UsernameField (for clean error messages)

import re

from django import forms
from django.forms import ValidationError
from django.contrib.auth.models import User
from django.utils.translation import ugettext_lazy as _

class UsernameField(forms.CharField):
    default_error_messages = {
        'username_too_short': _(u'Usernames must be at least three characters long.'),
        'username_too_long': _(u'Usernames can be at most thirty characters long.'),
        'bad_username_start': _(u'Usernames must start with a letter from the alphabet.'),
        'invalid_username': _(u'Usernames can only have letters, digits, and underscores.'),
    }
    
    MIN_USERNAME_SIZE = 3
    MAX_USERNAME_SIZE = 30
    
    def __init__(self, *args, **kwargs):
        super(UsernameField, self).__init__(*args, **kwargs)
        
    def clean(self, value):
        super_clean = super(UsernameField, self).clean(value)
        
        # We could do this all with one regular expression:
        #    ^[A-za-z]\w{1,29}$
        # but to be kind to our users we do it with increasingly 
        # restrictive tests that give increasingly more detailed error feedback.
        
        if len(super_clean) < UsernameField.MIN_USERNAME_SIZE:
            raise ValidationError(self.error_messages['username_too_short'])            
        if len(super_clean) > UsernameField.MAX_USERNAME_SIZE:
            raise ValidationError(self.error_messages['username_too_long'])        
        if re.match(r'^[A-Za-z]', super_clean) is None:
            raise ValidationError(self.error_messages['bad_username_start'])
        if re.match(r'^[A-Za-z]\w+$', super_clean) is None:
            raise ValidationError(self.error_messages['invalid_username'])
        return super_clean