derived classes and __getattr__

[Peter Otten]

Alexandru  Mosoi wrote:

> On Sep 5, 11:47 am, Peter Otten <__pete... at web.de> wrote:
>> Alexandru Moșoi wrote:
>> > i'm facing the following problem:
>>
>> > class Base(object):
>> > def __getattr__(self, attr): return lambda x: attr + '_' + x
>>
>> > def dec(callable):
>> > return lambda *args: 'dec_' + callable(*args)
>>
>> > class Derived(Base):
>> > what_so_ever = dec(Base.what_so_ever) # wrong, base doesn't have
>> > what_so_ever
>> > mumu = dec(Base.mumu)                          # wrong, base
>> > doesn't have mumu
>>
>> > any idea how to do this?
>>
>> __getattr__() is defined in the class to create instance attributes on
>> the fly. If you want class attributes you have to put the __getattr__()
>> method into the class of the class, or "metaclass":
>>
>> class Base(object):
>> class __metaclass__(type):
>> def __getattr__(self, attr):
>> return lambda self, x: attr + '_' + x
>>
>> def dec(callable):
>> return lambda *args: 'dec_' + callable(*args)
>>
>> class Derived(Base):
>> what_so_ever = dec(Base.what_so_ever)
>>
>> d = Derived()
>> print d.what_so_ever("42")
>>
>> I don't see how you can turn this into something useful...
>>
>> Peter
> 
> 10x. it works. however I have another small problem. now,
> d.third('blah') doesn't work because instance d doesn't have 'third'
> attribute. I was expecting derived class to inherit the metaclass as
> well, but it didn't.

That has nothing to do with inheritance:

>>> type(Derived)
<class 'classattr.__metaclass__'>

If Python doesn't find an attribute in the instance it looks it up in the
class but not in the metaclass:

>>> Base.third
<function <lambda> at 0x2b5f8028aa28>
>>> Base().third
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Base' object has no attribute 'third'

I think you should go back to the drawing board now and look for a simpler
approach to solve your actual problem...

Peter