internal processing still relying on assumptions from when str and bytes were the same object.

I'll attach as the first comment the "try-it-at-home" file to demonstrate this error.

Here is a file to try it out in an instant.

(lines 11–28 are not necessary; they are just "luxuries" allowing easier testing of the issue in a web browser)

Would this patch already solve? :

https://github.com/python/cpython/pull/19130

There seems to be another bug: The strange 'latin-1' default encoding of cgi.parse(), which only has effect in non-mulitpart:

    if hasattr(fp,'encoding'):
        encoding = fp.encoding
    else:
        encoding = 'latin-1'


( cgi.FieldStorage and the other functions in cgi and urllib.parse use a 'utf-8' default correctly - and do not try fp.encoding, which is usually not present and not reasonable in form handling WSGI. And 'application/x-www-form-urlencoded' implies  utf-8. )

=> that default should possibly become utf-8. Optionally cgi.parse() could take an extra parameter encoding=None  .