code: def get_signature(payment_request):
      
  
      vads_args = {}
      for field in payment_request._meta.fields:
          if field.name[:5] == 'vads_':
              field_value = field.value_from_object(payment_request)
              if field_value:
                  vads_args.update({
                      field.name: field_value
                  })
      base_str = ''
      for key in sorted(vads_args):
          base_str += str(vads_args[key]) + '+'
      base_str += app_settings.VADS_CERTIFICATE
      return hashlib.sha1(base_str.encode("utf-8")).hexdigest()
docs: Returns the signature for the transaction.
  
      To compute the signature, first you have to get the value of all
      the fields that starts by 'vads_', ordering them alphabetically.
      All the values are separated by the '+' character. Then you add
      the value of the payzen certificate.
      Finaly you hash the string using sha1.