WidgetRedirector.py

from tkinter import *

class WidgetRedirector:

    """Support for redirecting arbitrary widget subcommands.

    Some Tk operations don't normally pass through Tkinter.  For example, if a
    character is inserted into a Text widget by pressing a key, a default Tk
    binding to the widget's 'insert' operation is activated, and the Tk library
    processes the insert without calling back into Tkinter.

    Although a binding to <Key> could be made via Tkinter, what we really want
    to do is to hook the Tk 'insert' operation itself.

    When a widget is instantiated, a Tcl command is created whose name is the
    same as the pathname widget._w.  This command is used to invoke the various
    widget operations, e.g. insert (for a Text widget). We are going to hook
    this command and provide a facility ('register') to intercept the widget
    operation.

    In IDLE, the function being registered provides access to the top of a
    Percolator chain.  At the bottom of the chain is a call to the original
    Tk widget operation.

    """
    def __init__(self, widget):
        self._operations = {}
        self.widget = widget            # widget instance
        self.tk = tk = widget.tk        # widget's root
        w = widget._w                   # widget's (full) Tk pathname
        self.orig = w + "_orig"
        # Rename the Tcl command within Tcl:
        tk.call("rename", w, self.orig)
        # Create a new Tcl command whose name is the widget's pathname, and
        # whose action is to dispatch on the operation passed to the widget:
        tk.createcommand(w, self.dispatch)

    def __repr__(self):
        return "WidgetRedirector(%s<%s>)" % (self.widget.__class__.__name__,
                                             self.widget._w)

    def close(self):
        for operation in list(self._operations):
            self.unregister(operation)
        widget = self.widget; del self.widget
        orig = self.orig; del self.orig
        tk = widget.tk
        w = widget._w
        tk.deletecommand(w)
        # restore the original widget Tcl command:
        tk.call("rename", orig, w)

    def register(self, operation, function):
        self._operations[operation] = function
        setattr(self.widget, operation, function)
        return OriginalCommand(self, operation)

    def unregister(self, operation):
        if operation in self._operations:
            function = self._operations[operation]
            del self._operations[operation]
            if hasattr(self.widget, operation):
                delattr(self.widget, operation)
            return function
        else:
            return None

    def dispatch(self, operation, *args):
        '''Callback from Tcl which runs when the widget is referenced.

        If an operation has been registered in self._operations, apply the
        associated function to the args passed into Tcl. Otherwise, pass the
        operation through to Tk via the original Tcl function.

        Note that if a registered function is called, the operation is not
        passed through to Tk.  Apply the function returned by self.register()
        to *args to accomplish that.  For an example, see ColorDelegator.py.

        '''
        m = self._operations.get(operation)
        try:
            if m:
                return m(*args)
            else:
                return self.tk.call((self.orig, operation) + args)
        except TclError:
            return ""


class OriginalCommand:

    def __init__(self, redir, operation):
        self.redir = redir
        self.operation = operation
        self.tk = redir.tk
        self.orig = redir.orig
        self.tk_call = self.tk.call
        self.orig_and_operation = (self.orig, self.operation)

    def __repr__(self):
        return "OriginalCommand(%r, %r)" % (self.redir, self.operation)

    def __call__(self, *args):
        return self.tk_call(self.orig_and_operation + args)


def main():
    root = Tk()
    root.wm_protocol("WM_DELETE_WINDOW", root.quit)
    text = Text()
    text.pack()
    text.focus_set()
    redir = WidgetRedirector(text)
    global previous_tcl_fcn
    def my_insert(*args):
        print("insert", args)
        previous_tcl_fcn(*args)
    previous_tcl_fcn = redir.register("insert", my_insert)
    root.mainloop()
    redir.unregister("insert")  # runs after first 'close window'
    redir.close()
    root.mainloop()
    root.destroy()

if __name__ == "__main__":
    main()