Vfrdtyky

In Python 3.6, I can use the __set_name__ hook to get the class attribute name of a descriptor. How can I achieve this in python 2.x?

This is the code which works fine in Python 3.6:

class IntField:

    def __get__(self, instance, owner):

        if instance is None:

            return self

        return instance.__dict__[self.name]



    def __set__(self, instance, value):

        if not isinstance(value, int):

            raise ValueError('expecting integer')

        instance.__dict__[self.name] = value



    def __set_name__(self, owner, name):

        self.name = name



class Example:

    a = IntField()

You may be looking for metaclasses, with it you can process the class attributes at class creation time.

class FooDescriptor(object):

    def __get__(self, obj, objtype):

        print('calling getter')



class FooMeta(type):

    def __init__(cls, name, bases, attrs):

        for k, v in attrs.iteritems():

            if issubclass(type(v), FooDescriptor):

                print('FooMeta.__init__, attribute name is "{}"'.format(k))



class Foo(object):

    __metaclass__ = FooMeta

    foo = FooDescriptor()





f = Foo()

f.foo

Output:

FooMeta.__init__, attribute name is "foo"

calling getter

If you need to change the class before it is created you need to override __new__ instead of __init__ at your metaclass. See this answer for more information on this topic: Is there any reason to choose __new__ over __init__ when defining a metaclass?

There are various solutions with different degrees of hackishness. I always liked to use a class decorator for this.

class IntField(object):

    def __get__(self, instance, owner):            

        if instance is None:

            return self

        return instance.__dict__[self.name]



    def __set__(self, instance, value):            

        if not isinstance(value, int):

            raise ValueError('expecting integer')

        instance.__dict__[self.name] = value



def with_intfields(*names):

    def with_concrete_intfields(cls):

        for name in names:

            field = IntField()

            field.name = name

            setattr(cls, name, field)

        return cls

    return with_concrete_intfields

You can use it like this:

@with_intfields('a', 'b')

class Example(object):

    pass



e = Example()

Demo:

$ python2.7 -i clsdec.py

>>> [x for x in vars(Example) if not x.startswith('_')]

['a', 'b']

>>> Example.a.name

'a'

>>> e.a = 3

>>> e.b = 'test'

[...]

ValueError: expecting integer

Make sure to explicitly subclass from object in Python 2.7, that got me tripped up when I drafted the first version of this answer.