from tkinter import *
root = Tk()
def app(*args):
    m = montante.get()
    nome_c = variable.get()
    if nome_c == "Gasolina":
        p = 1.619
    elif nome_c == "Gasóleo 98":
        p = 1.516
    else:
        p = 1.419
    litros = m/p
    litrosarr = round(litros,2)
    resultado.set(litrosarr)
root.geometry("300x300")
root.resizable(0, 0)
root.config(bg="#d9ead3")
root.title("Bomba de Gasolina")
titulo = Label(text="Bomba de Gasolina",
               font=("Arial", "13", "bold"),bg="#d9ead3",fg="#bf9000")
titulo.place(relx=0.3, rely=0.05)
texto_sub1= Label(text="Preço a Meter:",
                    font=("Arial", "13", "bold"),bg="#d9ead3",fg="#bf9000")
texto_sub1.place(relx=0.08, rely=0.2)

texto_sub2= Label(text="Combustível:",
                    font=("Arial", "13", "bold"),bg="#d9ead3",fg="#bf9000")
texto_sub2.place(relx=0.09, rely=0.35)
montante = DoubleVar()
montante_entrada = Entry(textvariable=montante,
                             font=("Arial", "12", "bold"),
                             bg="white", fg="blue", justify='center')
montante_entrada.place(relx=0.55, rely=0.2, relwidth=0.25)


variable = StringVar(value='Gasolina')
lista = ["Gasolina", "Gasóleo 98", "Gasóleo 95"]
variable.set(lista[0])
variable.trace('w', app)
dropdown = OptionMenu(
    root,
    variable,
    *lista)
dropdown.place(relx=0.55,rely=0.35)
resultado = StringVar()
resultado_texto = Label(textvariable=resultado,
                           font=("Arial", 12, "bold"),bg="#d9ead3" )
resultado_texto.place(relx=0.5, rely=0.75, relwidth=0.3,relheight=0.08)
texto_sub3= Label(text="Litros Metidos:",
                    font=("Arial", "13", "bold"),bg="#d9ead3",fg="#bf9000")
texto_sub3.place(relx=0.15, rely=0.75)
root.mainloop()