Message widget not filling frame using tkinter - python-3.x

Question

I'm creating a simple user dialog window with a basic text on top and a tree view with one column below, that gives the user a couple of choices. A button at the bottom is used to confirm the selection.
Now I can't get the Message widget, which I use to display the instructions, to fill the Frame I've created for it. Meanwhile, the Treeview widget fills the Frame as I want it to.
Many proposed solutions on other StackOverflow questions state, that putting my_message.pack(fill=tk.X, expand=True) should work. It doesn't in my case.. In a different scenario it is recommended to put my_frame.columnconfigure(0, weight=1), which doesn't help either.
Here is the code:
import tkinter as tk
from tkinter import ttk
class MessageBox(object):
""" Adjusted code from StackOverflow #10057662. """
def __init__(self, msg, option_list):
root = self.root = tk.Tk()
root.geometry("400x400")
root.title('Message')
self.msg = str(msg)
frm_1 = tk.Frame(root)
frm_1.pack(expand=True, fill=tk.X, ipadx=2, ipady=2)
message = tk.Message(frm_1, text=self.msg)
message.pack(expand=True, fill=tk.X) # <------------------------------------ This doesn't show the desired effect!
frm_1.columnconfigure(0, weight=1)
self.tree_view = ttk.Treeview(frm_1)
self.tree_view.heading("#0", text="Filename", anchor=tk.CENTER)
for idx, option in enumerate(option_list):
self.tree_view.insert("", idx+1, text=option)
self.tree_view.pack(fill=tk.X, padx=2, pady=2)
choice_msg = "Long Test string to show, that my frame is unfortunately not correctly filled from side to side, as I would want it to."
choices = ["Test 1", "Test 2", "Test 3"]
test = MessageBox(choice_msg, choices)
test.root.mainloop()
I'm slowly going nuts, because I know that there is probably something very basic overruling the correct positioning of the widget, but I've been trying different StackOverflow solutions and browsing documentation for hours now with no luck.

Answer 1

Try to set a width of message in the tk.Message constructor, something like this:
message = tk.Message(frm_1, text=self.msg, width=400-10) # 400 - is your window width
message.pack() # In that case you can delete <expand=True, fill=tk.X>

Answer 2

The problem you are facing is a feature: The Message widget tries to lay out the text in one of two ways:
according to an aspect (width-to-height ratio in percent)
according to a maximum width (lines will be broken if longer)
Both of these goals seem not to cooperate well with the automatic resizing a Message widget experiences from a grid or pack layout manager. What can be done is to bind a handler to the resize event of the widget to adjusts the width option dynamically. Also, there are better options to use with the pack layout manager than shown in OP.
I derived an AutoMessage widget to get the event handler out of the way:
import tkinter as tk
from tkinter import ttk
class AutoMessage(tk.Message):
"""Message that adapts its width option to its actual window width"""
def __init__(self, parent, *args, **options):
tk.Message.__init__(self, parent, *args, **options)
# The value 4 was found by experiment, it prevents text to be
# displayed outside of the widget (exceeding the right border)
self.padx = 4 + 2 * options.get("padx", 0)
self.bind("<Configure>", self.resize_handler)
def resize_handler(self, event):
self.configure(width=event.width - self.padx)
class MessageBox(object):
"""Adjusted code from StackOverflow #10057662."""
def __init__(self, msg, option_list):
root = self.root = tk.Tk()
root.geometry("400x400")
root.title("Message")
self.msg = str(msg)
self.frm_1 = tk.Frame(root)
self.frm_1.pack(side=tk.TOP, fill=tk.X, padx=2, pady=2)
self.message = AutoMessage(self.frm_1, text=self.msg, anchor=tk.W)
self.message.pack(side=tk.TOP, fill=tk.X)
self.frm_1.columnconfigure(0, weight=1)
self.tree_view = ttk.Treeview(self.frm_1)
self.tree_view.heading("#0", text="Filename", anchor=tk.CENTER)
for idx, option in enumerate(option_list):
self.tree_view.insert("", idx + 1, text=option)
self.tree_view.pack(fill=tk.X, padx=2, pady=2)
choice_msg = "Long Test string to show, that my frame is unfortunately not correctly filled from side to side, as I would want it to."
choices = ["Test 1", "Test 2", "Test 3"]
test = MessageBox(choice_msg, choices)
test.root.mainloop()