How to get Django view to return form errors

Make your view something like this:

if form.is_valid():
    pass
    # actions
else:
    # form instance will have errors so we pass it into template
    return render(request, 'template.html', {'form': form})

And in the templates you can iterate over form.errors or simple:

{{ forms.as_p }}

It is redirecting you because you always return HttpResponseRedirect if the method is POST, even if the form is not vaild. Try this:

def main_page(request):
    form = RegistrationForm()
    if request.method == 'POST':
        form = RegistrationForm(request.POST)
        if form.is_valid():
            user = User.objects.create_user(
                username=form.clean_data['username'],
                password=form.clean_data['password1'],
                email=form.clean_data['email']
            )
            return HttpResponseRedirect('/')        
    variables = {
        'form': form
    }
    return render(request, 'main_page.html', variables)

That way, the form instance, on which is_valid was called, is passed to the template, and it has a chance to display the errors. Only if the form is valid, the user is redirected. If you want to be fancy, add a message using the messages framework before redirecting.

If you want it a little bit more concise:

def main_page(request):
    form = RegistrationForm(request.POST or None)
    if form.is_valid():
        user = User.objects.create_user(
            username=form.clean_data['username'],
            password=form.clean_data['password1'],
            email=form.clean_data['email']
        )
        return HttpResponseRedirect('/')        
    variables = {
        'form': form
    }
    return render(request, 'main_page.html', variables)

You can reformat your view to display the form errors in the console as below

def main_page(request):

    if request.method == 'POST':
        form = RegistrationForm(request.POST)
        if form.is_valid():
            user = User.objects.create_user(
                username=form.clean_data['username'],
                password=form.clean_data['password1'],
                email=form.clean_data['email']
            )
        else:
            print(form.errors)
            return HttpResponse("Form Validation Error")

        return HttpResponseRedirect('/')
    else:
        form = RegistrationForm()
        variables = {
            'form': form
        }
        return render(request, 'main_page.html', variables)

As always, keep the indent as above. If the form has any error , it will display in the console like

<ul class="errorlist"><li>date<ul class="errorlist"><li>Enter a valid date.</li></ul></li></ul>

Hope it helps