Possible bug in PL/Python?

Hi,

In a current project I've a PL/Python function that uses default
parameters, like this:

CREATE FUNCTION auth.create_user(
email text,
initial_password text,
display_name text DEFAULT NULL,
mobile_phone text DEFAULT NULL,
status auth.enum_user_status DEFAULT 'active'
) RETURNS text AS $$
... rest of function ...

Now I try to test if 'display_name' is actually passed or not, and if not
set it to something sensible, like so:
...
if display_name is None:
display_name = email[:email.find('@')]
...

And ... that fails with an error stating that 'display_name' is referenced
before assignment.
However, if I do it like this, it works:
...
if display_name is None:
real_display_name = email[:email.find('@')]
...

In straight Python the first example works, however in PL/Python only the
second works.

Is this how it is supposed to be and did I perhaps miss something in the
docs, or is it a bug?

Kind regards,
Jeroen