django adding element to list

I'm confused with how django adds elements to a list. consider the following:

    def add(request): 

        if request.method == "POST": 

        form = NewTaskForm(request.POST) 

            if form.is_valid(): 

                task = form.cleaned_data["task"]

                 request.session['tasks'].append(task) 

                # request.session['tasks'] += [task] 

                return HttpResponseRedirect(reverse("tasks:index"))

             else: 

                return render(request, "tasks/add.html",{

                      "form": form

                 }) 

        return render(request, "tasks/add.html",{ 

            "form": NewTaskForm() 

       })

if we add a print statement after request.session['tasks'].append(task) we get a list:

['check email']

we also get the same list if we comment the append line and use the correct way with +=

However, on the redirect to task/index the first way shows an empty list and the second way shows the list that's expected. Why? Whats going on?

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