How to make a window fullscreen in a secondary display with tkinter?

Try:

from Tkinter import *

rot = Tk()


wth,hgh = rot.winfo_screenwidth(),rot.winfo_screenheight()
#take desktop width and hight (pixel)
_w,_h = 800,600 #root width and hight
a,b = (wth-_w)/2,(hgh-_h)/2 #Put root to center of display(Margin_left,Margin_top)



def spann():
    def _exit():
        da.destroy()

    da = Toplevel()
    da.geometry('%dx%d+%d+%d' % (wth, hgh,0, 0))

    Button(da,text="Exit",command=_exit).pack()
    da.overrideredirect(1)
    da.focus_set()#Restricted access main menu




Button(rot,text="Exit",command=lambda rot=rot : rot.destroy()).pack()


but = Button(rot,text="Show SUB",command=spann)
but.pack()


rot.geometry('%sx%s+%s+%s'%(_w,_h,a,b))
rot.mainloop()
""" Geometry pattern 'WxH+a+b'
        W = Width
        H = Height
        a = Margin_left+Margin_Top"""

This works on Windows 7: If the second screen width and height are the same as the first one, you can use win1 or win2 geometry of the following code depending its relative position(leftof or rightof) to have a fullscreen in a secondary display:

from Tkinter import *

def create_win():
    def close(): win1.destroy();win2.destroy()
    win1 = Toplevel()
    win1.geometry('%dx%d%+d+%d'%(sw,sh,-sw,0))
    Button(win1,text="Exit1",command=close).pack()
    win2 = Toplevel()
    win2.geometry('%dx%d%+d+%d'%(sw,sh,sw,0))
    Button(win2,text="Exit2",command=close).pack()

root=Tk()
sw,sh = root.winfo_screenwidth(),root.winfo_screenheight()
print "screen1:",sw,sh
w,h = 800,600 
a,b = (sw-w)/2,(sh-h)/2 

Button(root,text="Exit",command=lambda r=root:r.destroy()).pack()
Button(root,text="Create win2",command=create_win).pack()

root.geometry('%sx%s+%s+%s'%(w,h,a,b))
root.mainloop()