Why does Django custom User Registration Form returns None instead of HttpResponse and no Form errors are showing?

I'm learning Django and trying to build a user registration page with a simple custom user form with just a few more fields then the default UserCreationForm. When I submit the registration form, I get a Django debug error message saying:

The view catalog.views.librarian_add_user didn't return an HttpResponse object. It returned None instead.

I'm using python 3.8.15 with Django 4.1.4.

I've created a UserRegistrationForm in forms.py:

from django.contrib.auth.forms import UserCreationForm
from .models import User  # I import from .models and not from django.contrib.auth.models for consistency's sake

class UserRegistrationForm(UserCreationForm):
    first_name = forms.CharField(max_length=101, required=True)
    last_name = forms.CharField(max_length=101, required=True)
    email = forms.EmailField(required=True)

    class Meta:
        model = User
        fields = (
            'username',
            'first_name',
            'last_name',
            'email',
            'password1',
            'password2',
        )

A function view librarian_add_user in views.py:

from django.contrib.auth import authenticate, login
from .forms import UserRegistrationForm

def librarian_add_user(request):
    if request.method == 'POST':
        form = UserRegistrationForm(request.POST)
        if form.is_valid():
            form.save()
            username = form.cleaned_data['username']
            password = form.cleaned_data['password1']
            user = authenticate(username=username, password=password)
            login(request, user)
            messages.success(request, ("User added successfully."))
            return HttpResponseRedirect(reverse('catalog:index'))
        # else:
        #     print(form.errors)  # I can see the errors printed in my terminal, which means `form.is_valid()` failed
    else:
        form = UserRegistrationForm()
        context = {
            'user_form': form,
        }
        return render(
            request,
            'catalog/librarian_add_user.html',
            context,
        )

and the librarian_add_user.html in my templates:

{% extends 'catalog/base_generic.html' %}

{% block content %}
{% if perms.catalog.can_mark_returned %}

    {% if user_form.errors %}
        {{ user_form.errors }}
    {% endif %}

    <h1>Add a new user</h1>
    <form action="" method="post">
        {% csrf_token %}
        <table>
            {{ user_form.as_p }}
        </table>
        <input type="submit" value="Register">
    </form>

{% endif %}

I will omit the path in urls.py for simplicity.

As commented inline in views.py, I've tried adding a nested else clause in librarian_add_user function view and can see the form.errors printed in my terminal in an HTML unordered list. I have hypothesized maybe this is due to UserRegistrationForm in forms.py did not validate whether password1 equaled password2. I was able to rule that out by deliberately typing different passwords between password1 and password2, and getting the appropriate form error message in my terminal.

I have also tried using the same nested else clause to create an "else" scenario where a unbounded form is shown along with an error message. Like this:

def librarian_add_user(request):
    if request.method == 'POST':
        form = UserRegistrationForm(request.POST)
        if form.is_valid():
            form.save()
            ...
            return HttpResponseRedirect(reverse('catalog:index'))
        else:
            form = UserRegistrationForm()  # <--- HERE!
            context = {
                'user_form': form
            }
    else:
        form = UserRegistrationForm()
        ...
        return render(
            request,
            'catalog/librarian_add_user.html',
            context,
        )

I know this is redundant but still the user_form.errors don't show up in the newly rendered unbounded form.

I have also tried putting the form validation work in the forms.py file according to the Stackoverflow suggested post here, with the following changes to forms.py:

class UserRegistrationForm(UserCreationForm):
    first_name = forms.CharField(max_length=101, required=True)
    last_name = forms.CharField(max_length=101, required=True)
    email = forms.EmailField(required=True)

    class Meta:
        model = User
        fields = (
            ...   # same as above and omitted for simplicity
        )
    
    def save(self, commit=True):   # <--- ENTIRE METHOD HERE
        user = super(UserRegistrationForm, self).save(commit=False)
        user.first_name = self.cleaned_data['first_name']
        user.last_name = self.cleaned_data['last_name']
        user.email = self.cleaned_data['email']
        
        if commit:
            user.save()
        
        return user

And these changes to views.py:

def librarian_add_user(request):
    if request.method == 'POST':
        form = UserRegistrationForm(request.POST)
        if form.is_valid():
            user = form.save()   # <--- HERE
            login(request, user)    # <--- HERE    
            messages.success(request, ("User added successfully."))   # <--- HERE
            return HttpResponseRedirect(reverse('catalog:index'))
    else:
        form = UserRegistrationForm()
        ...
        return render(
            request,
            'catalog/librarian_add_user.html',
            context,
        )

