Django form.is_valid() not calling method clean_<fieldname> - django

Question

I copied a simple example of a user registration page in Django 1.0 Web Site Development. I have defined the following form:
class RegistrationForm(forms.Form):
username = forms.CharField(label=u'Username', max_length=30)
email = forms.EmailField(label=u'Email')
password1 = forms.CharField(
label=u'Password',
widget=forms.PasswordInput()
)
password2 = forms.CharField(
label=u'Password (Again)',
widget=forms.PasswordInput()
)
def clean_password2(self):
if 'password1' in self.cleaned_data:
password1 = self.cleaned_data['password1']
password2 = self.cleaned_data['password2']
if password1 == password2:
return password2
raise forms.ValidationError('Passwords do not match.')
def clean_username(self):
print "Validating username..."
username = self.cleaned_data['username']
if not re.search(r'^\w+', username):
raise forms.ValidationError('Username can only contain '
'alphanumeric characters and the underscore.')
try:
User.objects.get(username=username)
except User.DoesNotExist:
return username
raise forms.ValidationError('Username is already taken')
In the case where a username is already taken, the field is validated using the normal form validation and my custom clean method is not called. When the form is submitted to register_page, form.is_valid() returns True.
Is there something missing that needs to be done in order for Django to know to call the clean_ methods?
def register_page(request):
if request.method == 'POST':
print "Posted to registration form"
form = RegistrationForm(request.POST)
if form.is_valid():
user = User.objects.create_user(
username=form.cleaned_data['username'],
password=form.cleaned_data['password1'],
email=form.cleaned_data['email']
)
return HttpResponseRedirect('/')
else:
form = RegistrationForm()
return render_to_response(
'users/registration.html',
context_instance=RequestContext(request, {
'form' : form
})

Answer 1

In case anybody else gets here via google and the above answer doesn't help:
I had this same problem and it turned out to be I was not setting exclude as a tuple in my forms meta:
class MyFormClass(ModelForm):
class Meta:
exclude = ('type_detail') #wrong
instead of:
class MyFormClass(ModelForm):
class Meta:
exclude = ('type_detail', ) #right

Answer 2

This was truly puzzling. I restored a auto-save version of forms.py and it magically started working, yet there is no difference that I can discern between the code snippet above, and the restored file.
Short answer: "Programmer Error"