How to use __getattribute__ to access a class attribute?

[Ruud de Jong]

I have the situation where I need to construct the name
of a static method, and then retrieve the corresponding
function from a class object.

I thought I could just use __getattribute__ for this purpose.
This works fine if I already have an instantiation of the class,
but not when I try this on the class object directly.

A bare bones example:
def a():
print "static method a"
a = staticmethod(a)

It works for a instantiation of class C:
static method a

But for the class object it works differently:

Traceback (most recent call last):
File "<pyshell#219>", line 1, in -toplevel-
C.__getattribute__(C,'a')()
TypeError: 'staticmethod' object is not callable

After some experimentation and documentation searching,
I found that to get at the actual function, the __get__
method for the staticmethod descriptor must be called:
static method a

If I use an class instance as the first argument
it works OK. But I ran into this problem when I tried
to use __getattribute__ in the __new__ function of a class
-- there is no class instance yet at that point,
and calling C() there leads to infinite recursion.

Another variant that worked is to call __getattribute__
static method a

But according to section 3.3.2.1 of the Language Reference
(More attribute access for new-style classes),
__getattribute__ "... should return the (computed) attribute value".
This could be interpreted to say that __getattribute__ should
return the function, not the staticmethod object.

Is there a reason for this difference in behavior?

 

[Chris]

Perhaps it might be easier if you made your own static method object:
.... def __call__(self):
.... print "static method a"
.... def __get__(self, cls, inst):
.... return self.__call__
....<bound method MyStaticMethod.__call__ of <__main__.MyStaticMethod object at
0x00E57390>><bound method MyStaticMethod.__call__ of <__main__.MyStaticMethod object at
0x00E57390>>static method a

As far as your question goes, __getattribute__ is following the "... should
return the (computed) attribute value" rule for retrieving the static method
object... unfortunatly the calculated value in this case is a reference to
the static method object rather then the __get__ method bound to the static
method object.

HTH

Chris

 

[Shalabh Chaturvedi]

I have the situation where I need to construct the name
of a static method, and then retrieve the corresponding
function from a class object.

If what you're trying to do is get C.a (where C is the class and a is
the name of the staticmethod) but all you have is the class C and a
string mname = "a", then just getattr(C, mname) should work. Generally
this works for any attribute access where you have the attribute name in
another variable.

Another way to get the naked function, which also works for
classmethods, is to do C.__dict__['a']. See comments below that explain
what you see.

I thought I could just use __getattribute__ for this purpose.
This works fine if I already have an instantiation of the class,
but not when I try this on the class object directly.

A bare bones example:

def a():
print "static method a"
a = staticmethod(a)

It works for a instantiation of class C:

static method a

When Python comes across x.a, it doesn't call x.__getattribute__('a'),
it calls type(x).__getattribute__(x, 'a'). Now type(x) is C, and it just
so happens (in this case) that x.__getattribute__ also ends up calling
the same method, since x doesn't have a __getattribute__ of its own.
This is why what you do above works.

But for the class object it works differently:


Traceback (most recent call last):
File "<pyshell#219>", line 1, in -toplevel-
C.__getattribute__(C,'a')()
TypeError: 'staticmethod' object is not callable

To repeat the same experiment, you should call
type(C).__getattribute__(C, 'a'). Note that in this case C *has* its own
__getattribute__ attribute (which is meant for instances of C, not C
itself). In fact you do this later, and it works. Hmm, at first glance
I'd expect what you do above to return an exception if passed C instead
of an instance of C (like any other well behaved method). I don't know
why that doesn't happen.

In conclusion, __getattribute__() does return the computed attribute
value, but for an object o it is called on the type(o) and not o itself.
Also, getattr() is usually all you need.

HTH,
Shalabh

 

[Shalabh Chaturvedi]

I have the situation where I need to construct the name
of a static method, and then retrieve the corresponding
function from a class object.

If what you're trying to do is get C.a (where C is the class and a is
the name of the staticmethod) but all you have is the class C and a
string mname = "a", then just getattr(C, mname) should work. Generally
this works for any attribute access where you have the attribute name
in another variable.

Another way to get the naked function, which also works for
classmethods, is to do C.__dict__['a']. See comments below that
explain what you see.

I thought I could just use __getattribute__ for this purpose.
This works fine if I already have an instantiation of the class,
but not when I try this on the class object directly.

A bare bones example:

def a():
print "static method a"
a = staticmethod(a)

It works for a instantiation of class C:

static method a

When Python comes across x.a, it doesn't call x.__getattribute__('a'),
it calls type(x).__getattribute__(x, 'a'). Now type(x) is C, and it
just so happens (in this case) that x.__getattribute__ also ends up
calling the same method, since x doesn't have a __getattribute__ of
its own. This is why what you do above works.

