from tkinter import *
from tkinter import ttk

root = Tk()
root.title("Distance conversion")

factors = { #all values are m / x
    "m" : 1,
    "cm" : 0.01,
    "mm" : 0.001,
    "km" : 1000,
    "au" : 1.496e11,
    "ly" : 9.461e15,
    "pc" : 3.086e16,
    "ft" : 0.3048,
    "yd" : 0.9144,
    "in" : 0.0254
}

def calculate(*args):
    try:
        value = float(entry.get())
        destUnit = 1/factors[destUnitString.get()]
        srcUnit = factors[srcUnitString.get()]
        factor = srcUnit*destUnit
        out.set(round(value*factor, 4))
    except ValueError:
        pass

bg = PhotoImage(file="M:\Pictures\hk stream\hornet siitt.png")
im = ttk.Label(root, image=bg)
im.grid(column=0,row=1)

mainframe = ttk.Frame(root, padding=(5,15,5,15))
mainframe.grid(column=0, row=0, sticky=(N,W,E,S))

entry = StringVar()
entryBox = ttk.Entry(mainframe, width=8, textvariable=entry)
entryBox.grid(column=2,row=1, sticky=(W,E))

out = StringVar()
ttk.Label(mainframe, textvariable=out).grid(column=2, row=2, sticky=(W,E))

calcButton = ttk.Button(mainframe, text="Convert", command=calculate)
calcButton.grid(column=3, row=3, sticky=W)

srcUnitString = StringVar()
srcUnitString.set("ft")
srcUnitEntry = ttk.Combobox(mainframe, textvariable=(srcUnitString), values=list(factors.keys()), state="readonly")
srcUnitEntry.grid(column=3, row=1, sticky=W)

ttk.Label(mainframe, text="is equivalent to").grid(column=1, row=2, sticky=E)

destUnitString = StringVar()
destUnitString.set("m")
destUnitEntry = ttk.Combobox(mainframe, textvariable=destUnitString, values=list(factors.keys()), state="readonly")
destUnitEntry.grid(column=3, row=2, sticky=W)

root.columnconfigure(0, weight=1)
root.rowconfigure(0, weight=1)	
mainframe.columnconfigure(2, weight=1)
for child in mainframe.winfo_children(): 
    child.grid_configure(padx=5, pady=5)
entryBox.focus()
root.bind("<Return>", lambda e: calcButton.invoke())

root.mainloop()