tkinter.Text: binding a variable to widget text contents

python, python-3.x, tkinter

Solution

If you're willing to live dangerously, it's possible to hook in to the internals of the text widget, and have it call a function whenever the contents change, regardless of how it changed.

The trick is to replace the underlying tk widget command with a proxy. This proxy is responsible for doing whatever the real text widget would do, then send a virtual event if what it did was insert or delete text.

With that in place, it's just a matter of setting up a binding to that event, and putting a read trace on the variable. Of course, if you try inserting widgets or images into the text they won't be reflected in the textvariable.

Here's a quick and dirty example, not tested at all in anything real. This uses the same technique that I used to implement line numbers in a text widget (see https://stackoverflow.com/a/16375233)

import Tkinter as tk
import random
import timeit

class TextWithVar(tk.Text):
    '''A text widget that accepts a 'textvariable' option'''
    def __init__(self, parent, *args, **kwargs):
        try:
            self._textvariable = kwargs.pop("textvariable")
        except KeyError:
            self._textvariable = None

        tk.Text.__init__(self, parent, *args, **kwargs)

        # if the variable has data in it, use it to initialize
        # the widget
        if self._textvariable is not None:
            self.insert("1.0", self._textvariable.get())

        # this defines an internal proxy which generates a
        # virtual event whenever text is inserted or deleted
        self.tk.eval('''
            proc widget_proxy {widget widget_command args} {

                # call the real tk widget command with the real args
                set result [uplevel [linsert $args 0 $widget_command]]

                # if the contents changed, generate an event we can bind to
                if {([lindex $args 0] in {insert replace delete})} {
                    event generate $widget <<Change>> -when tail
                }
                # return the result from the real widget command
                return $result
            }
            ''')

        # this replaces the underlying widget with the proxy
        self.tk.eval('''
            rename {widget} _{widget}
            interp alias {{}} ::{widget} {{}} widget_proxy {widget} _{widget}
        '''.format(widget=str(self)))

        # set up a binding to update the variable whenever
        # the widget changes
        self.bind("<<Change>>", self._on_widget_change)

        # set up a trace to update the text widget when the
        # variable changes
        if self._textvariable is not None:
            self._textvariable.trace("wu", self._on_var_change)

    def _on_var_change(self, *args):
        '''Change the text widget when the associated textvariable changes'''

        # only change the widget if something actually
        # changed, otherwise we'll get into an endless
        # loop
        text_current = self.get("1.0", "end-1c")
        var_current = self._textvariable.get()
        if text_current != var_current:
            self.delete("1.0", "end")
            self.insert("1.0", var_current)

    def _on_widget_change(self, event=None):
        '''Change the variable when the widget changes'''
        if self._textvariable is not None:
            self._textvariable.set(self.get("1.0", "end-1c"))


class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)

        self.textvar = tk.StringVar()
        self.textvar.set("Hello, world!")

        # create an entry widget and a text widget that
        # share a textvariable; typing in one should update
        # the other
        self.entry = tk.Entry(self, textvariable=self.textvar)
        self.text = TextWithVar(self,textvariable=self.textvar, 
                                borderwidth=1, relief="sunken", 
                                background="bisque")

        self.entry.pack(side="top", fill="x", expand=True)
        self.text.pack(side="top",fill="both", expand=True)

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).pack(fill="both", expand=True)
    root.mainloop()

Problem

Original source

Related problems

• Tkinter adding line number to text widget