But for the class object it works differently:


Traceback (most recent call last):
File "<pyshell#219>", line 1, in -toplevel-
C.__getattribute__(C,'a')()
TypeError: 'staticmethod' object is not callable

To repeat the same experiment, you should call
type(C).__getattribute__(C, 'a'). Note that in this case C *has* its
own __getattribute__ attribute (which is meant for instances of C, not
C itself). In fact you do this later, and it works. Hmm, at first
glance I'd expect what you do above to return an exception if passed C
instead of an instance of C (like any other well behaved method). I
don't know why that doesn't happen.

In conclusion, __getattribute__() does return the computed attribute
value, but for an object o it is called on type(o) and not o itself.
Also, getattr() is usually all you need.

HTH,
Shalabh

 

[Ivo]

A good read you can find : www.diveintopython.com

 

[Ruud de Jong]

Shalabh Chaturvedi schreef:

If what you're trying to do is get C.a (where C is the class and a is
the name of the staticmethod) but all you have is the class C and a
string mname = "a", then just getattr(C, mname) should work. Generally
this works for any attribute access where you have the attribute name in
another variable.

Erh, hmm, rrriight... Built-in functions. <blush/>
Completely forgot about that one -- too focussed
on trying to do everything with object methods and such.
I really need to pay more attention to those built-in functions.

Well, that solves my immediate problem.
I knew there had to be a simple solution

Another way to get the naked function, which also works for
classmethods, is to do C.__dict__['a']. See comments below that explain
what you see.

Well, that's not true, at least not on my system (XP, Python 2.3.3):
def a():
print "static method a"
a = staticmethod(a)

>>> C.__dict__['a']

> [snipped]

To repeat the same experiment, you should call
type(C).__getattribute__(C, 'a'). Note that in this case C *has* its own
__getattribute__ attribute (which is meant for instances of C, not C
itself).

C did not have its own __getattribute__ method. It inherited it
from object.

I still find this whole thing slightly ambiguous.
A class C is itself an object, an instance of 'type',
just as x = C() leads to x being an instance of class C.
But x.__getattribute__ and C.__getattribute__ return
different objects.

In fact you do this later, and it works. Hmm, at first glance
I'd expect what you do above to return an exception if passed C instead
of an instance of C (like any other well behaved method). I don't know
why that doesn't happen.

I would also have expected an exception. The strange thing, as I see
it, is that x.__getattribute__ and type(C).__getattribute__ both give
the correct result, but that C.__getattribute__ *almost* gives the
correct result, but stops at calling the __get__ function on the
desciptor object.

Anyway, now that you've kindly reminded me of the existance of getattr,
I'll not spend any more time trying to understand this.

Thanks,

Ruud

 

[Shalabh Chaturvedi]

Shalabh Chaturvedi schreef:

If what you're trying to do is get C.a (where C is the class and a is
the name of the staticmethod) but all you have is the class C and a
string mname = "a", then just getattr(C, mname) should work. Generally
this works for any attribute access where you have the attribute name
in another variable.

Erh, hmm, rrriight... Built-in functions. <blush/>
Completely forgot about that one -- too focussed
on trying to do everything with object methods and such.
I really need to pay more attention to those built-in functions.

Well, that solves my immediate problem.
I knew there had to be a simple solution

Another way to get the naked function, which also works for
classmethods, is to do C.__dict__['a']. See comments below that
explain what you see.

Well, that's not true, at least not on my system (XP, Python 2.3.3):
def a():
print "static method a"
a = staticmethod(a)

C.__dict__['a']

[snipped]

Sorry, my mistake. In fact it doesn't work for even classmethods. Only
for functions.

C did not have its own __getattribute__ method. It inherited it
from object.

True. Left out a little bit of information there (for brevity).

I still find this whole thing slightly ambiguous.
A class C is itself an object, an instance of 'type',
just as x = C() leads to x being an instance of class C.
But x.__getattribute__ and C.__getattribute__ return
different objects.

Only if you didn't first do x.__getattribute___ = None <wink>. then
x.__getattribute__ doesn't even return anything. x.__getattribute__ is
never meant to be called. It is only called on /type/ objects. If you
want to get an attribute on obj, you call __getattribute__ on type(obj).
(Well that's what Python does, you can just use getattr(obj, 'attr')).

I would also have expected an exception. The strange thing, as I see
it, is that x.__getattribute__ and type(C).__getattribute__ both give
the correct result, but that C.__getattribute__ *almost* gives the
correct result, but stops at calling the __get__ function on the
desciptor object.

What C.__getattribute__ doesn't do (maybe it should?) is to check that
the first argument is an instance of C, or at least an instance of
object. I'm just guessing here, but if it continues with the default
mechanism of __getattribute__, it is going to get the staticmethod object.