Form not always picklable due to template renderer

[Claude Paroz]

I'm confronted with a random pickling issue with some forms in Django 1.11.
The error is : PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

I tracked the pickling error to the Form.renderer.engine.engine.template_libraries dict.

I can more or less reproduce (different error message, but hopefully same cause) with:

from django.forms.renderers import get_default_renderer
import pickle
renderer = get_default_renderer()
renderer.engine
pickle.dumps(renderer)

PicklingError: Can't pickle <function add_preserved_filters at 0x7f2468bfc620>: it's not the same object as django.contrib.admin.templatetags.admin_urls.add_preserved_filters

[Carlton Gibson]

#29411 is a duplicate of this, with a slightly different reproduce case.

[Torsten Bronger]

To copy over important information from duplicate:

This broke in 1.11 and had worked in 1.10 and before.

[za]

Is this bug related with this ​https://docs.djangoproject.com/en/2.1/releases/1.11/#django-template-backends-django-template-render-prohibits-non-dict-context ?

[Tim Graham]

I believe template widget rendering (b52c73008a9d67e9ddbb841872dc15cdd3d6ee01 introduced this issue.