import tkinter
import locale

locale.setlocale(locale.LC_NUMERIC, 'de_DE.utf8')


class TestDoubleVar():
    def __init__(self):
        root = tkinter.Tk()
        self.var = tkinter.DoubleVar()
        self.var.set(0.8)
        number = tkinter.Scale(
            root,
            from_=0, to=1, resolution=0.1,
            textvariable=self.var,
            command=self.update,
            width=4
        )
        number.pack(side=tkinter.LEFT)
        root.mainloop()

    def update(self, *args):
        print(float(self.var.get()))


if __name__ == '__main__':
    TestDoubleVar()