"TypeError: 'set' object is not reversible" crash if form Media is a set rather than list/tuple

[Edwar Baron]

I have updated the version of Django to 2.0 in an existing project, and I have followed all the steps necessary for the project to be compatible.

but I'm having problems in the administrator when creating or editing a model that uses attributes from this django-markdownx library

the current configuration of the project of the usa packages:

amqp (2.2.2)
apipkg (1.4)
asn1crypto (0.23.0)
attrs (17.3.0)
Babel (2.5.1)
bcrypt (3.1.4)
billiard (3.5.0.3)
celery (4.1.0)
certifi (2017.11.5)
cffi (1.11.2)
chardet (3.0.4)
coreapi (2.3.3)
coreschema (0.0.4)
coverage (4.4.2)
cryptography (2.1.4)
Django (2.0)
django-celery-results (1.0.1)
django-colorfield (0.1.14)
django-constance (2.0.0)
django-cors-headers (2.1.0)
django-environ (0.4.4)
django-excel-response (2.0.2)
django-extensions (1.9.8)
django-filter (1.1.0)
django-markdownx (2.0.21)
django-picklefield (1.0.0)
django-redis (4.8.0)
django-redis-cache (1.7.1)
django-rest-swagger (2.1.2)
djangorestframework (3.7.3)
et-xmlfile (1.0.1)
execnet (1.5.0)
Fabric3 (1.13.1.post1)
factory-boy (2.7.0)
fake-factory (0.7.4)
flower (0.9.2)
idna (2.6)
isort (4.2.15)
itypes (1.1.0)
jdcal (1.3)
Jinja2 (2.10)
kombu (4.1.0)
Markdown (2.6.10)
MarkupSafe (1.0)
olefile (0.44)
opbeat (3.6.0)
openapi-codec (1.3.2)
openpyxl (2.4.9)
paramiko (2.4.0)
pep8 (1.7.1)
Pillow (4.3.0)
pip (9.0.1)
pluggy (0.6.0)
psycopg2 (2.7.3.2)
py (1.5.2)
pyasn1 (0.4.2)
pycparser (2.18)
pyflakes (1.6.0)
PyNaCl (1.2.1)
pytest (3.3.1)
pytest-cache (1.0)
pytest-cov (2.5.1)
pytest-django (3.1.2)
pytest-flakes (2.0.0)
pytest-isort (0.1.0)
pytest-pep8 (1.0.6)
pytest-sugar (0.9.0)
python-dateutil (2.6.1)
pytz (2017.3)
redis (2.10.6)
requests (2.18.4)
setuptools (36.4.0)
simplejson (3.13.2)
six (1.11.0)
sorl-thumbnail (12.4.1)
termcolor (1.1.0)
tornado (4.5.2)
typing (3.6.2)
uritemplate (3.0.0)
urllib3 (1.22)
vine (1.1.4)
Werkzeug (0.13)
wheel (0.30.0)

the error is caused in this file

File "/usr/local/lib/python3.6/site-packages/django/forms/widgets.py", line 114, in merge

...
        combined_list = list(list_1)
        last_insert_index = len(list_1)
        # Walk list_2 in reverse, inserting each element into combined_list if
        # it doesn't already exist.
        for path in reversed(list_2):

TypeError: 'set' object is not reversible
...

Local variables in frame

last_insert_index	0
combined_list	[]
list_2	                set(['markdownx/js/markdownx.js'])
list_1	                []

the variable list_2 is a list of set type

the simple solution that I think

is to convert that variable to a list

[Simon Charette]

I think that naively converting an unordered collection to an arbitrarily ordered one will cause more harm than good here as this might change the ordering of static files inclusions between Python interpreter launches (given PYTHONHASHSEED=random which is the default value on Python versions supported by Django 2.0) or ​generate hard to reproduce warnings

The third-party library should make sure to pass an ordered collection to Media.merge as hinted by the raised TypeError.