Why a file is empty after writing to it?

When I add print(img_list) to write_to_file() I see that this function is executed at start - without clicking button - even before create_img_list() runs (which creates list) so write_to_file() writes empty list.

You use command= incorrectly. It needs function name without () (so called "callback") but you run function and you assign its result to command=. Your code works like

result = _thread.start_new_thread(write_to_file, ()) # it executes function at startbutton = tk.Button(root, text="Click Me", command=result).pack()

but you need

def run_thread_later():    _thread.start_new_thread(write_to_file, ())button = tk.Button(root, text="Click Me", command=run_thread_later).pack()

Eventually you can uses lambda to create this function directly in command=

button = tk.Button(root, text="Click Me", command=lambda:_thread.start_new_thread(write_to_file, ())).pack()

BTW: you have common mistake

button = Button(...).pack()

which assign None to variable because pack()/grid()/place() return `None.

If you need access button later then you have to do it in two lines

button = Button(...)button.pack()

If you don't need access button later then you can skip `button()

Button(...).pack()