I've encountered what I can only describe as an extremely weird problem
with method resolution order that happens only in mod_wsgi on Ubuntu
24.04, using the Ubuntu 24.04 version of mod_wsgi (with their Apache and
Python 3), which appears to be an unpatched 5.0.0. The specific problem is
that if you have a class that inherits from 'list,<something else>', and
the second class normally supplies a __str__ method, under mod_wsgi you
get list.__str__ instead. This first manifested in a Django application[*]
but the minimal reproduction for us is a wsgi.py application that has:
import sys
class barney:
def render(self):
return "render() in barney"
__str__ = render
class fred(list, barney):
pass
print("fred.__str__ is", fred.__str__, file=sys.stderr)
(and then the documentation example 'hello world' WSGI application()
function so that mod_wsgi is happy to start this, and will produce
output so that I know the whole thing is working correctly)
On Ubuntu 24.04's mod_wsgi, this will report on startup:
fred.__str__ is <slot wrapper '__str__' of 'list' objects>
In any other environment, including Ubuntu's mod_wsgi on 20.04 and 22.04
(which use earlier versions of mod_wsgi), this reports what we expect:
fred.__str__ is <function barney.render at 0x782909bff2e0>
(Other tests show that the 'barney' class is one of fred's parents and
other attributes are being inherited from it, even though its __str__
isn't being used.)
The actual fred.__mro__ appears to be the same between environments,
listing 'fred', 'list', 'barney', and 'object' in that order.
I'm running this test wsgi.py with a very simple WSGI configuration
in Apache:
WSGIScriptAlias /accounts /<test location>/wsgi.py
WSGIDaemonProcess accounts user=<...> group=<...>
WSGIProcessGroup accounts
Does anyone have any idea what might be going on and either how to fix
it or how to debug it?
Thanks in advance. Please let me know if I can provide more information
that would be helpful.
- cks
[*: The specific symptom we saw was that empty django.form.util.ErrorList
instances using default rendering in templates would render as
'[]' instead of blank. This was traced to them using list.__str__
for rendering instead of their custom rendering, which is set up in
the same way that 'class barney' is in this example.
]
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