#32336: Allow readonly callables defined in get_readonly_fields
-----------------------------------------+--------------------------
Reporter: Tim McCurrach | Owner: nobody
Type: Bug | Status: assigned
Component: contrib.admin | Version: 3.1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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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).
--
Ticket URL: <https://code.djangoproject.com/ticket/32336>
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