forms.py

class RegisterForm(Form):
  name = TextField('Username', [Required()])
  email = TextField('Email address', [Required(), Email()])
  password = PasswordField('Password', [Required()])
  confirm = PasswordField('Repeat Password', [
    Required(),
    EqualTo('password', message='Passwords must match')
  ])
  accept_tos = BooleanField('I accept the TOS', [Required()])
  recaptcha = RecaptchaField()
  
  def __init__(self, *args, **kwargs):
    Form.__init__(self, *args, **kwargs)
    self.user = None

  def validate(self):
    rv = Form.validate(self)
    if not rv:
      return False

    user = User.query.filter_by(
      username=self.username.data).first()
    if user not is None:
      #username exists, some error info
      return False

    self.user = user
    return True