PEP 3115有以下使用__prepare__元类方法的示例（print语句是我的）：
# The custom dictionary
class member_table(dict):
def __init__(self):
self.member_names = []
def __setitem__(self, key, value):
print(f'in __setitem__{key, value}')
# if the key is not already defined, add to the
# list of keys.
if key not in self:
self.member_names.append(key)
# Call superclass
dict.__setitem__(self, key, value)
# The metaclass
class OrderedClass(type):
# The prepare function
@classmethod
def __prepare__(metacls, name, bases): # No keywords in this case
print('in __prepare__')
return member_table()
# The metaclass invocation
def __new__(cls, name, bases, classdict):
print('in __new__')
# Note that we replace the classdict with a regular
# dict before passing it to the superclass, so that we
# don't continue to record member names after the class
# has been created.
result = type.__new__(cls, name, bases, dict(classdict))
result.member_names = classdict.member_names
return result
print('before MyClass')
class MyClass(metaclass=OrderedClass):
print('in MyClass 1')
# method1 goes in array element 0
def method1(self):
pass
print('in MyClass 2')
# method2 goes in array element 1
def method2(self):
pass
print('in MyClass 3')
运行这个，打印这个：
before MyClass
in __prepare__
in __setitem__('__module__', '__main__')
in __setitem__('__qualname__', 'MyClass')
in MyClass 1
in __setitem__('method1', <function MyClass.method1 at 0x7fa70414da60>)
in MyClass 2
in __setitem__('method2', <function MyClass.method2 at 0x7fa70414daf0>)
in MyClass 3
in __new__
所以看起来什么时候MyClass执行，首先执行到类的元类__prepare__方法返回member_table()（谁/什么使用这个返回值？），然后设置类的__module__和__qualname__，然后执行类体，设置类的方法（method1和method2） ，然后__new__调用该方法，并将返回值__prepare__作为classdict参数值__new__（谁/什么传递了这个值？）。
我试图在 thonny 的调试器中逐步执行，但这引发了错误。我还尝试在 pythontutor.com 中逐步执行，但这还不够精细。我pdb'编辑了它，但很难理解发生了什么。最后，我添加了一些print语句，它们出现在上面的代码中。