CurrencyField for Django to enforce accuracy for monetary amounts

currency_field.py

class CurrencyField(DecimalField):
    """Special DecimalField with defaults for currency.

    Currency should be stored in the database with 4 decimal places so any calculations will not have rounding errors.
    Formfields will use 4 decimal places to avoid rounding errors. Any other display of currency fields uses 2 decimals.
    All fields that process currency should use this field, especially when used in calculations.
    """
    display_decimals = 2  # Representation decimals
    db_decimals = 4  # Database storage decimals
    max_digits = 10

    def __init__(self, *args, max_digits=None, decimal_places=None, **kwargs):
        """Some values are hard coded as they are supposed to be the same for all.
        """
        super().__init__(*args, **kwargs)
        self.decimal_places = type(self).display_decimals
        self.max_digits = type(self).max_digits

    def deconstruct(self):
        """Used in migrations to know the required fields arguments.
        """
        name, path, args, kwargs = super().deconstruct()
        del kwargs['decimal_places'], kwargs['max_digits']
        return name, path, args, kwargs

    @cached_property
    def validators(self):
        """Allow inserting 4 decimals in forms, they will be rounded to 2 decimals.
        """
        return super(DecimalField, self).validators + [
            validators.DecimalValidator(self.max_digits, self.db_decimals)
        ]

    def db_type(self, connection):
        """Columns in the db should always be created with more decimals than the display decimals.
        This seems to be the only fair way to do it, however is only compatible with PostgreSQL.
        """
        return f'numeric({self.max_digits}, {self.db_decimals})'

    def formfield(self, **kwargs):
        """Configure form fields for all CurrencyFields. The default form field is a DecimalField.
        It allows inserting up to 4 decimals in forms, which matches the decimal precision in the database.
        """
        return super(DecimalField, self).formfield(
            **{
                "max_digits": self.max_digits,
                "decimal_places": self.db_decimals,
                "form_class": forms.DecimalField,
                **kwargs,
            }
        )

Featured in this blog article: https://deepintodjango.com/keeping-accurate-amounts-in-django-with-currencyfield