每次在文本框中输入文本时，如何使用 tkinter 更新 label

Question

I'm trying to create an app where a part of it involves a label that dynamically changes and displays the number of characters typed within the textbox every time it gets updated. I couldn't find any information regarding this and unsure whether this is even possible.

Answer 1

The most foolproof way to get notified when the text widget changes is to intercept the low-level insert, delete, and replace commands. When they occur, an event can be generated, which you can then bind to.

This involves a tiny bit of tcl voodoo, but it works, and works whether you type into the widget, paste using the mouse, or directly call the insert or delete methods.

Here is a complete example:

import tkinter as tk

class CustomText(tk.Text):
    def __init__(self, *args, **kwargs):
        """A text widget that supports a <<TextChanged>> event"""
        tk.Text.__init__(self, *args, **kwargs)

        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 <<TextModified>> -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)))

def update_char_count(event):
    count = event.widget.count("1.0", "end-1c")
    # count is a tuple; the character count is the first element
    count = 0 if not count else count[0]
    label.configure(text=f"Characters: {count}")

root = tk.Tk()
text = CustomText(root)
label = tk.Label(root, anchor="w")
label.pack(side="bottom", fill="x")
text.pack(fill="both", expand=True)

text.bind("<<TextModified>>", update_char_count)
root.mainloop()

Answer 2

If you want a simpler solution that requires a bit more work by the caller, look at this:

import tkinter as tk

def callback(event):
    # After 1 ms call `_callback`
    # That is to make sure that tkinter has handled the keyboard press
    root.after(1, _callback)

def _callback():
    # The `-1` is there because when you have `text_widget.get(..., "end")`
    # It adds a "\n" character at then end
    number_of_chars = len(text_widget.get("1.0", "end")) - 1
    print(number_of_chars)

root = tk.Tk()
text_widget = tk.Text(root)
text_widget.pack()

text_widget.bind("<Key>", callback)

root.mainloop()

This basically binds all keyboard presses to the callback function. I can't think of any way to change the text in a text widget without pressing a keyboard button.

Please note that it will only run when a key is pressed so it wouldn't work if you called text_widget.insert or text_widget.delete (@acw1668 found that problem). It is the caller's responsibility to call _callback just after they call .insert / .delete .

Answer 3

An even simpler answer is always to use what someone has already created:D

# `idlelib` is part of the standard library
from idlelib.redirector import WidgetRedirector
import tkinter as tk

def my_insert(*args):
    original_insert(*args)
    update_label()

def my_delete(*args):
    original_delete(*args)
    update_label()

def my_replace(*args):
    original_replace(*args)
    update_label()

def update_label():
    number_of_chars = len(text.get("1.0", "end")) - 1
    print(number_of_chars)

root = tk.Tk()
text = tk.Text(root)
text.pack()

redir = WidgetRedirector(text)
original_insert = redir.register("insert", my_insert)
original_delete = redir.register("delete", my_delete)
original_replace = redir.register("replace", my_replace)

root.mainloop()

This solution works even if you use text.insert(...)

For more info on how it works just use help(WidgetRedirector) .