Steven D'Aprano wrote:
> On Fri, 12 Jun 2015 16:53:08 +0100, Paulo da Silva wrote:
>
>> I would like to do something like this:
>>
>> class C:
>> def __init__(self,**parms):
>> ...
>>
>> c=C(f1=1,f2=None)
>>
>> I want to have, for the object
>> self.f1=1
>> self.f2=None
>>
>> for an arbitrary number of parameters.
>>
>> What is the best way to achieve this?
>
>
> Others have suggested that you update the instance dict with the keyword
> parameters:
>
> self.__dict__.update(parms)
>
>
> But I suggest that you should be very careful with this technique,
> because it can lead to surprising problems and bugs in your code. If your
> class has methods (and what sort of class doesn't have methods?), this
> will override them and lead to mysterious failures in your code:
>
> instance = C(a=1, b=2, method=3)
> # much later
> result = instance.method(args) # will raise exception
>
>
> 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.
I sometimes do it anyway if only to get a meaningful class name.
> Instead, use composition: your class should *contain* a SimpleNamespace,
> not *be* one:
>
> class C:
> def __init__(self, **param):
> self.ns = SimpleNamespace(param)
**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"
>>> c = C(ns=42)
>>> assert c.ns == 42
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
Yes, it's less likely, but now that the OP has two convenient ways to
produce a name clash I think it's time to repeat that there is one bullet-
proof approach: use a dict ;)
> 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.
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