derived classes and __getattr__

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

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?

[Peter Otten]

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

[Fredrik Lundh]

Alexandru Mos,oi 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?

ever thought of using Python to write Python programs?

</F>

[Alexandru Mosoi]

On Sep 5, 11:47Â*am, Peter Otten <[e-mail]:

Alexandru Moșoi wrote:

i'm facing the following problem:

class Base(object):
Â* def __getattr__(self, attr): return lambda x: attr + '_' + x

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

class Derived(Base):
Â* Â*what_so_ever = dec(Base.what_so_ever) # wrong, base doesn't have
what_so_ever
Â* Â*mumu = dec(Base.mumu) Â* Â* Â* Â* Â*Â* Â* Â* Â* Â* Â* Â* Â*# 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):
Â* Â* class __metaclass__(type):
Â* Â* Â* Â* def __getattr__(self, attr):
Â* Â* Â* Â* Â* Â* return lambda self, x: attr + '_' + x

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

class Derived(Base):
Â* Â*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.

[Peter Otten]

Alexandru Mosoi wrote:

On Sep 5, 11:47Â*am, Peter Otten <[e-mail]:

>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) Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*# 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 <lambdaat 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

[Michele Simionato]

On Sep 5, 11:46*am, Alexandru Mosoi <[e-mail]:

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]

On Sep 5, 1:13Â*pm, Peter Otten <[e-mail]:

Alexandru Â*Mosoi wrote:

On Sep 5, 11:47Â*am, Peter Otten <[e-mail]:

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) Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â* Â*# 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 <lambdaat 0x2b5f8028aa28>>>Base().third

Traceback (most recent call last):
Â* 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__(self, attr):
return
functools.partial(Base.__metaclass__.__getattr__(s elf.__class__,
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]

Alexandru Mosoi a écrit :
(snip)

i somehow fixed the problem using:

def __getattr__(self, attr):
return
functools.partial(Base.__metaclass__.__getattr__(s elf.__class__,
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).