main.py

class Calculator:
    def __init__(self):
        self.question_number = 0
        self.total = 0

    def calculator(self):  # applies formula to the values
        self.rate = self.rate / 100
        self.monthly = self.monthly * 12
        for i in range(0, self.years):
            if self.total == 0:
                self.total = self.account
            self.total = (self.total + self.monthly) * (1 + self.rate)
        print("Total amount of money you would have in your account after " + str(self.years) + " years: " + str(round(self.total, 2)))

    def error(self):
        if self.question_number == 1:
            if self.account < 0:
                return True
            else:
                return False
        if self.question_number == 2:
            if self.monthly < 0:
                return True
            else:
                return False
        else:
            if 0 < self.rate < 100:
                return False
            else:
                return True

    def asker(self): # stores values from user inputs
        if self.question_number == 0:
            print("How many years will you be saving for? ")
            self.years = int(input("Enter years: "))
            self.question_number += 1

        if self.question_number == 1:
            print("How much money is currently in your account?")
            self.account = float(input("Enter current amount in account: "))
            if self.error():
                self.asker()
            else:
                self.question_number += 1

        if self.question_number == 2:
            print("How much money are you planning on investing monthly? ")
            self.monthly = int(input("Enter amount: "))
            if self.error():
                self.asker()
            else:
                self.question_number += 1

        if self.question_number == 3:
            print("What's the yearly interest rate of this investment?")
            self.rate = float(input('Enter interest rate in percentage: '))
            if self.error():
                self.asker()
            else:
                self.question_number += 1

        if self.question_number == 4:
            self.calculator()


c = Calculator()
c.asker()