__str__(self) 函数不适用于在 Python 中打印对象

Question

我一直致力于改进在 Python 中使用类和对象，但在我的程序中显示不同对象的值时遇到了麻烦。我的导师告诉我，只要将对象传递给 print() 函数，就会自动调用 str(self) 函数，但是，输出仍然显示对象在内存中的地址，而不是价值。我在下面记录了我的代码在两个不同的编译程序中运行它，但仍然找不到我的错误发生的位置。

# the class of the bank account
class BankAccount:
    def __init__(self, bal):
        self.__balance = bal

# deposits the amount given by the user
def deposit(self, amount):
    self.__balance += amount

# withdraws the amount given by the user
def withdraw(self, amount):
    if(self.__balance >= amount):
        self.__balance -= amount
    else:
        print("Error: Insufficient funds")

# returns the current balance of the user's account
def get_balance(self):
    return self.__balance

# sets the user's account given a specified balance
def set_balance(self, bal):
    self.__balance = bal

# prints the user's balance
def __str__(self):
    return "The balance is $" + format(self.__balance, ",.2f")

def main():
    start_bal = float(input("Enter your starting balance: ")) # retrieves balance from the user
    savings = BankAccount(start_bal) # creates an object that hold's the user's balance
    print(savings)

main()

程序输出<__main__.BankAccount object at 0x0000025EE6535A90>而不是获取帐户的值

请让我知道我可以更改以纠正问题的任何内容。谢谢。

Answer 1

这是一个缩进问题，你的类只定义为：

# the class of the bank account
class BankAccount:
    def __init__(self, bal):
        self.__balance = bal

尝试将本节下方的所有代码缩进一个缩进块，直到到达 def main() 函数，这样 BankAccount 就会获得您想要的方法：

# the class of the bank account
class BankAccount:
    def __init__(self, bal):
        self.__balance = bal

    # deposits the amount given by the user
    def deposit(self, amount):
        self.__balance += amount

    # withdraws the amount given by the user
    def withdraw(self, amount):
        if(self.__balance >= amount):
            self.__balance -= amount
        else:
            print("Error: Insufficient funds")

    # returns the current balance of the user's account
    def get_balance(self):
        return self.__balance

    # sets the user's account given a specified balance
    def set_balance(self, bal):
        self.__balance = bal

    # prints the user's balance
    def __str__(self):
        return "The balance is $" + format(self.__balance, ",.2f")

def main():
    start_bal = float(input("Enter your starting balance: ")) # retrieves balance from the user
    savings = BankAccount(start_bal) # creates an object that hold's the user's balance
    print(savings)

main()

Answer 2

当 main 被调用并在 print(savings) 被执行时，它会打印 BankAccount 类的 str 表示，这是储蓄内存地址，然后因为你的 str 实现是在类之外定义的它与类无关。

试试这个：

# the class of the bank account
class BankAccount:
    def __init__(self, bal):
        self.__balance = bal

    # deposits the amount given by the user
    def deposit(self, amount):
        self.__balance += amount

    # withdraws the amount given by the user
    def withdraw(self, amount):
        if(self.__balance >= amount):
            self.__balance -= amount
        else:
            print("Error: Insufficient funds")

    # returns the current balance of the user's account
    def get_balance(self):
        return self.__balance

    # sets the user's account given a specified balance
    def set_balance(self, bal):
        self.__balance = bal

    # prints the user's balance
    def __str__(self):
        return "The balance is $" + format(self.__balance, ",.2f")

def main():
    start_bal = float(input("Enter your starting balance: ")) # retrieves balance from the user
    savings = BankAccount(start_bal) # creates an object that hold's the user's balance
    print(savings)

main()

Answer 3

您需要覆盖 BankAccount 类的 __str__ 运算符：

class BankAccount:
    def __init__(self, bal):
        self.__balance = bal

    def __str__(self):
        return "The balance is $" + format(self.__balance, ",.2f")


In [25]: start_bal = 2.3
In [27]: savings = BankAccount(start_bal) 

In [28]: print(savings)
The balance is $2.30

只需将你的方法正确地嵌套在你的类中