Add an apply method to apply functions to elements

[philippjfr]

This PR implements the outcome of our API discussion to provide a more convenient API than hv.util.Dynamic. The main changes are twofold:

• .apply now defaults to ViewableElement for its spec keyword, which means it behaves exactly like an Operation
• Added support for supplying streams and keywords to .apply, which make it behave like hv.util.Dynamic, i.e. it provides support for chaining dynamic operations, but also making existing objects become dynamic if there is a stream or parameter dependency declared.

• Supercedes Apply method/operation for chaining dynamic operations #3473
• Add support for dynamic parameters
• Add unit tests

[philippjfr]

This is now working as I would like it to, the main problem now is that I've had to modify the default behavior slightly to get what we want since simply setting the specs to ViewableElement isn't quite the semantics we need. The difference between .map and the broadcasting that an operation does is that an operation stops early. For example when you have an Overlay of Curves the operation will stop when it encounters the Overlay, while map will apply to the Overlay and then continue traversing down to each Curve. I now have special handling for that when specs='default', but I wonder if I should add a flag that allows getting the early stopping behavior without special handling for specs='default'.

[jlstevens]

I dunno if this PR is ready yet but one thing I would like to see before merging is a concrete example of how to use map to effectively chain operations (with a screenshot).

Alternatively (or probably more sensibly) some examples in the documentation.

[philippjfr]

Here is a little example of .apply in action:

from scipy import ndimage

def image(freq=0):
    xs, ys = np.mgrid[-1:1:0.05, -1:1:0.05]*freq
    return hv.Image(np.sin(xs)*np.cos(ys))
    
dmap = hv.DynamicMap(image, kdims=('freq', 'Frequency')).redim.range(freq=(1, 10))

# Define an operation which applies gaussian filtering
def filter(obj, sigma, mode):
    return obj.clone(ndimage.gaussian_filter(obj.data, sigma, mode=mode))

# Define panel widgets which will control the function sigma and mode arguments
sigma = pn.widgets.FloatSlider(name='Sigma', start=1, end=10)
mode = pn.widgets.Select(name='Mode', value='mirror', options=['reflect', 'constant', 'mirror', 'nearest', 'wrap'])

# Map widget parameters to function arguments
filtered = dmap.apply(filter, sigma=sigma.param.value, mode=mode.param.value)

# Make a panel layout
hv_panel = pn.panel(filtered)
pn.Row(hv_panel[0], pn.Column(hv_panel[1], sigma, mode))

and here is a little example using param instances in operations:

import colorcet as ct

class Example(param.Parameterized):
    
    cmap = param.Selector(default=ct.bgy, objects={'bgy': ct.bgy, 'rainbow': ct.rainbow})

example = Example()
    
points = hv.Points(np.random.randn(10000000, 2))

pn.Row(datashade(points, cmap=example.param.cmap), example.param)

[philippjfr]

Tests are failing because the unpickling doesn't seem to be working:

AttributeError: 'Box' object has no attribute '_instance__params'

I tested it locally without issues, so don't quite know what's up with that.

[jlstevens]

I'm very happy with that API and I reviewed the code earlier. Happy to merge when the tests pass.

[jbednar]

That all looks great, though I'm not sure this is really a map function; seems more like apply, in that it accepts arguments and calls the function with those arguments. It's not purely apply because there is an extra argument snuck in there that in this case consists of the current value of the dmap. If called on a HoloMap that extra argument works out to it being worthy of the name map, in that the function then gets applied to every item in the container, as map would indicate. But if called on a DynamicMap its more like a dynamic apply call than a map, and if called on an element it's more like a regular apply call (just with this extra implicit argument). So I guess it can work both ways; up to @philippjfr.

[philippjfr]

Okay I'm actually much happier with this, .apply is now much simpler than it was when I was forcing the functionality into .map and it doesn't let you do as many stupid things. It also now neatly replaces the code that broadcasts an operation across a nested container, which eliminates some code there.

I'll work on a few more tests but it's working again, now with the .apply API.

[jbednar]

Does the example above still work, after replacing .map with .apply?

[philippjfr]

Does the example above still work, after replacing .map with .apply?

I'll push another fix shortly but yes, that should work.

[philippjfr]

I've just pushed a few fixes and a bunch more tests, so this should be ready to merge now.

[philippjfr]

Let's ignore the Windows failure for now, it's some regression in the latest dask 1.1.3 release which I'll have to look into independently.

[jlstevens]

The suggestion for updating .map is now mentioned in #3544. As the tests were passing earlier (except for Windows), I'll go ahead and merge.

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