def sign(self):
        """
        Returns 1 if a credit should increase the value of the
        account, or -1 if a credit should decrease the value of the
        account.

        This is based on the account type as is standard accounting practice.
        The signs can be derrived from the following expanded form of the
        accounting equation:

            Assets = Liabilities + Equity + (Income - Expenses)

        Which can be rearranged as:

            0 = Liabilities + Equity + Income - Expenses - Assets

        Further details here: https://en.wikipedia.org/wiki/Debits_and_credits

        """
        return -1 if self.type in (Account.TYPES.asset, Account.TYPES.expense) else 1
Returns 1 if a credit should increase the value of the
        account, or -1 if a credit should decrease the value of the
        account.

        This is based on the account type as is standard accounting practice.
        The signs can be derrived from the following expanded form of the
        accounting equation:

            Assets = Liabilities + Equity + (Income - Expenses)

        Which can be rearranged as:

            0 = Liabilities + Equity + Income - Expenses - Assets

        Further details here: https://en.wikipedia.org/wiki/Debits_and_credits