How to remove a field from the parent Form in a subclass?

I found that, please comment if interested.

(in Django 1.7.4) extending forms, with following code:

class MyForm(forms.ModelForm):

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

        for key, field in self.fields.iteritems():
            self.fields[key].required = False

    class Meta:
        model = MyModel
        exclude = []

    field_1 = forms.CharField(label="field_1_label")
    field_2 = forms.CharField(label="field_2_label", widget=forms.Textarea(attrs={'class': 'width100 h4em'}),)
    field_3 = forms.CharField(label="field_3_label", widget=forms.TextInput(attrs={'class': 'width100'}),)
    field_4 = forms.ModelChoiceField(label='field_4_label', queryset=AnotherModel.objects.all().order_by("order") )

class MyForm_Extended_1(MyForm):
    field_1 = None


class MyForm_Extended_2(MyForm):
    class Meta:
        model = MyModel
        exclude =[
                    'field_1',
                ]

MyForm_Extended_1 set field_1 as None, (the column in db is updated as Null)

MyForm_Extended_2 ignore the field (ignore the column in db during the save)

So, for my purpose, I use the second method.

You can alter the fields in a subclass by overriding the init method:

class LoginFormWithoutNickname(LoginForm):
    def __init__(self, *args, **kwargs):
        super(LoginFormWithoutNickname, self).__init__(*args, **kwargs)
        self.fields.pop('nickname')

Django 1.7 addressed this in commit b16dd1fe019 for ticket #8620. In Django 1.7, it becomes possible to do nickname = None in the subclass as the OP suggests. From the documentation changes in the commit:

It's possible to opt-out from a Field inherited from a parent class by shadowing it. While any non-Field value works for this purpose, it's recommended to use None to make it explicit that a field is being nullified.