[matpltlib]Basic question about callback function

[schniefen]

I'm learning about using the slider and button widgets and am studying the example given in the link.

Slider Demo

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.widgets import Slider, Button, RadioButtons

fig, ax = plt.subplots()
plt.subplots_adjust(left=0.25, bottom=0.25)
t = np.arange(0.0, 1.0, 0.001)
a0 = 5
f0 = 3
delta_f = 5.0
s = a0 * np.sin(2 * np.pi * f0 * t)
l, = plt.plot(t, s, lw=2)
plt.axis([0, 1, -10, 10])

axcolor = 'lightgoldenrodyellow'
axfreq = plt.axes([0.25, 0.1, 0.65, 0.03], facecolor=axcolor)
axamp = plt.axes([0.25, 0.15, 0.65, 0.03], facecolor=axcolor)

sfreq = Slider(axfreq, 'Freq', 0.1, 30.0, valinit=f0, valstep=delta_f)
samp = Slider(axamp, 'Amp', 0.1, 10.0, valinit=a0)


def update(val):
    amp = samp.val
    freq = sfreq.val
    l.set_ydata(amp*np.sin(2*np.pi*freq*t))
    fig.canvas.draw_idle()


sfreq.on_changed(update)
samp.on_changed(update)

resetax = plt.axes([0.8, 0.025, 0.1, 0.04])
button = Button(resetax, 'Reset', color=axcolor, hovercolor='0.975')


def reset(event):
    sfreq.reset()
    samp.reset()
button.on_clicked(reset)

rax = plt.axes([0.025, 0.5, 0.15, 0.15], facecolor=axcolor)
radio = RadioButtons(rax, ('red', 'blue', 'green'), active=0)


def colorfunc(label):
    l.set_color(label)
    fig.canvas.draw_idle()
radio.on_clicked(colorfunc)

plt.show()

I don't understand the parameters of the two first functions in the example nor the fact that they don't have any return statements. It seems like the parameters aren't used in the function, yet they're needed. I'd appreciate a clarification on what is going on. Thanks!

[Yoriz]

A callback function from a slider that's connected using on_changed must except a int which is the value that represents the slider position.

on_changed(self, func)
When the slider value is changed call func with the new slider value

Parameters:
func : callable
Function to call when slider is changed. The function must accept a single float as its arguments.

Returns:
cid : int
Connection id (which can be used to disconnect func)

The parameter val is not used in callback function update because both sliders are using the same callback, and then obtaining the value from both sliders using

    amp = samp.val
    freq = sfreq.val

A callback function from a button that's connected using on_clicked must except a connection id which can be used to disconnect the callback.

on_clicked(self, func)
Connect the callback function func to button click events.

Returns a connection id, which can be used to disconnect the callback.

The reset callback is not using the connection id to disconnect the callback because the button is required to stay active for future reset events.

Callback functions don't have return values because there is no where in the code for them to return to, in gui progams an event loop redraws the gui and waits for an event to happen like a button click, it then calls the connected event function to do whats required, and then redraws and waits for another event.

[schniefen]

So val gets assigned a value through the on_clicked function? How could I access the integer on_changed oron_clicked returns?

[Yoriz]

When the slider is altered it triggers an event that calls the connected event handler with the val of the slider,
that is why the event handler function must except the val argument.
To access val in the event handler function just use the val attribute like shown below.

sfreq = Slider(axfreq, 'Freq', 0.1, 30.0, valinit=f0, valstep=delta_f)
sfreq.on_changed(update)
 
def update(val):
    print(val)