formfield_callback in form_for_model also changes refering model

[Jure Cuhalev <gandalf@…>]

Using formfield_callback with newforms form.form_for_model will also change the model you are refering to.

def CategoryCallback(f, **kwargs):
    if not "cat_" in f.name:
      f.name = 'cat_'+f.name
    return f.formfield(**kwargs)

>>> w = Writer(name='Bob Woodward')
>>> Article(headline='Test article', pub_date=datetime.date(1988, 1, 4), writer=w, article='Hello.')
<Article: Test article>

>>> ArticleForm = form_for_model(Article, formfield_callback=CategoryCallback)
>>> f = ArticleForm(auto_id=False)
>>> print f
<tr><th>Headline:</th><td><input type="text" name="cat_headline" maxlength="50" /></td></tr>
<tr><th>Pub date:</th><td><input type="text" name="cat_pub_date" /></td></tr>
<tr><th>Writer:</th><td><select name="cat_writer">
<option value="" selected="selected">---------</option>
</select></td></tr>
<tr><th>Article:</th><td><textarea name="cat_article"></textarea></td></tr>
<tr><th>Categories:</th><td><select multiple="multiple" name="cat_categories">
</select><br /> Hold down "Control", or "Command" on a Mac, to select more than one.</td></tr>
>>> w = Writer(name='Bob Woodward')

but then doing this one (same one as in the beginning):

>>> Article(headline='Test article', pub_date=datetime.date(1988, 1, 4), writer=w, article='Hello.')

fails with:

    Traceback (most recent call last):
      File "/Users/gandalf/django/django_src/django/test/doctest.py", line 1243, in __run
        compileflags, 1) in test.globs
      File "<doctest modeltests.model_forms_callback.models.__test__.API_TESTS[16]>", line 1, in ?
        Article(headline='Test article', pub_date=datetime.date(1988, 1, 4), writer=w, article='Hello.')
      File "/Users/gandalf/django/django_src/django/db/models/base.py", line 166, in __init__
        raise TypeError, "'%s' is an invalid keyword argument for this function" % kwargs.keys()[0]
    TypeError: 'writer' is an invalid keyword argument for this function

I have attached more comprehensive test derived from modeltests/model_form. I belive that form_for_model with callback should not affect the actual model. It is also not clear if by doing my approach .save() should still work normally or is up to the developer to rename fields back to the way they were originally defined.

[Jure Cuhalev <gandalf@…>]

Test for described formfield_callback problem

[Collin Grady <cgrady@…>]

Doesn't the prefix argument to a Form do the same thing, but without the side-effects? :)

Editing the model fields to produce changes in the form output seems a bit backwards - perhaps a custom base form with init overrided would be better than trying to do certain modifications in the callback?

[Jure Cuhalev <gandalf@…>]

Yes, it probably would. But using it in this "wrong" way still shouldn't break the model in a running project.

[Philippe Raoult]

There are no object/class permissions in python, there is no way to "fix" this. The documentation should mention that the argument passed to the callback is the actual class field and should not be treated as a temporary variable you can modify.

[Philippe Raoult]

patch to newforms.txt to mention this problem

[Malcolm Tredinnick]

(In [6148]) Fixed #3922 -- Added a warning that fields passed to form_for_model callback function should not be modified. It's bad for your health. Thanks, Philippe Raoult.