Johann Cohen-Tanugi wrote:
> On 10/20/2010 11:10 PM, Robert Kern wrote:
>
>> On Wed, Oct 20, 2010 at 15:58, Johann Cohen-Tanugi<co...@lpta.in2p3.fr>
>> wrote:
>>
>>
>>> On 10/20/2010 10:35 PM, Nathaniel Smith wrote:
>>>
>>>
>>
>>
>>>> IMPORTANT USAGE NOTE: never do this :-)
>>>>
>>>>
>>>>
>>> What would you recommand? I do encounter situations where I need
>>> instantiation based on the name of the thing to instantiate, typically
>>> passed as an argument by the client code/user.....
>>>
>>>
>> Example?
>>
>>
>>
> Hi Robert,
> so in a big data analysis framework, that is essentially written in C++,
> exposed to python with SWIG, plus dedicated python modules, the user
> performs an analysis choosing some given modules by name,as in :
> myOpt="foo"
> my_analyse.perform(use_optimizer=myOpt)
>
> The attribute use_optimizer is then checked to perform the right
> calls/instantiations of python but also C++ objects..... and the actual
> crunching number is in the C++ part.
> But then I realize that I need to tweak this optimizer's state, and the
> optimizer object is accessible from a library pyOpt that has been
> swigified from a C++ library.
> Then I access the object by calling optObject =
> eval("pyOpt.%s(some_args)"%myOpt)
> where myOpt would be "foo" in this particular analysis. This is because
> what the attribute use_optimizer expects is also the object name in the
> library, of course.
> It goes without saying that I could do :
> if myOpt=="foo":
> optObject=pyOpt.foo(some_args)
> else:
> ....
> and if you guys tell me it is way safer, I will do that instead of the
> use of eval that I liked because of the compactness.....
>
In this case getattr fits... in general, in cases where one could do
if action == 'a':
a()
elif action == 'b':
b()
the nice (proper?) way of doing it is usually to populate a dictionary:
actions['a'] = a
actions['b'] = b
# automatically populate etc...
actions[action]()
And on the subject of passing strings around and eval-ing them:
http://cache.gawkerassets.com/assets/images/4/2010/03/for_traffic_cameras.jpg
Dag Sverre
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