The purpose of cleaned_data and response after form submit

> As I understand built_in django function 'cleaned_data' is used to ensure that forms do have something (not empty), right? If yes, then why to repeat, because django forms require non-empty input fields by default.

Cleaned_data is an object, not a function.

From the Django docs:

A Form instance has an is_valid() method, which runs validation routines for all its fields. When this method is called, if all fields contain valid data, it will:

• return True
• place the form’s data in its cleaned_data attribute.

So the cleaned_data is where to access the validated form data

> 'is_valid' checks if each input field has appropriate data types (e.g. email, char, int), which assumes that it is not empty, right? If yes, then again why we need cleaned_data feature?

This is correct. As above, cleaned_data hold the validated form data.

> Lets say, user submitted form and we want to show some message that it is successfully submitted and redirect back to previous page. It will be implemented using javascript or pure django forms? I didn't get that part in the video, it didnt work at first place.

Within the form.is_valid() code block, it is customary to add a redirect to a page upon successful form processing. There are Django function for this. There are also messaging functions that can be set up to display a one-time message after the redirect to confirm to the user the form was successfully processed.

Here is an example showing a redirect within the is_valid() block:

def product_form(request):
    form = forms.DigitalProductForm()
    if request.method == 'POST':
        form = forms.DigitalProductForm(request.POST)
        if form.is_valid():
            form.save()
            return HttpResponseRedirect(reverse("products:create"))
    return render(request, 'products/product_form.html', {'form': form})