Academic Search Application Python

# An academic search application written by Umut Bilgic..
# Version 1.0

import webbrowser
import sys
from tkinter import *
from tkinter.ttk import *

master = Tk()

search_var = IntVar()

def button_search_command():
    webbrowser.open("http://scholar.google.com.tr/scholar?hl=tr&q="+entry_search.get())
    webbrowser.open("www.ncbi.nlm.nih.gov/pubmed/?term="+entry_search.get())
    if search_var == 1:
        webbrowser.open("www.google.com/search?q="+entry_search.get())
    else:
        pass

def button_clear_command():
    entry_search.delete(0, END)

def button_quit_command():
    sys.exit("User hit 'Quit' button. ")

button_search = Button(master, text="Search", command = button_search_command)
button_search.grid(row = 0, column = 0)

button_clear = Button(master, text="Clear", command = button_clear_command)
button_clear.grid(row = 1, column = 0)

button_quit = Button(master, text="Quit", command = button_quit_command)
button_quit.grid(row = 1, column = 1)

entry_search = Entry(master)
entry_search.grid(row = 0, column = 1)

checkbox_google = Checkbutton(master, text="Search Google", variable = search_var)
checkbox_google.grid(row = 2, column = 0)

master.title("Search Tool")
mainloop()