def calculate_next_principal(self, current_principal, current_payment=0):
    next_principal = ((current_principal * (1 + (self.rate / self.frequency))) - current_payment)
    return next_principal
def calculate_next_principal(self, current_principal, current_payment=0):
    next_principal = ((current_principal * (1 + (self.rate / self.frequency))) - current_payment)
    return next_principal