How to set on/off value with checkbutton in Tk/Tcl

Question

I am currently learning about the usage of widgets in the tkinter package. And uncertain as to how the on/off value works in the checkbutton widget.

My written code is as follows:

from tkinter import *
from tkinter import ttk

root = Tk()
root.title("Checkbutton")

mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=0, row=0, sticky=(N, W, E, S))
mainframe.columnconfigure(0, weight=1)
mainframe.rowconfigure(0, weight=1)

measureSystem = StringVar()
cmd1 = lambda *args: print(str(measureSystem) + ' selected')
check = ttk.Checkbutton(mainframe, text='Use Metric',
                        command=cmd1, variable=measureSystem,
                        onvalue='metric', offvalue='imperial')

However when I do check and uncheck the checkbutton, what is returned is the following, regardless of the state the checkbutton is in:

PY_VAR1 selected

I understand that the onvalue and offvalue is supposed to be stored in the measureSystem variable depending on the state of the checkbutton.

Why is this not the case here?

Answer 1

Try changing:

str(measureSystem)

to:

str(measureSystem.get())

as StringVar uses .set() and .get() to access the value.