Meta classes - real example

[Mr. Wrobel]

W dniu 17.10.2016 o 23:23, Ethan Furman pisze:
> On 10/17/2016 09:23 AM, Mr. Wrobel wrote:
>> W dniu 17.10.2016 o 18:16, Chris Angelico pisze:
>>> On Tue, Oct 18, 2016 at 3:03 AM, Mr. Wrobel wrote:
>
>>>> I am looking for an example of metaclass usage. Especially I am
>>>> interestet
>>>> in manipulating instance variables, for example:
>>>>
>>>> My class:
>>>> class MrMeta(type):
>>>>         pass
>>>>
>>>> class Mr(object):
>>>>         __metaclass__ = MrMeta
>>>>
>>>>         def __init__(self):
>>>>                 self.imvariable = 'Zmienna self'
>>>>
>>>>         def aome method(self):
>>>>                 print 'I am in instance'
>>>>
>>>> So in general, I would like to create a mechanism, that is changing
>>>> value
>>>> for self.imvariable to capital letters and this should be included
>>>> in my
>>>> metaclass called MrMeta(type).
>>>>
>>>> Can you guide me please?
>>>
>>> Are you sure you can't just use a descriptor, such as @property?
>>>
>>> class Mr(object):
>>>     @property
>>>     def imvariable(self):
>>>         return self._imvariable
>>>     @imvariable.setter
>>>     def imvariable(self, value):
>>>         self._imvariable = str(value).upper()
>>
>> I am sure that I can do that with setter/getter, but I want to be
>> closer to black magic, that is why I wanted to inlcude Metaclasses.
>>
>> I know how to acomplish that for class variables, but don;t have any
>> idea how to instance.
>
> Metaclasses work on the class level, not the instance level.  The only*
> influnce a metaclass is going to have on instances is changes it makes
> while it's creating the class.
>
> For example, MrMeta could set descriptors in Mr when it is creating the
> Mr class; it could even put it's own __init__ in the class; but to
> affect things like instance variables that are added in methods -- well,
> it is possible, but it is *a lot* of work.
>
> --
> ~Ethan~
>
> * Okay, somebody prove me wrong!  :)

Ok,so in general, we could say that using Metclasses is ok for 
manipulating __new__ but not that what is setting by __init__. Am I right?

MrWrobel