id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
32336	Allow readonly callables defined in get_readonly_fields	Tim McCurrach	Tim McCurrach	"
Currently if you have a `get_readonly_fields` that defines a function within its scope, and then uses it such as the following:

{{{
def get_readonly_fields(self, request, obj=None):
    def my_field(obj):
        return ""This is readonly""
    return self.readonly_fields + (my_field, )
}}}
This will lead to an error:

{{{
File ""...lib/python3.8/site-packages/django/forms/models.py"", line 265, in __new__
    message = message % (', '.join(missing_fields),
TypeError: sequence item 0: expected str instance, function found
}}}

=== Motivation ===
Whilst the above may seem a bit of an edge case, the reason I came across it was wanting to use the `request` object for a readonly field. Readonly fields can be callables that accept only one parameter (obj). If you want a readonly field that behaves differently based on the request object, you need to do something like this:

{{{
def my_field(request, obj):
   # something here

class MyAdmin(admin.ModelAdmin):
    def get_readonly_fields(self, request, obj=None):
        readonly_field = functools.partial(my_field, request)
        return (readonly_field,)
}}}

But this obviously leads to the same error. Changing the behaviour of a specific readonly field based on the user seems like a reasonable use-case to cater for.

=== Cause of the error ===
 - `get_readonly_fields` is called several times per change view request. 
- Taking the example above, the value `my_field` stored in `fields` and `excluded` (local variables in `ModelAdmin.get_form`) refer to different objects since they come from separate calls to `get_readonly_fields`. 
- This is a problem further down the line when the ModelForm is being generated. At the end of `forms.models.fields_for_model` we return values in `fields` that are not in `exclude` but since there are different versions of `my_field` in both, it ends up not being filtered out when it should be.

=== Proposed Solution ===
Cache the return value of `get_readonly_fields` in some instance variable `self._readonly_fields`, and reset this value at the beginning of every request. This way the various variables that store readonly fields will all be referring to the same objects. For `InlineModelAdmin`s the value of `self._readonly_fields` would need to be set to `None` on instantiation (which happens for every new change view request).

=== Tests ===
I'm not sure if this needs a separate test. I have adapted `PostAdmin` (used in lots of admin tests, in particular the ones that test readonly_fields) to cater for this case. So although no tests are added, there are plenty of failing tests before the fix."	Bug	assigned	contrib.admin	3.1	Normal				Unreviewed	1	0	0	0	0	0