TabularInline not respecting form's custom label and help text if set in form's __init__ method

[nrogers64]

Here is my models.py file:

from django.db import models


class Author(models.Model):
    name = models.CharField(max_length=100)

    class Meta:
        ordering = ('name',)

    def __unicode__(self):
        return self.name


class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=100, help_text='The book\'s title.')

    class Meta:
        ordering = ('title',)

    def __unicode__(self):
        return self.title

Here is my forms.py file:

from django import forms

from .models import Book


class BookForm1(forms.ModelForm):
    title = forms.CharField(label='Foo', help_text='Bar')

    class Meta:
        model = Book
        fields = [
            'title',
        ]


class BookForm2(forms.ModelForm):
    class Meta:
        model = Book
        fields = [
            'title',
        ]

    def __init__(self, *args, **kwargs):
        super(BookForm2, self).__init__(*args, **kwargs)

        self.fields['title'].label = 'Baz'
        self.fields['title'].help_text = 'Qux'

Here is my admin.py file:

from django.contrib import admin

from .forms import BookForm1
from .models import Author, Book


class BookInline(admin.TabularInline):
    model = Book
    form = BookForm1


class AuthorAdmin(admin.ModelAdmin):
    inlines = [
        BookInline,
    ]

admin.site.register(Author, AuthorAdmin)
admin.site.register(Book)

Using the above code, if I go to /admin/name_of_app/author/add/ it works as expected, meaning that the field for the books is labeled "Foo" and its help text says "Bar". However, if I edit admin.py and replace both instances of BookForm1 with BookForm2, the field for the books is labeled "Title" and its help text says "The book's title.". In other words, it is falling back to the title and help text provided by the model instead of using what's defined in the BookForm2 class' __init__ method. I should also note that if I replace admin.TabularInline with admin.StackedInline then both BookForm1 and BookForm2 work as expected.

[Tim Graham]

Seems reasonable, although I haven't looked into what might be involved in solving it.

[Paulo]

I've done a little digging on this.
The problem is that the field labels need to be resolved before rendering the forms and so currently Django will look at the
base_fields attribute of the formset form class instead of creating an instance of the form class. This means that the __init__ method is never called when generating the table headers and so no modifications there will be considered.

As a solution we would create a throw away instance of the formset form class and fetch the field from its fields attribute but I don't like the idea of creating an empty form instance without the developer having a way to modify its initialization arguments.

I've attached a path for the suggested solution.

[Tim Graham]

I'd agree with that analysis. Did you look into why it works for StackedInline.

[Paulo]

Replying to timgraham:

I'd agree with that analysis. Did you look into why it works for StackedInline.

Yes, StackedInline does not need to get the field names prior to rendering the forms.
Instead, it just renders each form instance individually and thus uses a BoundField instance when rendering each field and label.

[Paulo]

Looking at this a bit more, I saw we have access to the empty form instance using formset.empty_form.
I feel much better about using that. Attached is the revised patch.

[Tim Graham <timograham@…>]

In 9c2d5a8d:

Fixed #26729 -- Allowed overriding a form field's label/help_text in Form.init() for TabularInline.

[Tim Graham <timograham@…>]

In c464cf88:

[1.10.x] Fixed #26729 -- Allowed overriding a form field's label/help_text in Form.init() for TabularInline.

Backport of 9c2d5a8d333277cc1b482a9d05f174cf4d09f24c from master