Changing the default value of a class property to an instance of the class itself

Maxxner · October 13, 2023, 11:49pm

This is a technical source code question in context of a future PR.

In Bokeh model properties with their default values are stored as class variables. For certain models a property of the model can be an instance of the class itself (see example below).
My question is now, how one would set a default value of this property that isn’t None?
Replacing the None with a constructor call to the respective class (i.e., default=DatetimeTickFormatter()) is invalid in Python. Thus I am wondering if there is an option around this within Bokeh (for the Python as well as JS part)?

Reference example:

github.com

bokeh/bokeh/blob/b0a59ca015568d8b6229cdb7ddb1508e4c7f504b/src/bokeh/models/formatters.py#L650


      
          
          strip_leading_zeros = Either(Bool, Seq(Enum(ResolutionType)), default=False, help="""
          Whether to strip any leading zeros in the formatted ticks.
          
          Valid values are:
          * ``True`` or ``False`` (default) to set stripping across all resolutions.
          * A sequence of resolution types, e.g. ``["microseconds", "milliseconds"]``, to enable
          scale-dependent stripping of leading zeros.
          """)
          
          context = Nullable(Either(String, Instance("bokeh.models.formatters.DatetimeTickFormatter")), default=None, help="""
          A format for adding context to the tick or ticks specified by ``context_which``.
          Valid values are:
          
          * None, no context is added
          * A standard :class:`~bokeh.models.DatetimeTickFormatter` format string, the single format is
            used across all scales
          * Another :class:`~bokeh.models.DatetimeTickFormatter` instance, to have scale-dependent
            context
          """)

Bryan · October 15, 2023, 9:16pm

@Maxxner the value of default can be a function or lambda that returns a per-instance default value in a deferred fashion. If there is currently a way to accomplish this kind of self-reference, then that would be the route (but I have not personally tried).