It turns out there are two entirely different problems here. To show text over windows, you’ll need to create an undecorated topmost window and chroma key the background. However, this won’t work when there’s a full-screen application running (such as a game). The only reliable way to show text over a full-screen application is to use a Direct3D hook.

I haven’t written up a Direct3D hook example, but I’ll give two different solutions to the first problem.

Solution 1: Tkinter + pywin32

In this example, I do the majority of the work with Tkinter and use win32api to prevent the text from blocking mouse clicks. If win32api isn’t available to you, then you can just remove that part of the code.

import Tkinter, win32api, win32con, pywintypes

label = Tkinter.Label(text="Text on the screen", font=('Times New Roman','80'), fg='black', bg='white')
label.master.overrideredirect(True)
label.master.geometry("+250+250")
label.master.lift()
label.master.wm_attributes("-topmost", True)
label.master.wm_attributes("-disabled", True)
label.master.wm_attributes("-transparentcolor", "white")

hWindow = pywintypes.HANDLE(int(label.master.frame(), 16))
# http://msdn.microsoft.com/en-us/library/windows/desktop/ff700543(v=vs.85).aspx
# The WS_EX_TRANSPARENT flag makes events (like mouse clicks) fall through the window.
exStyle = win32con.WS_EX_COMPOSITED | win32con.WS_EX_LAYERED | win32con.WS_EX_NOACTIVATE | win32con.WS_EX_TOPMOST | win32con.WS_EX_TRANSPARENT
win32api.SetWindowLong(hWindow, win32con.GWL_EXSTYLE, exStyle)

label.pack()
label.mainloop()

Solution 2: pywin32

This example does everything through pywin32. This makes it more complicated and less portable, but considerably more powerful. I’ve included links to the relevant parts of the Windows API throughout the code.

import win32api, win32con, win32gui, win32ui

def main():
    hInstance = win32api.GetModuleHandle()
    className="MyWindowClassName"

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms633576(v=vs.85).aspx
    # win32gui does not support WNDCLASSEX.
    wndClass                = win32gui.WNDCLASS()
    # http://msdn.microsoft.com/en-us/library/windows/desktop/ff729176(v=vs.85).aspx
    wndClass.style          = win32con.CS_HREDRAW | win32con.CS_VREDRAW
    wndClass.lpfnWndProc    = wndProc
    wndClass.hInstance      = hInstance
    wndClass.hCursor        = win32gui.LoadCursor(None, win32con.IDC_ARROW)
    wndClass.hbrBackground  = win32gui.GetStockObject(win32con.WHITE_BRUSH)
    wndClass.lpszClassName  = className
    # win32gui does not support RegisterClassEx
    wndClassAtom = win32gui.RegisterClass(wndClass)

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ff700543(v=vs.85).aspx
    # Consider using: WS_EX_COMPOSITED, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TOPMOST, WS_EX_TRANSPARENT
    # The WS_EX_TRANSPARENT flag makes events (like mouse clicks) fall through the window.
    exStyle = win32con.WS_EX_COMPOSITED | win32con.WS_EX_LAYERED | win32con.WS_EX_NOACTIVATE | win32con.WS_EX_TOPMOST | win32con.WS_EX_TRANSPARENT

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms632600(v=vs.85).aspx
    # Consider using: WS_DISABLED, WS_POPUP, WS_VISIBLE
    style = win32con.WS_DISABLED | win32con.WS_POPUP | win32con.WS_VISIBLE

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms632680(v=vs.85).aspx
    hWindow = win32gui.CreateWindowEx(
        exStyle,
        wndClassAtom,
        None, # WindowName
        style,
        0, # x
        0, # y
        win32api.GetSystemMetrics(win32con.SM_CXSCREEN), # width
        win32api.GetSystemMetrics(win32con.SM_CYSCREEN), # height
        None, # hWndParent
        None, # hMenu
        hInstance,
        None # lpParam
    )

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms633540(v=vs.85).aspx
    win32gui.SetLayeredWindowAttributes(hWindow, 0x00ffffff, 255, win32con.LWA_COLORKEY | win32con.LWA_ALPHA)

    # http://msdn.microsoft.com/en-us/library/windows/desktop/dd145167(v=vs.85).aspx
    #win32gui.UpdateWindow(hWindow)

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms633545(v=vs.85).aspx
    win32gui.SetWindowPos(hWindow, win32con.HWND_TOPMOST, 0, 0, 0, 0,
        win32con.SWP_NOACTIVATE | win32con.SWP_NOMOVE | win32con.SWP_NOSIZE | win32con.SWP_SHOWWINDOW)

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms633548(v=vs.85).aspx
    #win32gui.ShowWindow(hWindow, win32con.SW_SHOW)

    win32gui.PumpMessages()

def wndProc(hWnd, message, wParam, lParam):
    if message == win32con.WM_PAINT:
        hdc, paintStruct = win32gui.BeginPaint(hWnd)

        dpiScale = win32ui.GetDeviceCaps(hdc, win32con.LOGPIXELSX) / 60.0
        fontSize = 80

        # http://msdn.microsoft.com/en-us/library/windows/desktop/dd145037(v=vs.85).aspx
        lf = win32gui.LOGFONT()
        lf.lfFaceName = "Times New Roman"
        lf.lfHeight = int(round(dpiScale * fontSize))
        #lf.lfWeight = 150
        # Use nonantialiased to remove the white edges around the text.
        # lf.lfQuality = win32con.NONANTIALIASED_QUALITY
        hf = win32gui.CreateFontIndirect(lf)
        win32gui.SelectObject(hdc, hf)

        rect = win32gui.GetClientRect(hWnd)
        # http://msdn.microsoft.com/en-us/library/windows/desktop/dd162498(v=vs.85).aspx
        win32gui.DrawText(
            hdc,
            'Text on the screen',
            -1,
            rect,
            win32con.DT_CENTER | win32con.DT_NOCLIP | win32con.DT_SINGLELINE | win32con.DT_VCENTER
        )
        win32gui.EndPaint(hWnd, paintStruct)
        return 0

    elif message == win32con.WM_DESTROY:
        print 'Closing the window.'
        win32gui.PostQuitMessage(0)
        return 0

    else:
        return win32gui.DefWindowProc(hWnd, message, wParam, lParam)


if __name__ == '__main__':
    main()