how to add property "dynamically"?

[akonsu]

hello,

i need to add properties to instances dynamically during run time.
this is because their names are determined by the database contents.
so far i found a way to add methods on demand:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, f)
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

this seems to work and i can invoke method test() on an object. it
would be nice to have it as property though. so i tried:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, property(f))
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

but this does not work, instance.test returns a callable but does not
call it.

i am not an expert in python, would someone please tell me what i am
doing wrong?

thanks
konstantin

[Christian Heimes]

akonsu wrote:

i am not an expert in python, would someone please tell me what i am
doing wrong?

You can't add properties to an instance. They must be specified on the
type (aka new style class). Descriptors and magic methods are only
looked up on classes for various reasons - mostly performance reasons.
Old style classes behave differently but properties don't work with old
style classes.

You can archive the same behavior by hooking into __getattr__,
__setattr__ and __delattr__. Mind the speed penelty, though!

Christian

[Larry Bates]

akonsu wrote:

hello,

i need to add properties to instances dynamically during run time.
this is because their names are determined by the database contents.
so far i found a way to add methods on demand:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, f)
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

this seems to work and i can invoke method test() on an object. it
would be nice to have it as property though. so i tried:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, property(f))
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

but this does not work, instance.test returns a callable but does not
call it.

i am not an expert in python, would someone please tell me what i am
doing wrong?

thanks
konstantin

Are you sure you can't get by by adding attributes to the instance that hold the
values that the property would return?

class A(object):
def __init__(self, dbvaluedict):
self.__dict__.update(dbvaluedict)

>>dbvaluedict = dict('test': 'test')
a = A(dbvaluedict)
print a.test

test

If this doesn't help. You might want to start at the beginning and explain what
it is you are trying to accomplish. What you are trying to do is very unusual.

-Larry

[Rafe]

On Aug 17, 5:09 pm, Bruno Desthuilliers
<[e-mail]:

akonsu a écrit :hello,

i need to add properties to instances dynamically during run time.
this is because their names are determined by the database contents.
so far i found a way to add methods on demand:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, f)
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

this seems to work and i can invoke method test() on an object.

Nope. This adds per-instance *function* attributes - not *methods*.

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f(self) :
return "%s.test" % self
setattr(self, name, f)
return f
else :
raise AttributeError(
"'%s' object has no attribute '%s'" \
% (self.__class__.__name__, name)
)

a = A()
a.test()
=Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: f() takes exactly 1 argument (0 given)

To add methods on a per-instance basis, you have to manually invoke the
descriptor protocol's implementation of function objects:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f(self) :
return "%s.test" % self
m = f.__get__(self, type(self))
setattr(self, name, m)
return m
else :
raise AttributeError(
"'%s' object has no attribute '%s'" \
% (self.__class__.__name__, name)
)

it
would be nice to have it as property though. so i tried:

class A(object) :
def __getattr__(self, name) :
if name == 'test' :
def f() : return 'test'
setattr(self, name, property(f))
return f
else :
raise AttributeError("'%s' object has no attribute '%s'" %
(self.__class__.__name__, name))

but this does not work, instance.test returns a callable but does not
call it.

Properties must be class attributes. The only way (the only way I know)
to get them to work as instance-attributes is to overload
__getattribute__, which is tricky and may have pretty bad impact on
lookup perfs - and ruins the whole point of using properties FWIW.

i am not an expert in python, would someone please tell me what i am
doing wrong?

Wrong solution to your problem, I'd say. Let's start again:

"""

i need to add properties to instances dynamically during run time.
this is because their names are determined by the database contents.

"""

Care to elaborate ? I may be wrong, but I suspect you're trying to roll
your own python/database mapper. If so, there are quite a couple Python
ORMs around. Else, please tell us more.

I posted this to another thread, but...
You can dynamically add properties (or anything else) to a CLASS just
before returning the
instance using __new__():

class AClass(object):
def __new__(cls):
setattr(cls,"propName", property(fget = ...,
fset = ...,
fdel = ...,
doc = ...) )

obj = super(AClass, cls).__new__(cls)
return obj
- Rafe

[Gabriel Genellina]

En Tue, 19 Aug 2008 15:02:29 -0300, Rafe <ra*******@gmail.comescribió:

On Aug 17, 5:09 pm, Bruno Desthuilliers
<[e-mail]:

>akonsu a écrit :hello,

i need to add properties to instances dynamically during run time.

Properties must be class attributes. The only way (the only way I know)
to get them to work as instance-attributes is to overload
__getattribute__, which is tricky and may have pretty bad impact on
lookup perfs - and ruins the whole point of using properties FWIW.

You can dynamically add properties (or anything else) to a CLASS just
before returning the
instance using __new__():

class AClass(object):
def __new__(cls):
setattr(cls,"propName", property(fget = ...,
fset = ...,
fdel = ...,
doc = ...) )

obj = super(AClass, cls).__new__(cls)
return obj

If you modify the class, this makes the property available to all existing
instances, not just the one being created. Any previous property of the
same name is overriden too.

--
Gabriel Genellina

[Bruno Desthuilliers]

Rafe a écrit :
(snip)

I posted this to another thread, but...

And I answered there, explaining why it's not a proper solution.
http://groups.google.com/group/comp....51a8614a58082#
(snip about using __new__ to add class attributes, cf above link for more)