Whys is asterisk and random variable necessary in these functions?

[rrowhe4d]

In this basic Python Tkinter code, I'm trying to bind certain functions to trigger upon either a UI button press or a keyboard key press.

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import tkinter as tk from tkinter import ttk   main_window = tk.Tk() main_window.title('Test UI')   # Change text with "Enter" then flush def changeTextEnter():     text_label.configure(text=entry_bar.get())     entry_bar.delete(0, tk.END)   # Close program key function def quitApp():     main_window.destroy()   # Enter Button enter_button = ttk.Button(text='Enter', command=changeTextEnter) enter_button.grid(row=0, column=0)   # Entry bar entry_bar = ttk.Entry(width=30) entry_bar.grid(row=0, column=1)   # Quit Button quit_button = ttk.Button(text='Quit', command=main_window.destroy) quit_button.grid(row=0, column=2)   # Text label text_label = ttk.Label(text='TEST TEXT GOES HERE') text_label.grid(row=1, column=0, columnspan=2)   # Bind enter key main_window.bind('<Return>', changeTextEnter) # Bind quit key main_window.bind('<Escape>', quitApp)   main_window.mainloop()

After a while of trial and error, it seems to work the way I want if I add an *randomVariable in the declarations of def changeTextEnter(*randomVariable): and def quitApp(*randomVariable):
I get that one asterisk lets the function take an unknown number of arguments, and a double asterisk acts as a dictionary with key values.

My questions are:

1. Why do I need a parameter in those functions at all?
   
2. How does the variable *randomVariable get used, since it seems like I'm not actually using/ assigning anything to randomVariable anywhere within the function.
   
3. Why does the function not work as intended without the asterisk before the variable?
   

[Gribouillis]

When you bind the callback to a key, it receives a KeyPress event parameter every time the key is pressed. On the other hand, a function bound to a Button does not receive a parameter when the button is pressed. So if you want to bind the callbacks both to a key and to a Button, you need to give them a variable number of arguments. Try adding a print() in the functions to see what it does

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# Change text with "Enter" then flush def changeTextEnter(*args):     print('changeTextEnter:', args)     text_label.configure(text=entry_bar.get())     entry_bar.delete(0, tk.END)    # Close program key function def quitApp(*args):     print('quit:', args)     main_window.destroy()

Output:
changeTextEnter: (<KeyPress event keysym=Return keycode=36 char='\r' x=89 y=14>,)
changeTextEnter: ()
quit: (<KeyPress event keysym=Escape keycode=9 char='\x1b' x=181 y=14>,)

[DeaD_EyE]

The function signature is misleading. Don't use the asterisk, if you expect a static number of arguments. The callback sends one argument, which is the event itself. You should change the callbacks (example taken form Gribouillis).

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# Change text with "Enter" then flush def changeTextEnter(event):     print('changeTextEnter:', event)     text_label.configure(text=entry_bar.get())     entry_bar.delete(0, tk.END)     # Close program key function def quitApp(event):     print('quit:', event)     main_window.destroy()

[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.

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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.

1 2	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:

1	def keypress(self, *_):

But I cannot do this:

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

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

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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.

[DeaD_EyE]

But I cannot do this:

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

You should read the error messages:

Error:
SyntaxError: duplicate argument '_' in function definition

Fix:

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class Test:     def keypress(self, *args, **kwargs):         ...

[Gribouillis]

You could also do

1 2	def changeTextEnter(event=None):     ...

This allows zero or one argument.