Passing new fields to an object
Paulo da Silva
p_s_d_a_s_i_l_v_a_ns at netcabo.pt
Fri Jun 12 23:33:29 EDT 2015
On 13-06-2015 02:25, Steven D'Aprano wrote:
> On Fri, 12 Jun 2015 16:53:08 +0100, Paulo da Silva wrote:
>
...
>
> You should use SimpleNamespace, as Peter suggests, but *not* subclass it.
> If you subclass it and add methods:
>
> class C(SimpleNamespace):
> def foo(self, arg):
> print("called foo")
>
>
> then you risk overriding foo method, as above. If you don't add methods,
> there is no need to subclass.
>
> Instead, use composition: your class should *contain* a SimpleNamespace,
> not *be* one:
>
> class C:
> def __init__(self, **param):
> self.ns = SimpleNamespace(param)
> def __getattr__(self, attrname):
> return getattr(self.ns, attrname)
> def foo(self, arg):
> print("called foo")
>
>
> instance = C(a=1, b=2, foo=3)
> # later
> instance.foo("x") # prints "called foo"
>
>
> The special method __getattr__ only runs if the attribute name is not
> found in the usual way, so the method foo will continue to be found and
> not be overridden by the param foo.
Always learning!
Thanks a lot.
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