Django Credit Card Payment Form

class CreditCardField(forms.IntegerField):
  def get_cc_type(self, number):
    """
    Gets credit card type given number. Based on values from Wikipedia page
    "Credit card number".
    <a href="http://en.wikipedia.org/w/index.php?title=Credit_card_number<br />
" title="http://en.wikipedia.org/w/index.php?title=Credit_card_number<br />
">http://en.wikipedia.org/w/index.php?title=Credit_card_number<br />
</a>    """
    number = str(number)
    #group checking by ascending length of number
    if len(number) == 13:
      if number[0] == "4":
        return "Visa"
    elif len(number) == 14:
      if number[:2] == "36":
        return "MasterCard"
    elif len(number) == 15:
      if number[:2] in ("34", "37"):
        return "American Express"
    elif len(number) == 16:
      if number[:4] == "6011":
        return "Discover"
      if number[:2] in ("51", "52", "53", "54", "55"):
        return "MasterCard"
      if number[0] == "4":
        return "Visa"
    return "Unknown"
 
  def clean(self, value):
    """Check if given CC number is valid and one of the
    card types we accept"""
    if value and (len(value) < 13 or len(value) > 16):
      raise forms.ValidationError("Please enter in a valid "+\
          "credit card number.")
    elif self.get_cc_type(value) not in ("Visa", "MasterCard",
        "American Express", "Discover"):
 
      raise forms.ValidationError("Please enter in a Visa, "+\
          "Master Card, Discover, or American Express credit card number.")
 
    return super(CreditCardField, self).clean(value)
 
class PaymentForm(forms.Form):
  number = CreditCardField(required = True, label = "Card Number")
  first_name = forms.CharField(required=True, label="Card Holder First Name", max_length=30)
  last_name = forms.CharField(required=True, label="Card Holder Last Name", max_length=30)
  expire_month = forms.ChoiceField(required=True, choices=[(x, x) for x in xrange(1, 13)])
  expire_year = forms.ChoiceField(required=True, choices=[(x, x) for x in xrange(date.today().year, date.today().year + 15)])
  cvv_number = forms.IntegerField(required = True, label = "CVV Number",
      max_value = 9999, widget = forms.TextInput(attrs={'size': '4'}))
 
  def __init__(self, *args, **kwargs):
    self.payment_data = kwargs.pop('payment_data', None)
    super(PaymentForm, self).__init__(*args, **kwargs)
 
  def clean(self):
    cleaned_data = super(PaymentForm, self).clean()
    expire_month = cleaned_data.get('expire_month')
    expire_year = cleaned_data.get('expire_year')
 
    if expire_year in forms.fields.EMPTY_VALUES:
      #raise forms.ValidationError("You must select a valid Expiration year.")
      self._errors["expire_year"] = self.error_class(["You must select a valid Expiration year."])
      del cleaned_data["expire_year"]
    if expire_month in forms.fields.EMPTY_VALUES:
      #raise forms.ValidationError("You must select a valid Expiration month.")
      self._errors["expire_month"] = self.error_class(["You must select a valid Expiration month."])
      del cleaned_data["expire_month"]
    year = int(expire_year)
    month = int(expire_month)
    # find last day of the month
    day = monthrange(year, month)[1]
    expire = date(year, month, day)
 
    if date.today() > expire:
      #raise forms.ValidationError("The expiration date you entered is in the past.")
      self._errors["expire_year"] = self.error_class(["The expiration date you entered is in the past."])
 
    return cleaned_data