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: class C(object):
def a():
print "static method a"
a = staticmethod(a)
It works for a instantiation of class C:
x = C()
x.a
<function a at 0x00A981B0> x.__getattribut e__('a')
<function a at 0x00A981B0> x.__getattribut e__('a')()
static method a
But for the class object it works differently:
C.a
<function a at 0x00A981B0> C.__getattribut e__(C,'a')
<staticmethod object at 0x00AC6A10> C.a()
static method a C.__getattribut e__(C,'a')()
Traceback (most recent call last):
File "<pyshell#219>" , line 1, in -toplevel-
C.__getattribut e__(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:
C.__getattribut e__(C,'a').__ge t__(None, C)
<function a at 0x00A981B0> C.__getattribut e__(C,'a').__ge t__(None, C)()
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_ _
on the metaclass:
type(C).__getat tribute__(C,'a' )
<function a at 0x00A981B0> type(C).__getat tribute__(C,'a' )()
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? 6  3626 
Perhaps it might be easier if you made your own static method object: class MyStaticMethod( object):
.... def __call__(self):
.... print "static method a"
.... def __get__(self, cls, inst):
.... return self.__call__
.... class C(object):
.... a = MyStaticMethod( ) C.a
<bound method MyStaticMethod. __call__ of <__main__.MySta ticMethod object at
0x00E57390>> C.a()
static method a C.__getattribut e__(C,'a')
<__main__.MySta ticMethod object at 0x00E57390> C.__getattribut e__(C,'a')()
static method a c = C()
c.a()
static method a C.__getattribut e__(c,'a')
<bound method MyStaticMethod. __call__ of <__main__.MySta ticMethod object at
0x00E57390>> C.__getattribut e__(c,'a')()
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
"Ruud de Jong" <ru**********@c onsunet.nl> wrote in message
news:40******** **************@ dreader2.news.t iscali.nl... 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:
>>> class C(object): def a():
print "static method a"
a = staticmethod(a)
It works for a instantiation of class C:
>>> x = C()
>>> x.a <function a at 0x00A981B0> >>> x.__getattribut e__('a') <function a at 0x00A981B0> >>> x.__getattribut e__('a')() static method a
But for the class object it works differently:
>>> C.a <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a') <staticmethod object at 0x00AC6A10> >>> C.a() static method a >>> C.__getattribut e__(C,'a')()
Traceback (most recent call last):
File "<pyshell#219>" , line 1, in -toplevel-
C.__getattribut e__(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:
>>> C.__getattribut e__(C,'a').__ge t__(None, C) <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a').__ge t__(None, C)() 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_ _
on the metaclass:
>>> type(C).__getat tribute__(C,'a' ) <function a at 0x00A981B0> >>> type(C).__getat tribute__(C,'a' )()
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?
Ruud de Jong <ru**********@c onsunet.nl> wrote in message news:<40******* *************** @dreader2.news. tiscali.nl>...
Ruud de Jong wrote: 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:
>>> class C(object): def a():
print "static method a"
a = staticmethod(a)
It works for a instantiation of class C:
>>> x = C()
>>> x.a <function a at 0x00A981B0> >>> x.__getattribut e__('a') <function a at 0x00A981B0> >>> x.__getattribut e__('a')() static method a
When Python comes across x.a, it doesn't call x.__getattribut e__('a'),
it calls type(x).__getat tribute__(x, 'a'). Now type(x) is C, and it
just so happens (in this case) that x.__getattribut e__ 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:
>>> C.a <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a') <staticmethod object at 0x00AC6A10> >>> C.a() static method a >>> C.__getattribut e__(C,'a')()
Traceback (most recent call last):
File "<pyshell#219>" , line 1, in -toplevel-
C.__getattribut e__(C,'a')()
TypeError: 'staticmethod' object is not callable
To repeat the same experiment, you should call
type(C).__getat tribute__(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
Ruud de Jong wrote: 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:
>>> class C(object): def a():
print "static method a"
a = staticmethod(a)
It works for a instantiation of class C:
>>> x = C()
>>> x.a <function a at 0x00A981B0> >>> x.__getattribut e__('a') <function a at 0x00A981B0> >>> x.__getattribut e__('a')() static method a
When Python comes across x.a, it doesn't call x.__getattribut e__('a'),
it calls type(x).__getat tribute__(x, 'a'). Now type(x) is C, and it just
so happens (in this case) that x.__getattribut e__ 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:
>>> C.a <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a') <staticmethod object at 0x00AC6A10> >>> C.a() static method a >>> C.__getattribut e__(C,'a')()
Traceback (most recent call last):
File "<pyshell#219>" , line 1, in -toplevel-
C.__getattribut e__(C,'a')()
TypeError: 'staticmethod' object is not callable
To repeat the same experiment, you should call
type(C).__getat tribute__(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
A good read you can find : www.diveintopython.com
"Ruud de Jong" <ru**********@c onsunet.nl> wrote in message
news:40******** **************@ dreader2.news.t iscali.nl... 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:
>>> class C(object): def a():
print "static method a"
a = staticmethod(a)
It works for a instantiation of class C:
>>> x = C()
>>> x.a <function a at 0x00A981B0> >>> x.__getattribut e__('a') <function a at 0x00A981B0> >>> x.__getattribut e__('a')() static method a
But for the class object it works differently:
>>> C.a <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a') <staticmethod object at 0x00AC6A10> >>> C.a() static method a >>> C.__getattribut e__(C,'a')()
Traceback (most recent call last):
File "<pyshell#219>" , line 1, in -toplevel-
C.__getattribut e__(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:
>>> C.__getattribut e__(C,'a').__ge t__(None, C) <function a at 0x00A981B0> >>> C.__getattribut e__(C,'a').__ge t__(None, C)() 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_ _
on the metaclass:
>>> type(C).__getat tribute__(C,'a' ) <function a at 0x00A981B0> >>> type(C).__getat tribute__(C,'a' )()
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?
Shalabh Chaturvedi schreef: Ruud de Jong wrote:
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.
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):
class C(object):
def a():
print "static method a"
a = staticmethod(a)
C.__dict__['a']
<staticmethod object at 0x00AADD10>
[snipped]
To repeat the same experiment, you should call
type(C).__getat tribute__(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.__getattribut e__ and C.__getattribut e__ 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.__getattribut e__ and type(C).__getat tribute__ both give
the correct result, but that C.__getattribut e__ *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
Ruud de Jong wrote: Shalabh Chaturvedi schreef:
Ruud de Jong wrote:
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.
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):
>>> class C(object): def a():
print "static method a"
a = staticmethod(a)
>>> C.__dict__['a'] <staticmethod object at 0x00AADD10>
> [snipped]
Sorry, my mistake. In fact it doesn't work for even classmethods. Only
for functions.
To repeat the same experiment, you should call
type(C).__getat tribute__(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.
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.__getattribut e__ and C.__getattribut e__ return
different objects.
Only if you didn't first do x.__getattribut e___ = None <wink>. then
x.__getattribut e__ doesn't even return anything. x.__getattribut e__ 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')).
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.__getattribut e__ and type(C).__getat tribute__ both give
the correct result, but that C.__getattribut e__ *almost* gives the
correct result, but stops at calling the __get__ function on the
desciptor object.
What C.__getattribut e__ 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.
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 This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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