Supporting streams dictionaries in operations

[jlstevens]

WIP PR to address #4807.

For testing, I've defined the streams parameter of inspect_points as:

    streams = param.ClassSelector(default=dict(x=PointerXY.param.x,
                                               y=PointerXY.param.y),
                                  class_=(dict, list))

And here is the corresponding, working test example:

However, there are some serious limitations right now. The minor one is that I have to set hv.streams.PointerXY.x = 0 and
hv.streams.PointerXY.y = 0 due to an error processing dictionaries when allowing the None default. While I'm sure this can be fixed, this seems to be a far more fundamental problem right now:

As operations use stream classes, not instances, how can I get back to the correct class with something like Tap.param.x when the owner of the parameter may be a superclass? The code needs to instantiate the correct stream instance. Any ideas @jbednar @philippjfr ?

If this can be solved quickly then I can work on this PR to get it into the next minor release. However, if this is a deeper issue (as I suspect it is) then this PR will need to wait for updates in param at the very least. Assuming this has a quick solution, the next steps would be:

• Fix the dictionary keyword issue when the parameter values are initialized with None.
• See if this dictionary support can be pushed further down into Apply and/or Dynamic.

[jlstevens]

One possible solution to the owner problem is e75743f: simply make sure all parameters exist on each stream class without relying on inheritance.

The logic is that 1) this mechanism is primarily about linked streams, you may as well use parameters directly for the old non-linked case 2) linked streams have to be tied to the plotting backend which means they are unlikely to be user defined (and if they are, users would just need to know that they shouldn't rely on parameter inheritance) and 3) we can do something like the following to enforce that all the streams we provide are defined according to the rule:

objs = [getattr(hv.streams, el) for el in dir(hv.streams)]
stream_classes = [obj for obj in objs if isinstance(obj, type) and  issubclass(obj, hv.streams.Stream) ]
for stream_class in stream_classes:
    for name, p in stream_class.param.params().items():
        if name != 'name':
           if p.owner != stream_class:
              raise AssertionError('Parameter %s has owner %s when accessed from class %s' 
                                                      % (name, p.owner, stream_class))

Edit: Or better, I can use stream_classes = param.concrete_descendents(hv.streams.Stream). Should I test all streams or only linked streams?

[jbednar]

Sounds good. Seems like only linked streams need testing like that at this time.

[jlstevens]

@philippjfr I've pushed the changes we discussed in e71782f. Please do push better fixes to how the Params stream is handled!

[philippjfr]

Also happy with this now, just needs a few unit tests.

[jlstevens]

Added some unit tests which are now passing. @philippjfr happy to merge?

[jlstevens]

@philippjfr Addressed your latest suggestions and the tests are green again. Merge?