只绑定 tk/tkinter 上的重要键

Question

Can you please tell me if there's a convenient way to have a bind only on "significant" keys with tk/tkinter?

So, the situation is I have a bind on a tk.Entry (because I need a callback to trigger while keys are being pressed), like this:

myEntry.bind('<Key>', ...)

Right now, the bind callback is triggered whatever key is pressed (even if it is the shift!) but I'd want it to trigger only on "significant" keys ... By "significant" I mean every key that involve changes on the text in the Entry, so "significant" keys are for sure letters, numbers, symbols and the backspace or delete keys, but NOT arrow keys, home, end, pgUp/Down or also the tab, caps-lock, shift, ctrl, etc... (I'm still thinking if I will need it to trigger on return key or not but that's not a problem, because if I need it too I could add a specific bind on it or otherwise have it ignored in the callback later)

I don't know if perhaps is there something different from <Key> I could bind to get what I need, is it? Otherwise, if not, I know I can get which key was pressed looking at event keycode... is that the way to go? If yes, can you suggest me some suitable keycode interval please?

Thank you

Answer 1

In a comment, you say you are doing real-time filtering of a listbox. For that, it would arguably be better to set a trace on a variable rather than setting a binding on the entry widget. The trace will only be called when the value changes, so you don't have to distinguish which key triggered the change.

Here's a simple example that illustrates the concept:

import tkinter as tk

widgets = [w.__name__ for w in tk.Widget.__subclasses__()]

root = tk.Tk()
entryvar = tk.StringVar()
entry = tk.Entry(root, textvariable=entryvar)
listbox = tk.Listbox(root)
entry.pack(side="top", fill="x")
listbox.pack(side="bottom", fill="both", expand=True)
listbox.insert("end", *sorted(widgets))

after_id = None

def filter_changed(*args):
    global after_id
    if after_id:
        root.after_cancel(after_id)

    # delay actual filtering slightly, in case the user is typing fast.
    after_id = root.after(500, filter_listbox)

def filter_listbox(*args):
    pattern = entryvar.get()
    filtered_widgets = [w for w in widgets if w.startswith(pattern) ]
    listbox.delete(0, "end")
    listbox.insert("end", *filtered_widgets)

entryvar.trace("wu", filter_changed)

root.mainloop()

Answer 2

Here is my solution, it uses event.char and evaluates against a string with characters (can add more characters if needed):

from tkinter import Tk, Entry


string = r"""0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"""


def key_press(event):
    if event.char and event.char in string:
        print(event.char)


root = Tk()

entry = Entry(root)
entry.pack()
entry.bind('<Key>', key_press)

root.mainloop()