Termometro Tkinter

termometro.py

from tkinter import *
from tkinter import ttk

class Termometro(Tk):

    def __init__(self):
        Tk.__init__(self)
        self.geometry("210x350")
        self.title("Termometro")
        self.configure(bg="SystemButtonFace")
        self.resizable(0,0)

        self.old_temp = "0.0"
        self.temp_string = StringVar(value="0.0")
        self.temp_string.trace("w", self.validate_temp)
        self.temp_unit = StringVar(value="C")

        self.style = ttk.Style()
        self.style.configure("new.TRadiobutton", background="SystemButtonFace")
        self.style.configure("new.TLabel", background="SystemButtonFace")

        self.make_layout()

    def make_layout(self):
        self.temp_input = ttk.Entry(self, textvariable=self.temp_string).place(x=10, y=10)

        self.lb_unidad = ttk.Label(self, text="Grados:", style="new.TLabel").place(x=10, y=40)
        self.rb_c = ttk.Radiobutton(self, text="Celsius", variable=self.temp_unit, value="C", style="new.TRadiobutton", command=self.select).place(x=20, y=60)
        self.rb_f = ttk.Radiobutton(self, text="Fahrenheit", variable=self.temp_unit, value="F", style="new.TRadiobutton", command=self.select).place(x=20, y=80)

    def select(self):
        try:
            degree = float(self.temp_string.get())
            unit = self.temp_unit.get()

            if unit == "C":
                out = (degree - 32)*5/9
            else:
                out = degree*9/5 + 32
            self.temp_string.set(out)
            self.old_tmp = out
        except:
            pass

    def validate_temp(self, *args):
        new_tmp = self.temp_string.get()
        try:
            float(new_tmp)
            self.old_temp = new_tmp
        except:
            if new_tmp:
                self.temp_string.set(self.old_temp)

app = Termometro()
app.mainloop()