Singletons configurable allow multiple instances #867

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1. azjps commented on Sep 19, 2023

   

   azjps

   on Sep 19, 2023

   Last edited by azjps

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   This is intentional I believe, the singleton API is only accessed via the instance() / clear_instance() / instantiated() methods: https://traitlets.readthedocs.io/en/stable/config.html#the-main-concepts

   Singletons are not strictly enforced - you can have many instances of a given singleton class, but the instance() method will always return the same one.

   There's an example in the docs under examples/docs/multiple_apps.py, although I added it based on related threads in this tracker.

   I'm not sure about the ordering issue, normally SubApp.instance() would raise if another subapp is already instantiated. See for example related discussion in #474 and the (inactive) PR which aims to handles the case of a non-singleton subcommand.

   Edit: I do agree the docs could use further clarification on this, it definitely is a bit confusing and the docstring for SingletonConfigurable is misleading as per the current design. But I think trying to change Application.__init__() to be truly a singleton would break a significant amount of downstream code.
2. Carreau commented on Sep 19, 2023

   

   Carreau

   on Sep 19, 2023

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   Yeah, the problem is I believe that a few places are not using .instance() which can lead to assumptions about having singletons.

   👍React with 👍1Reacted by azjps