4.魔法函数
4.1 __new__ 、__init__ 、__call __、__del__
__new__ 实例化对象（1.创建对象 2.分配内存）
__init__ 构造方法,实例化对象时自动调用(1.可以没有 2.如果有方法必须返回None,默认不写return语句)
__call __ 对象可以被调用时触发执行
__del__ 析构方法,当对象被回收时触发执行(程序结束、对象引用计数为零称为垃圾时)
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| class MyClass(object):
def __init__(self):
print("__init__ is running...")
def __new__(cls):
print("__new__ is running...")
return super().__new__(cls)
def __call__(self, *args, **kwargs):
print("__call__ is running...")
def __del__(self):
print("__del__ is running...")
MyClass()
print("----------------------")
a = MyClass()
assert hasattr(a, "__del__")
print(callable(a))
assert hasattr(lambda x, y: x + y, "__call__")
print(callable(lambda x, y: x + y))
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4.2 __str__ 和 __repr__
两个方法都只是为了自定义对象的打印信息
对象被打印时执行,一般默认先找str, str没有则使用repr
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| class A(object):
def __init__(self):
self.name = "李四"
def __str__(self):
print("__str__ is running ...")
return "str"
def __repr__(self):
print("__repr__ is running ...")
return ""
print(A())
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4.3 compare系列
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| class Student:
def __init__(self, age):
self.age = age
def __eq__(self, other):
print("__eq__ is running ... 可自定义比较逻辑")
if isinstance(other, Student):
return self.age == other.age
return False
print(Student(18) == Student(18))
print(Student(18) != 18)
print(dir(Student(18)))
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4.4 attr系列
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| class MyClass(object):
def __init__(self, name, age):
self.name = name
self.age = age
def __getattr__(self, item):
print("getattr 获取不存在的对象属性时触发")
return self.__dict__.get(item)
def __setattr__(self, key, value):
print("setattr 设置修改对象属性时触发")
super().__setattr__(key, value)
def __delattr__(self, item):
print("delattr 删除对象属性时触发")
if item == "name":
pass
else:
super().__delattr__(item)
def __getattribute__(self, name):
print("__getattribute__ called")
return super().__getattribute__(name)
a = MyClass("李四", 18)
print(a.id)
del a.age
print(f"查看对象属性:{a.__dict__}")
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4.5 item系列
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class Person(object):
def __setitem__(self, key, value):
print("setitem []设置值时触发")
setattr(self, key, value)
def __getitem__(self, item):
print("getitem []取值时触发")
return getattr(self, item)
def __delitem__(self, key):
print("delitem del p[key]时触发", key)
p = Person()
p['id'] = 1
print(p['id'])
del p['id']
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4.6 __enter__ 和 __exit__
上下文管理器: 支持”上下文管理协议”的对象,包含 enter() 和 exit() 方法
with 可以操作一个 支持上下文管理协议的对象
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| class MyOpen:
def __init__(self, file_name: str, mode="r"):
self.file = open(file_name, mode)
def __enter__(self):
print("进入with语句块时触发")
return self.file
def __exit__(self, exc_type, exc_val, exc_tb):
print("退出with语句块时触发,不论with语句块是否有异常报错，__exit__都会被执行")
self.file.close()
with MyOpen("test", "w") as f:
f.write("hello world")
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4.7 __slots__
该类实例只能创建__slots__中声明的属性，否则报错, 具体作用就是节省内存
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| from memory_profiler import profile
class Test(object):
__slots__ = ['a', 'name']
def __init__(self, name):
self.name = name
Test.c = 3
t = Test("xx")
t.a = 1
print(t.c)
class TestA(object):
__slots__ = ['a', 'b', 'c']
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
class TestB(object):
def __init__(self, a, b, c):
self.a = a
self.b = b
self.c = c
@profile
def func_02():
temp = [TestA(i, i + 1, i + 2) for i in range(10000)]
del temp
temp = [TestB(i, i + 1, i + 2) for i in range(10000)]
del temp
func_02()
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4.8 __add__、 __dict__、 __bases__、 __all__
__add__: 手动实现相加操作
__dict__: 获取对象的属性
__bases__: 获取类继承的元素
__all__: 当其它文件以“from 模块名 import *”的形式导入该模块时，该文件中只能使用 __all__ 列表中指定的成员
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| class MyClass(object):
def __init__(self, value):
self.value = value
def __add__(self, other):
return self.value + other.value
a = MyClass(10)
print(a + MyClass(20))
print(MyClass.__dict__)
class A(object):
pass
print(A.__bases__)
__all__ = ["MyClass"]
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