This is probably some thing simple and really stupid but I have spend last afternoon, evening, and whole day today looking at it with no avail. Could someone help take a look and help answer the following questions?

1. Do I need a nested else clause for the case when form.is_valid() fails? Even the tutorial on MDN and Django doesn't do this.
2. Why is it that my form.errors does not show in the template and how is this related (which I think it is) to form.is_valid() failing and returning None?
3. How can I fix my code so I can register a new test user?

Thanks so much for helping a new comer!

SOLUTION:

I was able to solve the issue thanks to the suggestion from @srn, through the final result is a little different.

There are two ways to solve this. One way is to build a nested else clause that deals with the case when form.is_valid() is false. It keeps the data already entered (but not yet bounded to the form object) and just renders the form with the data.*

*For those who are confused about this, I learned from this error that althought I have filled out the form and pressed the "register" button, data binding to the form object DOES NOT HAPPEN until form.is_valid().

The views.py function view looks like this:

def librarian_add_user(request):
    if request.method == 'POST':
        form = UserRegistrationForm(request.POST)
        if form.is_valid():
            user = form.save()
            messages.success(request, ("User added successfully."))
            return HttpResponseRedirect(reverse('catalog:librarian-manage-member', kwargs={'username': user.username}))
        else:   # <--- HERE!
            return render(request, 'catalog/librarian_add_user.html', {'user_form': form})
    else:
        form = UserRegistrationForm()

        context = {
            'user_form': form,
        }
        return render(
            request,
            'catalog/librarian_add_user.html',
            context,
        )

The second way is more elegant IMO and is where my code deviated from all the examples in the official Django documentation. All I needed to do is to fix an indentation error after the outer else clause so that the context variable and return command covers the scenario where form.is_valid() is false. This fixed all my issues as shown below:

def librarian_add_user(request):
    if request.method == 'POST':
        form = UserRegistrationForm(request.POST)
        if form.is_valid():
            user = form.save()
            messages.success(request, ("User added successfully."))
            return HttpResponseRedirect(reverse('catalog:librarian-manage-member', kwargs={'username': user.username}))
    else:
        form = UserRegistrationForm()

    context = {   # <--- Outdented from here to the end
        'user_form': form,
    }
    return render(
        request,
        'catalog/librarian_add_user.html',
        context,
    )

As shown above, the outer if/else clause now only deals with whether the request is POST or GET, and the inner if clause only returns and exits the program if form.is_valid() is true. I can't believe I was on this for two days...(facepalm)

So to answer my own questions:

1. Yes, there needs to be coverage for the scenario where form.is_valid() is false. It could be explicit with an else clause, or outdenting the code following the if/else conditions to catch that scenario.
2. Since form.is_valid() case was not covered, there was no page rendered and hence no error information.
3. See the two approaches above.

At the end, this goes to show after three years of programming and building apps (non-production) as a hobby, I can still make simple mistakes. So don't ever get discouraged, stupid mistakes will happen years down the road...lol

If something returns None, it usually means that nothing was returned.

In your case this can happen in librarian_add_user().

if form.is_valid() is not True, the function will return nothing at all, None.

This happens because you only provide what it shall do in case it is True. If it isn't, the code won't magically jump back to the "outer" if/else clause.

Example:

def foo(baz):
    if baz == 1:
        return(1)

>>> x = foo(2)
>>> type(x)
<class 'NoneType'>

So yes, you should try to return something if the form is not valid, for readability from within an else-condition on the same level as the matching 'if'.