staticmethod and setattr

[Michael.Lausch]

Hi,

I managed to get confused by Python, which is not such an easy task.

The problem i have is rooted in marshalling, JSON and Dojo.
I need some static class in function with the name "$ref"
i tried:
class Foo(object):
@staticmethod
def _ref(o):
pass

setattr(Foo, "$ref", Foo._ref)

but when i do something like this in code:

class B(object):
pass
b = Bar
f = Foo.getattr("$ref")
f(b)

if get the following error
ypeError
unbound method _ref() must be called with Foo instance as first
argument (got Bar instance instead)

I managed to impelemnt it with a unbound, "normal" _ref() function,
but i want one _ref function per
class, because they are a little bit different for each class.

One aproach which i have not tried is:
setattr(Foo, "$ref", Foo._ref.im_func).
maybe this works, but i think this is not a clean aproach.
Any ideas?

 

[Steven D'Aprano]

Hi,

I managed to get confused by Python, which is not such an easy task.

The problem i have is rooted in marshalling, JSON and Dojo. I need some
static class in function with the name "$ref" i tried:
class Foo(object):
@staticmethod
def _ref(o):
pass

setattr(Foo, "$ref", Foo._ref)

That doesn't work as expected:

Foo.__dict__['_ref'] is Foo.__dict__['$ref']

False


Try this instead:

setattr(Foo, "$ref", Foo.__dict__['_ref'])
Foo.__dict__['_ref'] is Foo.__dict__['$ref']

True

 

[Michael.Lausch]

Hi,

I managed to get confused by Python, which is not such an easy task.

The problem i have is rooted in marshalling, JSON and Dojo. I need some
static class in function with the name "$ref" i tried:
class Foo(object):
    @staticmethod
    def _ref(o):
         pass

setattr(Foo, "$ref", Foo._ref)

That doesn't work as expected:

Foo.__dict__['_ref'] is Foo.__dict__['$ref']

False

Try this instead:

setattr(Foo, "$ref", Foo.__dict__['_ref'])
Foo.__dict__['_ref'] is Foo.__dict__['$ref']

True

Now I'm trying to understand why this is the case.
How is Foo.__dict__['_ref'] different from Foo._ref?
Shouldn't it return the same attribute?

And after further experiments i found out that a making
Foo._ref a classmethod does work with my first approach.

 

[Bruno Desthuilliers]

Michael.Lausch a écrit :
(snip)

Now I'm trying to understand why this is the case.
How is Foo.__dict__['_ref'] different from Foo._ref?
Shouldn't it return the same attribute?

It's an application of the descriptor protocol:

http://wiki.python.org/moin/FromFunctionToMethod