如何存储每个类别对象的总提款金额？

如何解决如何存储每个类别对象的总提款金额？

我有一个Category类，并且该类的每个实例都有一个分类帐属性。该分类帐属性实际上是词典的列表，其中包含取款和存款金额以及以{"amount" : amount,"description" : description}形式的描述。现在，我想定义一个函数create_spend_chart，该函数将对象列表作为参数，并找到提款的总额。我已经能够成功做到这一点：

    def create_spend_chart(categories):
        total_withdrawn = 0
        for i in categories:
            for p in i.ledger:
                if p["amount"] < 0:
                    total_withdrawn += -p["amount"]

但是我在这里面临的问题是，我似乎无法分别存储每个类别对象的总提款金额。我该怎么办？

我的代码库可能会帮助您ins回答问题：

    class Category:
        def __init__(self,name):
            self.name = name
            self.ledger = list()
    
        def get_balance(self):
            total_balance = 0
            for i in self.ledger:
                total_balance += i["amount"]
            return total_balance
    
        def check_funds(self,amount):
            if self.get_balance() >= amount:
                return True
            else:
                return False
        
        def deposit(self,amount,description = "Deposit"):
            form = {"amount" : int(amount),"description" : description}
            self.ledger.append(form)
        
        def withdraw(self,description = "Withdrawal"):
            if description == None:
                description = "Withdrawal"
    
            form = {"amount" : -int(amount),"description" : description}
            if self.check_funds(amount):
                self.ledger.append(form)
                return True
            else:
                return False
        
        def transfer(self,category_object):
            form1 = {"amount" : -int(amount),"description" :  f"Transfer to {category_object.name}"}
            form2 = {"amount" : int(amount),"description" :  f"Transfer from {self.name}"}
            
            if self.check_funds(amount):
                self.ledger.append(form1)
                category_object.ledger.append(form2)
                return True
            else:
                return False
        def __repr__(self):
            Ledger = ""
            for i in self.ledger:
                if len(i["description"]) > 23:
                    des = i["description"][:23]
                else:
                    des = i["description"]
                Ledger += des.ljust(23) + str(round(i["amount"],2)).rjust(7) + "\n"
            Ledger = Ledger + "Total: " + str(round(self.get_balance(),2))
            receipt = f"{self.name}".center(30,"*") + "\n" + Ledger
            return receipt
    
    def create_spend_chart(categories):
        total_withdrawn = 0
        withdrawals = list()
        for i in categories:
            for p in i.ledger:
                if p["amount"] < 0:
                    total_withdrawn += -p["amount"]

PS ：此函数不是方法，是在类声明之外定义的。

解决方法

使用collections.defaultdict来简化诸如馅饼之类的聚合。

import collections

# ...

withdrawn_per_category = collections.defaultdict(int)
for i in categories:
    for p in i.ledger:
        if p["amount"] < 0:
            withdrawn_per_category[i.name] += -p["amount"]

（我选择使用int作为默认数据类型，但在这里并没有什么关系，只要它是可转换的数字类型即可。）

没有collections

如果由于某种原因您不想使用方便的内置collections模块，则可以使用常规命令自己模拟相同的行为：

withdrawn_per_category = {}
for i in categories:
    for p in i.ledger:
        if p["amount"] < 0:
            withdrawn_per_category[i.name] = withdrawn_per_category.get(i.name,0) - p["amount"]