old_contents: from django import forms
  from django.core import validators
  from django.contrib.auth.models import User
  from django.contrib.auth.forms import UserCreationForm
  
  class RegistrationForm(UserCreationForm):
  
  	username = forms.CharField(label='username',
  							max_length=30,
  							required=True,) #validators=[self.isValidUserName])		
  	class Meta:
  		model = User 
  		fields = ('username','first_name', 'last_name', 'email',)
  
  #	def isValidUserName(self, field_data, all_data):
  #		try:
  #			User.objects.get(username=field_data)
  #		except User.DoesNotExist:
  #			return
  #		raise validators.ValidationError('The username "%s" is already taken.' % field_data)
  
  	def save(self, commit=True):
  		new_user = super(RegistrationForm, self).save(commit=False)
  
  		new_user.is_active = False
  		
  		if commit:
  			new_user.save()
  
  		return new_user
  
new_contents: from django import forms
  from django.core.exceptions import ValidationError
  from django.contrib.auth.models import User
  from django.contrib.auth.forms import UserCreationForm
  
  def isValidUserName(username):
  	try:
  		User.objects.get(username=username)
  	except User.DoesNotExist:
  		return
  	raise ValidationError('The username "%s" is already taken.' % username)
  
  
  class RegistrationForm(UserCreationForm):
  
  	username = forms.CharField(label='username',
  							max_length=30,
  							required=True, validators=[isValidUserName])		
  	class Meta:
  		model = User 
  		fields = ('username','first_name', 'last_name', 'email',)
  
  
  	def save(self, commit=True):
  		new_user = super(RegistrationForm, self).save(commit=False)
  
  		new_user.is_active = False
  		
  		if commit:
  			new_user.save()
  
  		return new_user
  
  
subject: Implement validation to the username field.