Python 3.4 tkinter real-time temperature converter

Question

I wrote complicated program in Python 3.4 (with tkinter gui). It's temperature converter (Celsius to Fahrenheit and reverse) in real-time. It works almost good, but there is one problem. I have to add a space after input value every time. Anyone have idea what's wrong in this program?

from tkinter import *

def cel_na_fahr(event):
    e = ".0"
    a = float(ent1.get())
    a = round((32+9/5*a),2)
    a = str(a)
    if a.endswith(e):
        a=a.replace(".0","")
        ent2.delete(0,END)
        ent2.insert(0,a+event.char)
    else:
        ent2.delete(0,END)
        ent2.insert(0,a+event.char)

def fahr_na_cel(event):
    e = ".0"
    a = float(ent2.get())
    a = round(5/9*(a-32),2)
    a = str(a)
    if a.endswith(e):
        a=a.replace(".0","")
        ent1.delete(0,END)
        ent1.insert(0,a+event.char)
    else:
        ent1.delete(0,END)
        ent1.insert(0,a+event.char)

root = Tk()

root.geometry("300x180+400+400")

fr1 = Frame(root, padx=5, pady=40)
fr1.pack(side=TOP)

fr2 = Frame(root)
fr2.pack(side=TOP)

lbl1 = Label(fr1, text="cel to fahr   ")
lbl1.pack(side=LEFT)

ent1 = Entry(fr1) 
ent1.pack(side=RIGHT)

lbl2 = Label(fr2, text="fahr to cel   ")
lbl2.pack(side=LEFT)

ent2 = Entry(fr2) 
ent2.pack(side=RIGHT)

ent1.bind('<Key>', cel_na_fahr)
ent2.bind('<Key>', fahr_na_cel) 

root.mainloop()

Answer 1

You have to type a space because when the <Key> callback triggers, the key that the user most recently pressed hasn't yet been added to the entry. This is probably what you're trying to compensate for by adding event.char, although you're doing it in the wrong place anyway.

Change your bindings to KeyRelease, so that the callbacks trigger after the entry is updated, and remove the +event.char stuff, as you don't need it any more.

from tkinter import *

def cel_na_fahr(event):
    print(ent1.get())
    e = ".0"
    a = float(ent1.get())
    a = round((32+9/5*a),2)
    a = str(a)
    if a.endswith(e):
        a=a.replace(".0","")
        ent2.delete(0,END)
        ent2.insert(0,a)
    else:
        ent2.delete(0,END)
        ent2.insert(0,a)

def fahr_na_cel(event):
    print(ent2.get())
    e = ".0"
    a = float(ent2.get())
    a = round(5/9*(a-32),2)
    a = str(a)
    if a.endswith(e):
        a=a.replace(".0","")
        ent1.delete(0,END)
        ent1.insert(0,a)
    else:
        ent1.delete(0,END)
        ent1.insert(0,a)

root = Tk()

root.geometry("300x180+400+400")

fr1 = Frame(root, padx=5, pady=40)
fr1.pack(side=TOP)

fr2 = Frame(root)
fr2.pack(side=TOP)

lbl1 = Label(fr1, text="cel to fahr   ")
lbl1.pack(side=LEFT)

ent1 = Entry(fr1) 
ent1.pack(side=RIGHT)

lbl2 = Label(fr2, text="fahr to cel   ")
lbl2.pack(side=LEFT)

ent2 = Entry(fr2) 
ent2.pack(side=RIGHT)

ent1.bind('<KeyRelease>', cel_na_fahr)
ent2.bind('<KeyRelease>', fahr_na_cel) 

root.mainloop()