Only show certain fields when editing in Django

certain_fields_editing.py

class SomeForm(forms.ModelForm):
    class Meta:
        model = SomeModel

    def __init__(self, *args, **kwargs):
        super(SomeForm, self).__init__(*args, **kwargs)
        if self.instance:
            print "There is an instance, so were editing."
            self.fields['anewfieldonlywhenediting'] = forms.CharField()