# ###### IBAN Field
# Write a Django custom Field model to store IBANs. The Field must meet the following requirements:
# 1. The stored value should never be fully visible - given an IBAN like "GR96 0810 0010 0000 0123 4567 890",
#    the value should be displayed as "---7890" everywhere
# 2. Superusers should be able to see the full value when needed

"""
Approach:
Create a custom field for IBAN to handle data display. Field iban must be private to prevent direct access.
And must only be accessible using via a method.

Implement a method to mask iban number when required. Mask method must take user details into account and hide iban number
for all the users except super-user.

A template tag is required to access iban from django template. Template tag utlilizes mask method with logged user info.
And display the iban number.

USE:
// load template tag
{% load mask_utils %}

// display
{% mask row.iban user %}
// user is instance of current logged in user

Below is the implementation:
"""

# mask_utils.py
# start
from django import template
register = template.Library()

@register.simple_tag(name='mask')
def mask(iban, user):
    return iban(user)

# end

# models.py
# start
class IBANField(models.TextField):

    MASK_CHAR = '-'
    MASK_CHAR_LENGTH = 8
    DISPLAY_NUMBER_LENGTH = 4

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def from_db_value(self, value, expression, connection, context):
        if value is None:
            return value
        return self.display(value)

    @staticmethod
    def _clean(value):
        return value.replace(' ', '')

    def _mask(self, value):
        value = value[-self.DISPLAY_NUMBER_LENGTH:]
        return value.rjust(self.MASK_CHAR_LENGTH, self.MASK_CHAR)

    def display(self, value):
        def mask_wrapper(user):
            if user.is_superuser:
                return value
            return self._mask(self._clean(value))
        return mask_wrapper


class SomeModel(models.Model):

    # using underscore to keep it private and accessible only via method for lazy evaluation
    _iban = IBANField()

    def iban(self):
        return self._iban


# end