def available_credit(context):
    ''' Calculates the sum of unclaimed credit from this user's credit notes.

    Returns:
        Decimal: the sum of the values of unclaimed credit notes for the
            current user.

    '''

    notes = commerce.CreditNote.unclaimed().filter(
        invoice__user=user_for_context(context),
    )
    ret = notes.values("amount").aggregate(Sum("amount"))["amount__sum"] or 0
    return 0 - ret
Calculates the sum of unclaimed credit from this user's credit notes.

    Returns:
        Decimal: the sum of the values of unclaimed credit notes for the
            current user.