More generic value snapping for Slider widgets #18569

ianhi · 2020-09-25T04:49:58Z

PR Summary

Adds a valsnap argument to the Slider widget to allow specifying the values the slider should snap to. This overrides the valstep argument.

I also made a new slider demo to show the use of valstep and valsnap as I think they would get lost in the current slider demo. For a point of context I have read through the existing tutorial multiple times, but never managed to encode that valstep existed until I started working on this PR

I used searchsorted instead of argmin as it is significantly faster.
This works for idx=0 because then the indexing just becomes -1 and we compare the largest and smallest elements of the array.

PR Checklist

I have a few questions here:

Documentation:
a. There isn't any narrative docs on widgets to add this to?
b. Does this deserve it's own example (perhaps named slider valstep and valsnap demo) it feel weird to cram into the current
naming. I chose valsnap because it was consistent with the other kwargs but if there are better ideas I'm happy to change it.
Should this get both a whats_new and a next_api_changes?
To get the number for next_api_changes is the best strategy to look through both open and closed PRs right before opening this?
- Or I guess do what I've done here and wait until after opening the PR

Has pytest style unit tests (and pytest passes).
Is Flake 8 compliant (run flake8 on changed files to check).
New features are documented, with examples if plot related.
Documentation is sphinx and numpydoc compliant (the docs should build without error).
Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
[N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

tacaswell · 2020-09-25T05:11:46Z

lib/matplotlib/widgets.py

@@ -330,6 +334,11 @@ def __init__(self, ax, label, valmin, valmax, valinit=0.5, valfmt=None,
        self.valmin = valmin
        self.valmax = valmax
        self.valstep = valstep
+        self.valsnap = valsnap
+        if self.valsnap is not None:


This isn't a great pattern because now if valsnap is None, we silently drop the user's input for valstep on the floor. This is the sort of thing that causes the greatest anger at libraries, you change the thing AND NOTHING HAPPENS.

We should find a way to either fold valstep and valsnap into one parameter or raise if both are passed.

Folding them into one parameter seems ok (there is a bit of ambiguity else where about plural) and an the code shows, you can generate the snap location from a step. In other cases where we have colliding kwargs we raise (e.g. norm vs density in hist).

Ah indeed. Are you expressing a preference for a single kwarg here? I'm 55/45 towards two kwargs (appropriately ordered) and raising an error if both are passed. I think there is also an opportunity for user anger when they discover that a feature exists but was not very discoverable.

I just pushed a change so it is two kwargs and raises an error if both are given. If need be I'm happy to move to a single valstep kwarg

You can just do this above, before the assignment, as if valstep is not None and valsnap is not None:...

I moved the ValueError up to be with the other ValueErrors

tacaswell · 2020-09-25T05:13:31Z

lib/matplotlib/widgets.py

@@ -255,7 +255,7 @@ class Slider(AxesWidget):
    def __init__(self, ax, label, valmin, valmax, valinit=0.5, valfmt=None,
                 closedmin=True, closedmax=True, slidermin=None,
                 slidermax=None, dragging=True, valstep=None,
-                 orientation='horizontal', **kwargs):
+                 valsnap=None, orientation='horizontal', **kwargs):


If you are adding a new argument it has to go at the end (or better yet be keyword only!). This in an API break as if there is someone (admittedly making a poor coding choice) of passing in 13 positional arguments, valsnap would get the string intended for orientation and likely blow up.

Yikes. But yes ok, I suppose having millions of users nearly guarantees that that is happening somewhere.

tacaswell

👍 in principle, but the API needs some tweaking.

tacaswell · 2020-09-25T05:16:03Z

Just needs a whats_new (assuming we un-break the signature change) as no existing code should be broken by it.
if you are changing the API odds are close to one it will take atleast 1 round of review so open the PR, get number, then add file
adding narrative docs about widgets would be great (but maybe in a different PR)

ianhi · 2020-09-25T21:47:47Z

While I'm at it would it be ok to add a kwarg that would allow for eliminating the red line that marks the initial value?

Something like markinit as a boolean. It could also be nice to be able to change the color of that line so another approach could be to have init_mark_color that accepts a color or None and if None then the line is not drawn?

The color and whether is is drawn is currently hardcoded here:

matplotlib/lib/matplotlib/widgets.py

Lines 338 to 343 in 5173cda

    
           if orientation == 'vertical': 
        
               self.poly = ax.axhspan(valmin, valinit, 0, 1, **kwargs) 
        
               self.hline = ax.axhline(valinit, 0, 1, color='r', lw=1) 
        
           else: 
        
               self.poly = ax.axvspan(valmin, valinit, 0, 1, **kwargs) 
        
               self.vline = ax.axvline(valinit, 0, 1, color='r', lw=1)

ianhi · 2020-10-23T03:57:56Z

I also added another kwarg initcolor that can be use to specify the previous hardcoded color of the line marking the initial slider position. It also accepts None to not draw the line.

lib/matplotlib/widgets.py

ianhi · 2020-10-31T21:57:34Z

To keep things more manageable, I propose to defer changing set_val() to a separate PR.

