Python doesn't updated global list - RectangleSelector

Question

I am trying to draw a square Region of Interest on a processed TIFF image and to get coords of the rectangle.

To do this, I wrote this code :

click = [0,0]
release = [0,0]

def line_select_callback(eclick, erelease):  
    
    x1 = eclick.xdata
    y1 = eclick.ydata
    x2 = erelease.xdata
    y2 = erelease.ydata
    print(x1, x2, y1, y2)
    click.append(x1)
    a =len (click)
    print ("the lenght of click list is",a )
    #click[:] = x1, y1
    #release[:] = x2, y2
     
def toggle_selector(event):
   
    print(' Key pressed.')
    if event.key == 't':
        if toggle_selector.RS.active:
            print(' RectangleSelector deactivated.')
            toggle_selector.RS.set_active(False)
        else:
            print(' RectangleSelector activated.')
            toggle_selector.RS.set_active(True)


#fig = plt.figure('Image filtered and normalized')
fig, ax = plt.subplots()

#cursor = Cursor(ax, useblit=True, color='blue', linewidth=2 )
plt.imshow(im_filt_norm, cmap=plt.cm.rainbow)
figManager = plt.get_current_fig_manager()
figManager.window.showMaximized()
plt.title('Image filtered and normalized', fontsize=25)


# drawtype is 'box' or 'line' or 'none'
toggle_selector.RS = RectangleSelector(ax, line_select_callback,
                                       drawtype='box', useblit=True,
                                       button=[1, 3],  # disable middle button
                                       minspanx=5, minspany=5,
                                       spancoords='pixels',
                                       interactive=True, rectprops = dict(facecolor='black', edgecolor = 'black', alpha=0.8, fill=True))
fig.canvas.mpl_connect('key_press_event', toggle_selector)
plt.show()
plt.colorbar()

start_roi = np.array (click) 
b = len (start_roi)
print ("the lenght of start_roi is", b )

print ("this is start_ROI", start_roi)

Mainly I defined a global list called Click , made by two zeros, and I was expecting to be updated with the x1 appended to it. The code is not complete yet, since I am troubleshooting.

Using append gives a weird result : Spyder sees size = 2 , but opening it I found three columns , so it seems x1 was correctly appended. See pic below.

Click list with append

When I convert the list into array the size is again 2 , and it's made by two zeros, as defined out of line_select_callback.

using : click[:] = x1, y1 doesn't work too, giving a list made by two zeros, but actually if I open it in the IDE console I can see the actual values taken from rectangle selector.

See pic below :

Click list with click[:] = x1, y1

I feel a bit stuck, I also tried to define x1, x2, y1 and y2 as global variables just after def line_select_callback(eclick, erelease):

but also in this case I was not able to find them back out of the function.

Thanks in advance to anyone who wants to help,

Andrea