On 3/14/07, Chris MacRaild <[EMAIL PROTECTED]> wrote:
On Tue, 2007-03-13 at 19:51 +1100, Edward d'Auvergne wrote: > Gary, > > I've written some unit tests which are located in the file > 'test_suite/unit_tests/data/__init__.py'. Would you know what > modifications I need to apply to make these tests run? These tests > are for the methods of the Data singleton class (the relax data > storage object) which is located in 'data/__init__.py'. I haven't > used the statement 'from x import Data' for this class so it is the > legal and ubiquitous usage of an __init__.py file. Would > 'unit_test_runner.py' handle this code? > > Thanks, > > Edward > > > P.S. These tests demonstrate the utility of subclassing the singleton > and is one reason I was arguing for the use of the class import rather > than reference import. >It is of course correct that the Singleton implimentation I proposed does not allow for subclassing. This however might well be seen as a design feature, given the well known conceptual problems with subclassing Singleton. Essentially, subclassing violates the Singleton Design Pattern: class Singleton: def __new__(self): ... class Sub(Singleton): pass >>> a = Singleton() >>> b = Sub() >>> isinstance(a,Singleton) True >>> isinstance(b,Singleton) True >>> a is b False >>> a == b False ie. we have 2 instances of Singleton that are not the same thing, and not even equal! If this is the behaviour you want, then you need something other than Singleton. On the other hand, if you really want Singleton, then you can't also hope for subclassability.
Actually one is an instance of Singleton and the other is an instance of Sub. This can be useful for two reasons. Firstly new properties and functionality can be added to the object and the new singleton object used for completely different purposes (the Data class doesn't contain that much, it's mainly the PipeContainer objects rather than the Data singleton which makes it interesting as the relax data storage object). So by subclassing Data (and replacing or removing the add_item() method), you can instantly have a dictionary type singleton which can be used for anything. Secondly a new and second singleton can be created for comparison and for isolated testing of the methods of that singleton without having to worry about corrupting the object. As this issue appears to be an impasse, I propose a vote on the subject. Any relax developer can vote on the issue by responding to this message. The parent thread of all these discussions is my message with the subject "The singleton design pattern for the old 'self.relax.data' data structure" located at https://mail.gna.org/public/relax-devel/2007-03/msg00012.html (Message-id: <[EMAIL PROTECTED]>). A second minor thread was started at https://mail.gna.org/public/relax-devel/2007-03/msg00038.html (Message-id: <[EMAIL PROTECTED]>). The two options to vote for will be labelled A and B. They are as follows: A: The relax data storage structure is a singleton which is imported as a class from the 'data' module. The instantiation in the calling module then gives a reference to this single object. This code can be seen in the 1.3 repository line. It is described in the parent thread (see above). B: The relax data storage structure is a singleton which is instantiated upon loading of the 'data' module, the reference of which overwrites the singleton 'Data' class name in the module. The instance reference variable is then imported into the calling module and renamed. A working implementation does not yet exist. It is described in Chris' message at https://mail.gna.org/public/relax-devel/2007-03/msg00013.html (Message-id: <[EMAIL PROTECTED]>). Please read all of the messages in the relevant threads before voting. All the posts are accessible from https://mail.gna.org/public/relax-devel/2007-03/threads.html . Cheers, Edward P.S. My vote is for option A. _______________________________________________ relax (http://nmr-relax.com) This is the relax-devel mailing list [email protected] To unsubscribe from this list, get a password reminder, or change your subscription options, visit the list information page at https://mail.gna.org/listinfo/relax-devel
