Question

How can I know in MyForm.clean() whether the data is new, or if already saved data is being modifed?

What should is_this_new_data() look like in the following code?

class MyForm(forms.ModelForm):
    def clean(self):
        cleaned_data = self.cleaned_data
        if is_this_new_data(self):
            # perform some checks if this is new data
        else:
            # do nothing if this is data being modifed
            return cleaned_data

Answer 1

Check self.cleaned_data['some_field'] against self.instance.some_field.

A quick way to check if the object is new is to see if self.instance.pk has a value. It will be None unless the object already exists.