derived classes and __getattr__
__peter__ at web.de
Fri Sep 5 12:13:17 CEST 2008
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...
> 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:
If Python doesn't find an attribute in the instance it looks it up in the
class but not in the metaclass:
<function <lambda> at 0x2b5f8028aa28>
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...
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