深入了解python中元类的相关知识

class ObjectCreator:
  pass  
my_object = ObjectCreator()
print(my_object)
"""
输出结果：
<__main__.ObjectCreator object at 0x037DACD0>
"""

class ObjectCreator:
  pass

class ObjectCreator:
  pass
# 把它赋值给一个变量
a = ObjectCreator
print(a) # <class '__main__.ObjectCreator'>
# 作为函数参数传递
def echo(o):
  print(o)
  
echo(ObjectCreator) # <class '__main__.ObjectCreator'>

def choose_class(name):
  if name == 'foo':
    class Foo(object):
      pass
    return Foo   # 返回的是类，不是类的实例
  else:
    class Bar(object):
      pass
    return Bar
MyClass = choose_class("foo")
print(MyClass) # 打印类对象
# 输出结果
<class '__main__.choose_class.<locals>.Foo'>
print(MyClass()) # 打印实例对象
# 输出结果
<__main__.choose_class.<locals>.Foo object at 0x0368CFD0>

MyClass = type("MyClass",(),{})
print(MyClass)
# 输出结果：
<class '__main__.MyClass'>

Foo = type("Foo",(),{'bar':True})

class Foo:
  bar = True

FooChild = type("FooChild",(Foo,),{})
print(FooChild.bar) # # bar属性是由Foo继承而来
# 输出结果：
True

def test_f(self):
  print("添加的实例方法")
Foo = type("Foo",(),{"test_f":test_f})
f = Foo()
f.test_f()
# 输出结果：
添加的实例方法

@staticmethod
def test_static():
  print("添加的静态方法")
Foo = type("Foo",(),{"test_static":test_static})
Foo.test_static()
Foo.test_static()
# 输出结果：
添加的静态方法

@classmethod
def test_class(cls):
  print("添加的类方法")
Foo = type("Foo",(),{"test_class":test_class})
Foo.test_class()
# 输出的结果：
添加的类方法

MyClass =  MetaClass() # 使用元类创建类对象
MyObject = MyClass() # 使用类对象创建实例对象

class A:
  pass
print(A.__class__)
a = A()
print(a.__class__)
print(a.__class__.__class__)
# 输出结果：
<class 'type'>
<class '__main__.A'>
<class 'type'>

age = 18
print(age.__class__)
print(age.__class__.__class__)
# 输出结果：
<class 'int'>
<class 'type'>

name = "张三"
print(name .__class__)
print(name .__class__.__class__)
# 输出结果：
<class 'str'>
<class 'type'>

def f():
  pass
print(f.__class__)
print(f.__class__.__class__)
# 输出结果：
<class 'function'>
<class 'type'>

def upper_attr(future_class_name: str,future_class_parents: tuple,future_class_attr: dict):
  newAttr = {}
  for key,value in future_class_attr.items():
    if not key.startswith("__"):
      newAttr[key.upper()] = value
  return type(future_class_name,future_class_parents,newAttr)
class Foo(metaclass=upper_attr):
  name = "张三"
  age = 18

hasattr(Foo,"name") # 判断是否有该类属性 False
hasattr(Foo,"NAME") # True
hasattr(Foo,"age") # False
hasattr(Foo,"AGE") # True

class MyMetaClass(type):
  def __new__(cls, class_name: str, class_parents: tuple, class_attr: dict):
    newAttr = {}
    for key, value in class_attr.items():
      if not key.startswith("__"):
        newAttr[key.upper()] = value
    # 方法1：通过'type'来做类对象的创建
    # return type(class_name, class_parents, newAttr)  
    # 方法2：复用type.__new__方法
    # 这就是基本的OOP编程，没什么魔法
    # return type.__new__(cls, class_name, class_parents, newAttr)  
    # 方法3：使用super方法
    return super(MyMetaClass, cls).__new__(cls, class_name, class_parents, newAttr)
class Foo(metaclass=MyMetaClass):
  name = "张三"
  age = 18
hasattr(Foo,"name") # 判断是否有该类属性 False
hasattr(Foo,"NAME") # True
hasattr(Foo,"age") # False
hasattr(Foo,"AGE") # True