How to detect that a key is being pressed, not HAS been pressed earlier!??

Discussion in 'Python' started by Rune, Jan 28, 2004.

  1. Rune

    Rune Guest


    I'm trying to build a gui application and need to know if the user is
    actually holding down the shift or ctrl key. That is, if the user
    currently is holding down the shift key. In pseudo code this will boil
    down to something like this:

    def test:
    if user presses shift:
    elif user presses ctrl:
    return CTRL_IS_PRESSED
    return false

    It's important to notice here that I'm not interested if the user has
    already pressed shift or ctrl. I'm only interested in knowing if he is
    currently holding down one of these keys. (I have looked into msvcrt
    and the like but have found no answer..) The function should also work
    in both windows and Linux.

    Any help is appriciated :)
    Rune, Jan 28, 2004
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  2. You should have a look at the wxPython demo (wxKeyEvents section) that shows
    how this is handled in a cross platform fashion.

    Search for wxPython from
    Gilles Lenfant, Jan 28, 2004
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  3. Rune

    Jarek Zgoda Guest

    This event *is* available in WinAPI, but don't ask me where. I'm sure,
    because VCL (which is built on top of WinAPI and uses standard Windows
    messages) distinguishes KeyUp and KeyDown events.

    If you want to have it available also in linux, use Qt/PyQt.
    Jarek Zgoda, Jan 28, 2004
  4. Rune

    Jeff Epler Guest

    In Tk, each event has a 'state' field, which can tell you whether
    modifier keys or mouse buttons were pressed when an event was received.
    Note that for the KeyPress event which is a modifier key, the state
    field will not reflect the newly pressed key (the same goes for button
    presses), as implied by this section of the XKeyEvent manpage:

    The state member is set to indicate the logical state of the pointer
    buttons and modifier keys just prior to the event, which is the bitwise
    inclusive OR of one or more of the button or modifier key masks: But-
    ton1Mask, Button2Mask, Button3Mask, Button4Mask, Button5Mask, Shift-
    Mask, LockMask, ControlMask, Mod1Mask, Mod2Mask, Mod3Mask, Mod4Mask,
    and Mod5Mask.

    from Tkinter import *

    # for pre-2.3 versions
    #def enumerate(l):
    # for i in range(len(l)): yield i, l

    # Set up a mapping from bits to names
    # (I'm not sure if these values match on Windows)
    names = "Shift Caps Control Alt Num Mod3 Mod4 Mod5 1 2 3 4 5".split()
    mods = [(1<<i, v) for i, v in enumerate(names)]

    # Here's a handler that gets various events and looks at the state field
    def setModifiers(event):
    s = []
    for i, v in mods:
    if event.state & i: s.append(v)
    if not s: s = ['(none)']
    var.set(" ".join(s))

    # Set up a minimal user interface
    t = Tk()
    var = StringVar(t)
    l = Label(t, textv = var, width=32)

    # including a couple of bindings (the text will update when
    # one of these bindings is triggered)
    for binding in "<Motion> <1> <ButtonRelease-1>".split():
    l.bind(binding, setModifiers)

    Jeff Epler, Jan 28, 2004
  5. No idea about linux, but on windows you use win32api.GetKeyState().

    Thomas Heller, Jan 28, 2004
  6. In wxPython you could do it like this ...

    1. Bind the key press events to the two functions

    EVT_KEY_DOWN(self, self.OnKeyDown)
    EVT_KEY_UP(self, self.OnKeyUp)

    2. Set or unset a flag on keydown and keyup

    def OnKeyDown(self, event):
    if (event.GetKeyCode() == WXK_SHIFT):
    self.Shift = True
    elif (event.GetKeyCode() == WXK_CONTROL):
    self.Control = True

    def OnKeyUp(self, event):
    if (event.GetKeyCode() == WXK_SHIFT):
    self.Shift = False
    elif (event.GetKeyCode() == WXK_CONTROL):
    self.Control = False

    Vennlig hilsen, Sean
    Sean Richards, Jan 28, 2004
  7. [Rune]
    You mean GetAsyncKeyState: "The GetAsyncKeyState function determines
    whether a key is up or down at the time the function is called"

    Not GetKeyState: "The key status returned from this function changes as a
    thread reads key messages from its message queue. The status does not
    reflect the interrupt-level state associated with the hardware."

    ....assuming, Rune, that you *really do* want the current state. If your
    program is event driven, and you're responding to Shift+Click or Ctrl+Drag
    or similar, then you want GetKeyState. Imagine the machine is running
    very slowly. I Shift+Click your app and release the Shift key before your
    app responds. You call GetAsyncKeyState and see that Shift is not
    pressed. Had you used GetKeyState, it would tell you that the Shift key
    was pressed at the time of the click.
    Richie Hindle, Jan 29, 2004
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