Django Form field initial value on failed validation

The documentation says:

If you have a bound Form instance and want to change the data somehow, or if you want to bind an unbound Form instance to some data, create another Form instance. There is no way to change data in a Form instance. Once a Form instance has been created, you should consider its data immutable, whether it has data or not.

I cannot really believe that your code works. But ok. Based on the documentation I would do it this way:

if request.method == 'POST':
    data = request.POST.copy()
    form = MyForm(data)
    if form.is_valid(): 
        form.save() 
    else: 
        data['myField'] = 'some different data'
        form = MyForm(initial=data)

I ended up doing

if request.method == 'POST':
    new_data = request.POST.copy()
    form = MyForm(data=new_data)
    if form.is_valid(): 
        form.save() 
    else: 
        new_data['myField'] = 'some different data'

Hope this helps someone