👍 I dropped that commit. Otherwise I can't think of anything else to change here.

timhoffm

Only minor style comments.

examples/widgets/slider_snap_demo.py

lib/matplotlib/widgets.py

ianhi · 2020-11-01T00:13:40Z

I also added changing the color of the bar for one of the sliders to increase the discoverability of that option.

ianhi · 2020-11-01T00:18:38Z

Does this need another review from @tacaswell given their prior changes requested?

timhoffm · 2020-11-01T00:34:33Z

Every code PR needs 2 positive reviews. Ideally, @tacaswell would do the second review because he has requested changes. But if he doesn't have time any other reviewer is fine. I take the authority to clear the change request if needed.

examples/widgets/slider_snap_demo.py

QuLogic · 2020-11-07T08:13:29Z

lib/matplotlib/widgets.py

+        initcolor : color or None, default: 'r'
+            The color of the line at the *valinit* position. If None then
+            no line will be drawn.


Usually we use None to mean default. We also already have 'none' to mean no colour; do we really need to support None here?

I find using None to mean "don't draw" and colors otherwise to be the most intuitive api. I'd also worry about the discoverability of using 'none' for example: I'm an advanced user but did not know that you could specify 'none' as a color.

Usually we use None to mean default

I made the default 'r' as that's what was hardcoded, but I think it would make more sense for None to be the default and specifiying a color otherwise. I don't really see the usecase for drawing a line at the starting position, but I guess this edges into personal preference rather than logic and additionally is maybe backwards incompatible. What about switching the default to None?

None means "default" unless we really can't help it. I you want the default to be "none" I guess that could work for you....

Gotcha - thanks. It probably is the least friction to as 'r' and not accept None so I'll do that + leave a note in the docstring to use 'none' if you don't want it to be visible.

lib/matplotlib/widgets.py

ianhi · 2020-11-08T04:03:20Z

I add a reference to the new example - and while doing so added the references to the other widget examples except for menu.py which doesn't actually use any widgets and i was less sure what to reference.

plus lint snapping example

API break is fixed.

tacaswell reviewed Sep 25, 2020

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tacaswell previously requested changes Sep 25, 2020

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tacaswell added this to the v3.4.0 milestone Sep 25, 2020

ianhi force-pushed the slider-snap-values branch 2 times, most recently from 7334e31 to 1055bce Compare September 25, 2020 21:45

QuLogic added topic: widgets/UI New feature labels Oct 1, 2020

ianhi force-pushed the slider-snap-values branch from 1055bce to 51feb94 Compare October 23, 2020 03:33

timhoffm reviewed Oct 30, 2020

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ianhi force-pushed the slider-snap-values branch 2 times, most recently from 24fdcd4 to 70ff4cd Compare October 30, 2020 19:42

timhoffm reviewed Oct 31, 2020

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ianhi force-pushed the slider-snap-values branch 2 times, most recently from ade813e to fc4acd6 Compare October 31, 2020 21:56

timhoffm approved these changes Oct 31, 2020

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ianhi force-pushed the slider-snap-values branch from fc4acd6 to 41bdc05 Compare November 1, 2020 00:11

ianhi force-pushed the slider-snap-values branch from 41bdc05 to caa3f42 Compare November 1, 2020 00:14

QuLogic reviewed Nov 7, 2020

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ianhi added 3 commits November 7, 2020 22:47

allow slider to snap to an array of values

c0c1768

Create example of slider snapping to values

ee8f9d0

allow not drawing the line marking initial slider position

0baa16b

ianhi force-pushed the slider-snap-values branch from caa3f42 to 0baa16b Compare November 8, 2020 03:48

add references to widget examples

bafcca9

plus lint snapping example

ianhi force-pushed the slider-snap-values branch from 18b6d1d to bafcca9 Compare November 12, 2020 03:17

QuLogic approved these changes Dec 3, 2020

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QuLogic merged commit c1861b0 into matplotlib:master Dec 3, 2020

ianhi deleted the slider-snap-values branch December 3, 2020 15:35

ianhi mentioned this pull request Apr 27, 2021

Passing an array to valstep in Slider throws a ValueError. #20089

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More generic value snapping for Slider widgets #18569

More generic value snapping for Slider widgets #18569

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ianhi commented Nov 8, 2020

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ianhi commented Sep 25, 2020 •

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