[Python] Matplotlib Slider: function set_val does not work following standard prescriptions?

I am a beginner with the Slider tool and retrieved a code which used to work but does not anymore. I found a similar code here to test the Slider tool in this answer which provides a self-contained example:

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.widgets import Slider

class ChangingPlot(object):
def __init__(self):
self.inc = 0.5

self.fig, self.ax = plt.subplots()
self.sliderax = self.fig.add_axes([0.2, 0.02, 0.6, 0.03],
facecolor='yellow')

self.slider = DiscreteSlider(self.sliderax, 'Value', 0, 10,
increment=self.inc, valinit=self.inc)
self.slider.on_changed(self.update)

x = np.arange(0, 10.5, self.inc)
self.ax.plot(x, x, 'ro')
self.dot, = self.ax.plot(self.inc, self.inc, 'bo', markersize=18)

def update(self, value):
self.dot.set_data([[value],[value]])
self.fig.canvas.draw()

def show(self):
plt.show()

class DiscreteSlider(Slider):
"""A matplotlib slider widget with discrete steps."""
def __init__(self, *args, **kwargs):
"""Identical to Slider.__init__, except for the "increment" kwarg.
"increment" specifies the step size that the slider will be discritized
to."""
self.inc = kwargs.pop('increment', 0.5)
Slider.__init__(self, *args, **kwargs)
self.val = 1

def set_val(self, val):
discrete_val = int(val / self.inc) * self.inc
# We can't just call Slider.set_val(self, discrete_val), because this
# will prevent the slider from updating properly (it will get stuck at
# the first step and not "slide"). Instead, we'll keep track of the
# the continuous value as self.val and pass in the discrete value to
# everything else.
xy = self.poly.xy
xy[2] = discrete_val, 1
xy[3] = discrete_val, 0
self.poly.xy = xy
self.valtext.set_text(self.valfmt % discrete_val)
if self.drawon:
self.ax.figure.canvas.draw()
self.val = val
if not self.eventson:
return
for cid, func in self.observers.items():
func(discrete_val)


p = ChangingPlot()
p.show()


If I copy and paste the example given in this other page in my Python 3.12.9/Matplotlib 3.10.1 environment, I get a failure :

Cell In[9], line 47, in DiscreteSlider.set_val(self, val)


45 xy = self.poly.xy
46 print(type(self.poly.xy),len(xy),xy)
---> 47 xy[2] = discrete_val, 1
48 xy[3] = discrete_val, 0
49 self.poly.xy = xy




TypeError: 'tuple' object does not support item assignment
​

The print command returns :

<class 'tuple'> 2 (0, 0.25)
​

I understand that tuples cannot be modified but this was supposed to work before ? If I test xy length and content, it returns only 2 values while here it addresses the tuple beyond xy[1].

What has become wrong ? The few examples I found on internet all still use the above syntax.

I could convert xy to a list, but I still have the problem that xy length is 2 not 4 and these are simple values, not tuples. So I don't understand the structure of self.poly.xy and I don't understand why the standard example does not work, so I cannot play with it to understand its use...

Thanks for any help

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