29 内置方法 eval | exec 元类 单例

dic_str = "{‘a‘: 1, ‘b‘: 2, ‘c‘: 3}"
print(eval(dic_str))

list_str = "[1, 2, 3, 4, 5]"
print(eval(list_str))

tuple_str = "(1, 2, 3, 4, 5)"
print(eval(tuple_str))

source = ‘‘‘
name = ‘Bob‘
age = 20
‘‘‘
class A:
    pass
a = A()

dic = {}

# 可以操作全局与局部两个名称空间，一般不用关心全局名称空间
exec(source, {}, dic)
a.__dict__ = dic
print(a.__dict__)   # {‘name‘: ‘bob‘, ‘age‘: 20}
print(a.name)
print(a.age)

local_str = """
def __init__(self, name, age):
    self.name = name
    self.age = age
def study(self):
    print(self.name + ‘在学习‘)
"""
local_dic = {}
exec(local_str, {}, local_dic)
Student = type(‘Student‘, (), local_dic)
print(Student)

# type(name, bases, namespace)

s = ‘‘‘
my_a = 10
my_b = 20
def __init__(self):
    pass
@classmethod
def print_msg(cls, msg):
    print(msg)
‘‘‘
namespace = {}
exec(s, {}, namespace)

Student = type(‘Student‘, (object, ), namespace)

stu = Student()

# 元类：所有自定义的类本身也是对象，是元类的对象，所有自定义的类本质上是由元类实例化出来了
Student = type(‘Student‘, (object, ), namespace)

class MyMeta(type):
    # 在class Student时调用：Student类的创建 => 来控制类的创建
    
    # 自定义元类，重写init方法的目的：
    # 1.该方法是从type中继承来的，所以参数同type的init
    # 2.最终的工作(如果开辟空间，如果操作内存)还是要借助type
    # 3.在交给type最终完成工作之前，可以对类的创建加以限制 *****
    def __init__(cls, class_name, bases, namespace):
        # 目的：对class_name | bases | namespace加以限制 **********************
        super().__init__(class_name, bases, namespace)
    
    # 在Student()时调用：Student类的对象的创建 => 来控制对象的创建
    
    # 自定义元类，重写call方法的目的：
    # 1.被该元类控制的类生成对象，会调用元类的call方法
    # 2.在call中的返回值就是创建的对象
    # 3.在call中
    #       -- 通过object开辟空间产生对象
    #       -- 用被控制的类回调到自己的init方法完成名称空间的赋值
    #       -- 将修饰好的对象反馈给外界
    def __call__(cls, *args, **kwargs):
        # 目的：创建对象，就可以对对象加以限制 **********************
        obj = object.__new__(cls)  # 通过object为哪个类开辟空间
        cls.__init__(obj, *args, **kwargs)  # 调回当前被控制的类自身的init方法，完成名称空间的赋值
        return obj

# 问题：
# 1.继承是想获得父级的属性和方法，元类是要将类的创建于对象的创建加以控制
# 2.类的创建由元类的__init__方法控制
#        -- 元类(class_name, bases, namespase) => 元类.__init__来完成实例化
# 3.类的对象的创建由元类的__call__方法控制
#         -- 对象产生是需要开辟空间，在__call__中用object.__new__()来完成的
class Student(object, metaclass=MyMeta):
    pass

# class Student:  <=>  type(class_name, bases, namespace)

class MyMeta(type):
    # 控制类的创建过程
    def __init__(cls, class_name, bases, namespace):
        print(cls, class_name, bases, namespace)
        super().__init__(class_name, bases, namespace)

    def __call__(cls, *args, **kwargs):
        obj = object.__new__(cls)
        cls.__init__(obj, *args, **kwargs)
        return obj

# Student类与Student类的对象产生都可以备元类MyMeta控制
class Student(metaclass=MyMeta):
    def __init__(self, name, age):
        self.name = name
        self.age = age
stu = Student(‘owen‘, 18)
print(stu.name)

# 单例1
class MyMeta(type):
    def __call__(cls, *args, **kwargs):
        if not hasattr(cls, ‘instance‘):
            cls.instance = cls.__new__(cls)
            cls.instance.__init__(*args, **kwargs)
        return cls.instance


class A(metaclass=MyMeta):
    def __init__(self):
        self.ip = ‘1.1.1.0‘

# 单例2
class Song:
    __instance = None
    def __init__(self):
        pass
    @classmethod
    def getInstance(cls):
        if cls.__instance == None:
            cls.__instance = cls()
        return cls.__instance
s1 = Song.getInstance()
s2 = Song.getInstance()
print(s1, s2)

def singleton(cls):
    _instance = None
    def getInstance(*args, **kwargs):
        nonlocal _instance
        if _instance == None:
            _instance = cls(*args, **kwargs)
        return _instance
    return getInstance

@singleton
class A:
    def __init__(self, num):
        self.num = num
print(A(1), A(2), A(3))

class A:
    __instance = None
    def __new__(cls, *args, **kwargs):
        if cls.__instance == None:
            cls.__instance = super().__new__(cls)
        return cls.__instance
print(A(), A())

# single_module.py
class Single:
    pass
singleton = Single()

# 测试文件
from single_module import singleton
print(singleton)
print(singleton)