Dynamically adjusting floating point spinbox increment in tkinter

I want to be able to dynamically adjust the increment for my floating point numerical spinbox (spin1) below. To do so, I've created a second spin box (spin2) that has values (0.1, 0.01, 0.001, 0.0001), which determine the step size of spin1.

This works fine as long as I always move from coarser to finer increments. It fails when I attempt to move from a finer to a coarser increment because the value gets truncated to the precision of the increment.

Using the code below, here is how the problem can be reproduced:

1. Move spin1 to 0.2:

   

2. Increment spin2, say to 0.01:

   

3. Increment spin1 to 0.21:

   

4. Decrement spin2 back to 0.1:

   

5. Decrement spin1:

   

I would expect to see the value 0.11 in spin1, but instead I see 0.1. Why is this happening, and how can I fix it?

from tkinter import Tk, Spinbox

def update():
    spin1['increment'] = float(spin2.get())

window = Tk()
spin1 = Spinbox(window, from_=0.0, to=10.0, increment=0.1)
spin2 = Spinbox(window, values=(0.1, 0.01, 0.001, 0.0001), command=update)
spin1.pack()
spin2.pack()
window.mainloop()

Additional observation:

• The spinner always truncates values rather than rounding. So if spin1 were set to 2.6 in step 3, the final result would still be 0.1, not 0.2.

You can add format="%.4f" (based on the maximum decimal points of 0.0001 from spin2) to spin1:

spin1 = Spinbox(window, from_=0.0, to=10.0, increment=0.1, format="%.4f")