python 魔法函数学习

1.什么是魔法函数

    以__开始和结束，不需要专门调用，在特定场合下，python会自己调用，如__init__, __str__等

2.常用的魔法函数

    __str__: 返回类对象友好的提示，例子

class Test:
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return self.name

obj = Test(name="Hello, World")
print(obj)

    __len__:计算对象容器中元素的个数      # 应用场景？？？，将类当容器处理

class TestLen:
    def __init__(self, mylist):
        self.mylist = mylist

    def __len__(self):
        return len(self.mylist)

obj = TestLen([1,2,3,4])
print(len(obj)) # 去掉__len__, 则报错

__setitem__(新建字典),__getitem__（通过key获取value）,__delitem__(删除key对应的value)，应用场景，将类当字典处理  

class TestDict:
    def __init__(self):
        self.dict = {}

    # 新建字典
    def __setitem__(self, key, value):
        self.dict[key] = value

    # 通过字典key获取value
    def __getitem__(self, item):
        return self.dict[item]

    # 删除对应key的记录
    def __delitem__(self, key):
        del self.dict[key]

obj = TestDict()
obj.dict["name"] = "hyh" # 调用__setitem__
obj["age"] = 18 # 效果同上
print(obj.dict)
print(obj["name"]) # 调用__getitem__
del obj["age"] # 调用__delitem__
print(obj.dict)

__iter__,__next__:迭代器 ,将类对象当作可迭代对象

class TestIterator:
    def __init__(self, value):
        self.start = 0
        self.end = value

    def __iter__(self):
        return self

    def __next__(self):
        if self.start < self.end:
            self.start += 1
            return self.start
        else:
            raise StopIteration

obj = TestIterator(5)
# 可遍历
for i in obj:
    print(i)

# 使用next函数遍历
#print(next(obj))
#print(next(obj))

 __enter__,__exit__:实现with上下文管理  

class TestWith:
    def __init__(self, filename, mode="r", encoding="utf-8"):
        self.filename = filename
        self.__file = open(filename, mode, encoding=encoding)

    def __enter__(self):
        print("enter")
        return self

    def write(self, content):
        print("write")
        self.__file.write(content)

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("exit")
        self.__file.close()

with TestWith(r"E:\python_apps\bk-pipline\haoyong_test\hyh.txt", mode="w", encoding="utf-8") as f:
    f.write("nihao\n hello world")

实现数据库连接释放