python metaclass 详细说明

>>> class example(object):
...     pass
...
>>> object1 = example()
>>> print(object1)
<__main__.example object at 0x102e26990>

# print a class since it's an object
>>> print(example)
<class '__main__.example'>

# assign an attribute to the class
>>> print(hasattr(example, 'new_attribute'))
False
>>> example.new_attribute = 'assign an attribute to the class'
>>> print(hasattr(example, 'new_attribute'))
True
>>> print(example.new_attribute)
assign an attribute to the class

# assign the class to a variable
>>> example_mirror = example
>>> print(example_mirror)
<class '__main__.example'>
>>> print(example_mirror())
<__main__.example object at 0x102e26a90>

# pass class as a parameter
>>> def echo(cls):
...     print(cls)
...
>>> echo(example)
<class '__main__.example'>

>>> def dynamic_class_creater(name):
...     if name == 'name1':
...         class class1(object):
...             pass
...         return class1
...     else:
...         class class2(object):
...             pass
...         return class2
...
>>> first_class = dynamic_class_creater('name1')
>>> print(first_class)
<class '__main__.class1'>
>>> print(first_class())
<__main__.class1 object at 0x10e4149d0>

>>> print(type(1))
<type 'int'>

>>> print(type('str'))
<type 'str'>

>>> print(type(example()))
<class '__main__.example'>

>>> print(type(example))
<type 'type'>

type(class_name, tuple_of_parent_class, dict_of_attribute_names_and_values)

>>>class class_example(object):
...     pass

>>>class_example = type('class_example', (), {}) # create a class on the fly
>>>print(class_example)
<class '__main__.class_example'>
>>> print(class_example()) # get a instance of the class
<__main__.class_example object at 0x10e414b10>

>>> class_example = type('class_example', (), {'attr': 1})
>>> print(class_example)
<class '__main__.class_example'>
>>> print(class_example.attr)
1
>>> print(class_example())
<__main__.class_example object at 0x10e414a90>
>>> print(class_example().attr)
1

>>> class class_example(object):
...     attr = 1

>>> child_example = type('child_example', (class_example,), {})
>>> print(child_example)
<class '__main__.child_example'>
>>> print(child_example.attr)
1

>>> class child_example(class_example):
...     pass

>>> def echo(self):
...     print(self.attr)
...
>>> child_example = type('child_example', (class_example,), {'echo': echo})
>>> hasattr(class_example, 'echo')
False
>>> hasattr(child_example, 'echo')
True
>>> child_example().echo()
1

>>> child_example = type('child_example', (class_example,), {})
>>> def another_method(self):
...     print('another method')
...
>>> child_example.another_method = another_method
>>> hasattr(child_example, 'another_method')
True
>>> child_example().another_method()
another method

class = metaclass()
object = class()

>>> age = 24
>>> age.__class__
<type 'int'>

>>> name = 'bob'
>>> name.__class__
<type 'str'>

>>> def foo(): pass
>>> foo.__class__
<type 'function'>

>>> class Bar(object): pass
>>> bar = Bar()
>>> bar.__class__
<class '__main__.Bar'>

>>> age.__class__.__class__
<type 'type'>
>>> name.__class__.__class__
<type 'type'>
>>> foo.__class__.__class__
<type 'type'>
>>> bar.__class__.__class__
<type 'type'>

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

class Foo(Bar):
    pass

# the metaclass will automatically get passed the same argument 
# that is passed to `type()`
def upper_attr(class_name, class_parents, class_attr):
    '''Return a class object, with the list of its attribute turned into 
    uppercase.
    '''
    # pick up any attribute that doesn't start with '__' and turn it into uppercase.
    uppercase_attr = {}
    for name, val in class_attr.items():
        if name.startswith('__'):
            uppercase_attr[name] = val
        else:
            uppercase_attr[name.upper()] = val

    # let `type` do the class creation
    return type(class_name, class_parents, uppercase_attr)

class Foo(object):
    # this __metaclass__ will affect the creation of this new style class
    __metaclass__ = upper_attr
    bar = 'bar'

print(hasattr(Foo), 'bar')
# False

print(hasattr(Foo), 'BAR')
# True

f = Foo()
print(f.BAR)
# 'bar'

# remember that `type` is actually a just a class like int or str
# so we can inherit from it.

class UpperAttrMetaclass(type):
    '''
    __new__ is the method called before __init__
    It's the function that actually creates the object and returns it.
    __init__ only initialize the object passed as a parameter.
    We rarely use __new__, except when we want to control how the object
    is created.
    For a metaclass, the object created is a class. And since we want to 
    customize it, we need to override __new__.
    We can also do something by overriding __init__ to get customized initialization
    process as well.
    Advanced usage involves override __call__, but we won't talk about this here.
    '''
    def __new__(upperattr_metaclass, class_name, class_parents, class_attr):
        uppercase_attr = {}
        for name, val in class_attr.items():
            if name.startswith('__'):
                uppercase_attr[name] = val
            else:
                uppercase_attr[name.upper()] = val
        return type(class_name, class_parents, uppercase_attr)

class UpperAttrMetaclass(type):
    def __new__(upperattr_metaclass, class_name, class_parents, class_attr):
        uppercase_attr = {}
        for name, val in class_attr.items():
            if name.startswith('__'):
                uppercase_attr[name] = val
            else:
                uppercase_attr[name.upper()] = val
        # basic OOP. Reuse the parent's `__new__()`
        return type.__new__(upperattr_metaclass, class_name, class_parents, uppercase_attr)

class UpperAttrMetaclass(type):
    def __new__(cls, cls_name, bases, attr_dict):
        uppercase_attr = {}
        for name, val in attr_dict.items():
            if name.startswith('__'):
                uppercase_attr[name] = val
            else:
                uppercase_attr[name.upper()] = val
        return type.__new__(cls, cls_name, bases, uppercase_attr)

class UpperAttrMetaclass(type):
    def __new__(cls, cls_name, bases, attr_dict):
        uppercase_attr = {}
        for name, val in attr_dict.items():
            if name.startswith('__'):
                uppercase_attr[name] = val
            else:
                uppercase_attr[name.upper()] = val
        return super(UpperAttrMetaclass, cls).__new__(cls, cls_name, bases, uppercase_attr)

class Person(models.Model):
    name = models.CharField(max_length=30)
    age = models.IntegerField()

guy = Person(name='bob', age='35')
print(guy.age)

>>> class Foo(object):
...     pass
...
>>> id(Foo)
4299321816