无法弄清楚如何使用 Python 中的 class 创建 "list" 实例变量

Question

这是我的代码：

class Category():

    
    def __init__(self,category,balance=0,ledger=[]):
        self.category = category
        self.balance = balance
        self.ledger = ledger

    def check_funds(self,amt):
        if amt > self.balance:
            return False
        else:
            return True
    

    def deposit(self, dep_amt, description=""):
        self.balance +=dep_amt
        dep_note = "{\"amount\": "+str(dep_amt)+", \"description\": "+description+"}"
        self.ledger.append(dep_note)
        
    def withdraw(self, wd_amt, description=""):
        if not self.check_funds(wd_amt):
            return False
        else:
            self.balance -= wd_amt
            wd_note = "{\"amount\": "+"-"+str(wd_amt)+", \"description\": "+description+"}"
            self.ledger.append(wd_note)
            return True
        
    def get_balance(self):
        return self.balance
    
    def transfer(self,tf_amt,category):
        wd_description = "Transfer to "+category.category
        dp_description = "Transfer from "+self.category
        self.withdraw(tf_amt,wd_description)
        category.deposit(tf_amt,dp_description)

正在创建实例：

food = Category('food')
food.deposit(1000,"food initial deposit")
food.withdraw(300,"groceries")

clothing = Category('clothing')
clothing.deposit(1000,"clothes initial deposit")
clothing.withdraw(20,"pants")
food.transfer(200,clothing)

虽然运行代码 clothing.ledger 这是输出：

['{"amount": 1000, "description": 食品初始存款}', '{"amount": -300, "description": 杂货}', '{"amount": 1000, " description": 衣服初始存款}', '{"amount": -20, "description": pants}', '{"amount": -200, "description": 转入衣服}', '{"amount" : 200, "description": 从食物中转移}']

这就是我要找的：

['{"amount": 1000, "description": 衣服初始定金}', '{"amount": -20, "description": pants}', '{"amount": -200, "description": 转移到服装}']

基本上我的 ledger 属性占用了所有我不想要的实例。

Answer 1

在 Python 中传递动态或可变（列表、字典）默认值时，最好将它们设置为 None 并指定它们函数内的预期值。

def __init__(self,category,balance=0,ledger=None):
    If ledger is None:
        ledger = []
    self.category = category
    self.balance = balance
    self.ledger = ledger

否则 ledger 只会计算一次。参考：https://medium.com/python-features/how-to-avoid-classic-pitfall-while-passing-default-values-in-python-7002c0dc4c7c

无法弄清楚如何使用 Python 中的 class 创建 "list" 实例变量

Can't figure out how to create a "list" instance variable using class in Python

class

class-instance-variables

python-3.x