Try This One:
def ventanaPrincipal():
top = Tkinter.Tk()
var = Tkinter.StringVar()
L1 = Tkinter.Label(top, text="Introduzca el nombre"+"
"+ "completo del remitente:")
E1 = Tkinter.Entry(top,textvariable = var)
def buscar():
buscarPorNombreYApellidos(var.get())
B1 = Tkinter.Button(top, text ="Buscar", command = buscar)
L1.pack(side = Tkinter.LEFT)
B1.pack(side = Tkinter.RIGHT)
E1.pack( side = Tkinter.RIGHT)
top.mainloop()