How do I dynamically exclude a non-model field from the admin?

Question

I've added a custom field to a ModelForm for use in the admin:

class MyModel(models.Model):
    name = models.CharField(max_length=64)
     ...etc...

class MyModelAdminForm(forms.ModelForm):
    dynamicfield = forms.IntegerField()

    def __init__(self, *args, **kwargs):
        super(MyModelAdminForm, self).__init__(*args, **kwargs)
        if 'instance' in kwargs:
            #remove dynamicfield from the form, somehow?

    def save(self, *args, **kwargs):
        #do something with dynamicfield
        super(MyModelAdminForm, self).save(*args, **kwargs)

    class Meta:
        model = MyModel

class MyModelAdmin(admin.ModelAdmin):
    form = MyModelAdminForm

admin.site.register(MyModel, MyModelAdmin)

The point of this field is to collect additional data needed during initial object creation. Therefore it should only appear when a new object is being created (the check for 'instance').

The problem I'm having is making the field go away when editing an existing object. I've tried:

self.fields['dynamicfield'].widget = self.fields['dynamicfield'].hidden_widget()

but that only gets rid of the text input field, not the label.

I've also tried variations on these (also using base_fields in place of fields, before the super() call):

del self.fields['dynamicfield'] #yields error:

TemplateSyntaxError at /admin/db/myapp/mymodel/6/
Caught an exception while rendering: Key 'dynamicfield' not found in Form


self.fields['dynamicfield'].hidden = True #Does nothing.

self.fields['dynamicfield'] = None #yields error:

TemplateSyntaxError at /admin/db/catalog/product/6/
Caught an exception while rendering: 'NoneType' object has no attribute 'label'    

Doing these before the call to super() also tried, but also fail:

del self.base_fields['dynamicfield'] #yields error:
TemplateSyntaxError at /admin/db/myapp/mymodel/6/
Caught an exception while rendering: Key 'dynamicfield' not found in Form

Answer 1

Try this:

    def __init__(self, *args, **kwargs):

        if 'instance' in kwargs:
            del self.base_fields['dynamicfield']
        super(MyModelAdminForm, self).__init__(*args, **kwargs)

Answer 2

If the removal does not work (which I don't know why) what about dynamic adding?

def __init__(self, *args, **kwargs):
    super(MyModelAdminForm, self).__init__(*args, **kwargs)
    if 'instance' not in kwargs:
       self.fields['dynamicfield'] = forms.IntegerField()

Here is also a link on how to create dynamic forms. (I think it is even the guy who started Django?)