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.
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