Python元类（Metaclass）原理与用法详解

MyClass = MetaClass()
MyObject = MyClass()

MyClass = type('MyClass', (), {})

>>> age = 35
>>> age.__class__
<type 'int'>
>>> name = 'bob'
>>> name.__class__
<type 'str'>
>>> def foo(): pass
>>> foo.__class__
<type 'function'>
>>> class Bar(object): pass
>>> b = Bar()
>>> b.__class__
<class '__main__.Bar'>

>>> a.__class__.__class__
<type 'type'>
>>> age.__class__.__class__
<type 'type'>
>>> foo.__class__.__class__
<type 'type'>
>>> b.__class__.__class__
<type 'type'>

class Foo(object):
 __metaclass__ = something...
 [...]

class Foo(Bar):
 pass

def upper_attr(future_class_name, future_class_parents, future_class_attr):
 """
  Return a class object, with the list of its attribute turned
  into uppercase. """
 # pick up any attribute that doesn't start with '__'
 attrs = ((name, value) for name, value in future_class_attr.items() if not name.startswith('__'))
 # turn them into uppercase
 uppercase_attr = dict((name.upper(), value) for name, value in attrs)
 # let `type` do the class creation
 return type(future_class_name, future_class_parents, uppercase_attr)
__metaclass__ = upper_attr # this will affect all classes in the module
class Foo(): # global __metaclass__ won't work with "object" though
 # but we can define __metaclass__ here instead to affect only this class
 # and this will work with "object" childrend
 bar = 'bip'
print hasattr(Foo, 'bar')
# Out: False
print hasattr(Foo, 'BAR')
# Out: True
f = Foo()
print f.BAR
# Out: 'bip'

# remember that `type` is actually a class like `str` and `int`
# so you can inherit from it
class UpperAttrMetaclass(type):
  # __new__ is the method called before __init__
  # it's the method that creates the object and returns it
  # while __init__ just initializes the object passed as parameter
  # you rarely use __new__, except when you want to control how the object
  # is created.
  # here the created object is the class, and we want to customize it
  # so we override __new__
  # you can do some stuff in __init__ too if you wish
  # some advanced use involves overriding __call__ as well, but we won't
  # see this
  def __new__(upperattr_metaclass, future_class_name,
        future_class_parents, future_class_attr):
    attrs = ((name, value) for name, value in future_class_attr.items() if not name.startswith('__'))
    uppercase_attr = dict((name.upper(), value) for name, value in attrs)
    return type(future_class_name, future_class_parents, uppercase_attr)

class UpperAttrMetaclass(type):
  def __new__(upperattr_metaclass, future_class_name,
        future_class_parents, future_class_attr):
    attrs = ((name, value) for name, value in future_class_attr.items() if not name.startswith('__'))
    uppercase_attr = dict((name.upper(), value) for name, value in attrs)
    # reuse the type.__new__ method
    # this is basic OOP, nothing magic in there
    return type.__new__(upperattr_metaclass, future_class_name,
              future_class_parents, uppercase_attr)

class UpperAttrMetaclass(type):
  def __new__(cls, name, bases, dct):
    attrs = ((name, value) for name, value in dct.items() if not name.startswith('__'))
    uppercase_attr = dict((name.upper(), value) for name, value in attrs)
    return super(UpperAttrMetaclass, cls).__new__(cls, name, bases, uppercase_attr)