__new__() does not return anything, on singletong pattern

[Mario Figueiredo]

On Thu, 12 Mar 2015 16:31:12 +1100, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:

>Mario Figueiredo wrote:
>
>
>If this is supposed to be a singleton, you can't create more instances. The 
>point of a singleton that there is only one instance (or perhaps a small 
>number, two or three say). Why do you need two different ways to create 
>instances if you only have one instance?
>

The map is instantiated from the contents of a saved map file on the
main application.

A different application, a map editor, needs to also instantiate an
object of the class Map. But in this case the map needs to either be
empty (if the user wants to create a new map), or loaded from the
saved map file (if the user wants to edit an existing map).

>
>> I added the following method to the class definition, above:
>> 
>>     @classmethod
>>     def generate(cls, width, height, fill=terrain[6]):
>>         if Map._instance is None:
>>             Map._instance = super(Map, cls).__new__(cls)
>>         else:
>>             raise Exception('Cannot generate an instance of Map.')
>> 
>>         Map._instance.author = None
>>         Map._instance.name = None
>
>Since this method modifies the singleton instance in place, it doesn't 
>generate a new instance. It shouldn't be called generate().

No sure what you mean. That method either creates a new instance or
raises an exception. It doesn't modify an instance in-place.

>
>>         Map._instance.description = None
>>              # etc...
>>              self.cells = [Cell(fill)] * width * height
>>         return Map._instance
>
>That's not your actual code, since the indentation is wrong.

     Map._instance.description = None
     # etc...
     self.cells = [Cell(fill)] * width * height
     return Map._instance