How can I programmatically authenticate a user in Django?

Alsways be careful when programmatically logging users in, you might get the error ´user has no attribute "backend". You have to set the backend too if that has no happened previously. Project that uses this and some sample code:

def splash_register(request):
  if request.session.get('beta'):

    if request.method=='POST':
        userform=MyUserCreationForm(request.POST)
        if userform.is_valid():
            #username of <30 char is required by Django User model.  I'm storing username as a hash of user email 

            user=userform.save(commit=False)
            user.username=hash(user.email)
            user.backend='django.contrib.auth.backends.ModelBackend'
            user.save()


            username=user.username
            password=str(userform.cleaned_data['password'])
            auth.login(request, user)
            request.session['first_visit']=True
            return HttpResponseRedirect("/")
        else:
            userform=MyUserCreationForm(request.POST)
            return render_to_response("website/splash_register.html", {'userform':userform}, context_instance=RequestContext(request))
    return render_to_response("website/splash_register.html", context_instance=RequestContext(request))     
else:
    return HttpResponseRedirect('/splash/')

The accepted answer definitely works but, I prefer to use the Django built in auth forms, like django.contrib.auth.forms.AuthenticationForm

Here is a snippet that shows the important part

form = AuthenticationForm(request, data=request.POST)
if form.is_valid():
    try:
        form.clean()
    except ValidationError:
        # handle error

    login(request, form.get_user())

The major difference in this approach is that AuthenticationForm.clean method calls authentication function for you and checks User.is_active for you as well.

There is no other way than "programmatically". Of course, this is documented.

from django.contrib.auth import authenticate, login

user = authenticate(username=username, password=password)
if user is not None:
    login(request, user)