use checkboxes to update a counter

I have some labels in a frame with checkboxes and I would like to update a counter each time a checkbox is ticked (or unticked). So far I failed to implement it... I would like the output to appear in the text: number of flag1/flag2 selected (/10): with the overal number corresponding to how many flag1/flag2 boxes have been ticked.

I'd like the update to be triggered as soon as a box is checked.

I am adding the code below. Any help would be greatly appreciated! Regards, Pierre.

import tkinter as tk
import webbrowser


class Pierre:
    def __init__(self, master, urls, chk_lbl):
        self.urls = urls
        self.chk_lbl = chk_lbl
        self.counter = 0
        self.vars = []

        i=0
        for url in self.urls:
            lbl = tk.Label(master, text=url, fg="blue", cursor="hand2")
            lbl.grid(row=i, column=1)
            lbl.bind("<Button-1>", self.callback)


            ic = 2
            for lbl in self.chk_lbl:
                var = tk.IntVar()
                chk = tk.Checkbutton(master, text=lbl, variable=var)
                chk.grid(row=i, column=ic)
                self.vars.append(var)
                ic += 1
            i+=1

    def state(self):
        return map((lambda var: var.get()), self.vars)

    def callback(self, event):
        webbrowser.open_new(self.url)

class TestClass(tk.Tk):
    def __init__(self, **kwargs):
        root = tk.Tk.__init__(self, **kwargs)
        self.title('Test')

        # --- Changes starts from here --- #
        def onFrameConfigure(canvas):
            canvas.configure(scrollregion=canvas.bbox('all'))

        self.topframe = tk.Frame(root)
        self.topframe.pack(side=tk.TOP, pady=30)

        self.canvas = tk.Canvas(root)
        self.bottomframe = tk.Frame(self.canvas)
        # self.bottomframe.pack( side = tk.BOTTOM )
        self.scrollbar = tk.Scrollbar(root, orient='vertical', command=self.canvas.yview)
        self.canvas.configure(yscrollcommand=self.scrollbar.set)
        self.canvas.pack(side=tk.LEFT, fill='y')
        self.scrollbar.pack(side=tk.LEFT, fill='y')
        self.canvas.create_window(0, 0, window=self.bottomframe, anchor='nw')
        self.bottomframe.bind('<Configure>', lambda event, canvas=self.canvas: onFrameConfigure(self.canvas))
        # --- Changes ends here --- #

        self.button = tk.Button(self.topframe, text='Click', command=self.output_value)
        self.button.pack(side="left", fill="both", expand=True)

        iai = 1 # hardcoded value. should be set to the number of time flag1 has been ticked over all labels
        ihf = 2 # hardcoded value. should be set to the number of time flag2 has been ticked over all labels

        tk.Label(self.topframe, text='number of flag1 selected (/10):%i' % iai).pack() 
        tk.Label(self.topframe, text='number of flag2 selected (/10):%i' % ihf).pack() 


    def output_value(self):
        urls = ["http://www.google.com", "http://www.facebook.com", "http://bbc.co.uk"]
        chk_lbl = ['flag1', 'flag2']
        Pierre(self.bottomframe, urls, chk_lbl)

if __name__ == "__main__":
    root = TestClass()
    root.mainloop()