allow slider to snap to an array of values

ianhi · ianhi · commit c0c1768a0aa2 · 2020-11-07T22:47:47.000-05:00
diff --git a/doc/users/next_whats_new/slider_snap_to_array.rst b/doc/users/next_whats_new/slider_snap_to_array.rst
@@ -0,0 +1,6 @@
+Sliders can now snap to arbitrary values
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The `~matplotlib.widgets.Slider` UI widget now accepts arrays for *valstep*.
+This generalizes the previous behavior by allowing the slider to snap to
+arbitrary values.
diff --git a/lib/matplotlib/tests/test_widgets.py b/lib/matplotlib/tests/test_widgets.py
@@ -267,6 +267,17 @@ def test_slider_valmin_valmax():
     assert slider.val == slider.valmax
 
 
+def test_slider_valstep_snapping():
+    fig, ax = plt.subplots()
+    slider = widgets.Slider(ax=ax, label='', valmin=0.0, valmax=24.0,
+                            valinit=11.4, valstep=1)
+    assert slider.val == 11
+
+    slider = widgets.Slider(ax=ax, label='', valmin=0.0, valmax=24.0,
+                            valinit=11.4, valstep=[0, 1, 5.5, 19.7])
+    assert slider.val == 5.5
+
+
 def test_slider_horizontal_vertical():
     fig, ax = plt.subplots()
     slider = widgets.Slider(ax=ax, label='', valmin=0, valmax=24,
diff --git a/lib/matplotlib/widgets.py b/lib/matplotlib/widgets.py
@@ -11,7 +11,7 @@
 
 from contextlib import ExitStack
 import copy
-from numbers import Integral
+from numbers import Integral, Number
 
 import numpy as np
 
@@ -295,8 +295,9 @@ def __init__(self, ax, label, valmin, valmax, valinit=0.5, valfmt=None,
         dragging : bool, default: True
             If True the slider can be dragged by the mouse.
 
-        valstep : float, default: None
-            If given, the slider will snap to multiples of *valstep*.
+        valstep : float or arraylike, default: None
+            If a float, the slider will snap to multiples of *valstep*.
+            If an array the slider will snap to the values in the array.
 
         orientation : {'horizontal', 'vertical'}, default: 'horizontal'
             The orientation of the slider.
@@ -401,9 +402,16 @@ def observers(self):
 
     def _value_in_bounds(self, val):
         """Makes sure *val* is with given bounds."""
-        if self.valstep:
+        if isinstance(self.valstep, Number):
             val = (self.valmin
                    + round((val - self.valmin) / self.valstep) * self.valstep)
+        elif self.valstep is not None:
+            valstep = np.asanyarray(self.valstep)
+            if valstep.ndim != 1:
+                raise ValueError(
+                    f"valstep must have 1 dimension but has {valstep.ndim}"
+                )
+            val = valstep[np.argmin(np.abs(valstep - val))]
 
         if val <= self.valmin:
             if not self.closedmin:
