derived classes and __getattr__

[=?UTF-8?Q?Alexandru_Mo=C8=99oi?=]

i'm facing the following problem:

class Base(object):
def __getattr__(sel f, attr): return lambda x: attr + '_' + x

def dec(callable):
return lambda *args: 'dec_' + callable(*args)

class Derived(Base):
what_so_ever = dec(Base.what_s o_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?

[Peter Otten]

Re: derived classes and __getattr__

Alexandru Moșoi wrote:

i'm facing the following problem:
>
class Base(object):
def __getattr__(sel f, attr): return lambda x: attr + '_' + x
>
def dec(callable):
return lambda *args: 'dec_' + callable(*args)
>
class Derived(Base):
what_so_ever = dec(Base.what_s o_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__(t ype):
def __getattr__(sel f, 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_s o_ever)

d = Derived()
print d.what_so_ever( "42")

I don't see how you can turn this into something useful...

Peter

[Fredrik Lundh]

Re: derived classes and __getattr__

Alexandru Mos,oi wrote:

i'm facing the following problem:
>
class Base(object):
def __getattr__(sel f, attr): return lambda x: attr + '_' + x
>
def dec(callable):
return lambda *args: 'dec_' + callable(*args)
>
class Derived(Base):
what_so_ever = dec(Base.what_s o_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?

ever thought of using Python to write Python programs?

</F>

[Alexandru Mosoi]

Re: derived classes and __getattr__

On Sep 5, 11:47 am, Peter Otten <__pete...@web. dewrote:

Alexandru Moșoi wrote:

i'm facing the following problem:

>

class Base(object):
  def __getattr__(sel f, attr): return lambda x: attr + '_' + x

>

def dec(callable):
  return lambda *args: 'dec_' + callable(*args)

>

class Derived(Base):
   what_so_ever = dec(Base.what_s o_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__(t ype):
        def __getattr__(sel f, 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_s o_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.

[Peter Otten]

Re: derived classes and __getattr__

Alexandru Mosoi wrote:

On Sep 5, 11:47 am, Peter Otten <__pete...@web. dewrote:

>Alexandru Moșoi wrote:

i'm facing the following problem:

>>

class Base(object):
def __getattr__(sel f, attr): return lambda x: attr + '_' + x

>>

def dec(callable):
return lambda *args: 'dec_' + callable(*args)

>>

class Derived(Base):
what_so_ever = dec(Base.what_s o_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__(t ype):
>def __getattr__(sel f, 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_s o_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(Derive d)

<class 'classattr.__me taclass__'>

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 <lambdaat 0x2b5f8028aa28>

>>Base().thir d

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

[Michele Simionato]

Re: derived classes and __getattr__

On Sep 5, 11:46 am, Alexandru Mosoi <brtz...@gmail. comwrote:

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.

Yep, it is explained here: https://www.ibm.com/developerworks/l...ary/l-pymeta2/
It would help if you told us the real problem you are trying to solve.

[Alexandru Mosoi]

Re: derived classes and __getattr__

On Sep 5, 1:13 pm, Peter Otten <__pete...@web. dewrote:

Alexandru  Mosoi wrote:

On Sep 5, 11:47 am, Peter Otten <__pete...@web. dewrote:

Alexandru Moșoi wrote:
i'm facing the following problem:

>

class Base(object):
def __getattr__(sel f, attr): return lambda x: attr + '_' + x

>

def dec(callable):
return lambda *args: 'dec_' + callable(*args)

>

class Derived(Base):
what_so_ever = dec(Base.what_s o_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__(t ype):
def __getattr__(sel f, 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_s o_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.__me taclass__'>
>
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 <lambdaat 0x2b5f8028aa28> >>Base().thir d
>
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

i somehow fixed the problem using:

def __getattr__(sel f, attr):
return
functools.parti al(Base.__metac lass__.__getatt r__(self.__clas s__,
attr), self)

however this looks ugly enough to look for another solution. i still
have one more question: why do I have to bind self? (without which
functions fail expecting an instance)

[Bruno Desthuilliers]

Re: derived classes and __getattr__

Alexandru Mosoi a écrit :
(snip)

i somehow fixed the problem using:
>
def __getattr__(sel f, attr):
return
functools.parti al(Base.__metac lass__.__getatt r__(self.__clas s__,
attr), self)
>
however this looks ugly enough to look for another solution. i still
have one more question: why do I have to bind self? (without which
functions fail expecting an instance)

Because the partial type doesn't implement the descriptor protocol
(which is implemented by function type to return a method).