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<cohen@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.jp... Dag Sverre