Whys is asterisk and random variable necessary in these functions?

[deanhystad]

You should read the documentation. This describes what arguments are passed to the bound event callback.

https://docs.python.org/3/library/tkinte...d-events-1

In your example you are binding a key code (<Return>, <Escape>), so the argument passed to your functions is an event that contains the key that was pressed.

This is a little program that displays the event in an Entry widget. You can select the entry and type and it works like normal, but it you press <Return> or <Escape> an text is replaced by the event info.

import tkinter as tk

class MainWindow(tk.Tk):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.title("Test UI")
        self.bind("<Return>", self.keypress)
        self.bind("<Escape>", self.keypress)
        self.text = tk.StringVar(self, "")
        tk.Entry(self, textvariable=self.text, width=80).pack(padx=20, pady=20)

    def keypress(self, event):
        self.text.set(str(event))


MainWindow().mainloop()

If you don't use the event you can ignore it. The linter in my IDE does not like functions arguments that aren't used in the function, so I use a place holder instead of an argument.

   def keypress(self, _):
        self.text.set("something happened")

The "_" in the function declaration tells the reader that one argument is passed, but not used by the function. Unfortunately you can only use one "_". If there are multiple arguments to ignore I might do this:

def keypress(self, *_):

But I cannot do this:

def keypress(self, *_, **_):

I can also use a lambda expressing to remove or replace the event argument.

self.bind("<Return>", lambda event:self.keypress("Return was pressed")
self.bind("<Escape>", lambda event:self.keypress("Escape was pressed")

    def keypress(self, msg):
        self.text.set(msg)

Here the lambda expression consumes the "event" argument and passes along a different argument.