How to populate listbox from file input lines in python

I have been trying to open a file read from it, modify it and save that file again. I'm having trouble reading the file through the list box I've created to contain its content. I am not too sure what the problem is here. Here is the code below.

import Tkinter
import Tkinter as tk  # gives tk namespace
from Tkinter import *
from ScrolledText import *
import tkFileDialog
import tkMessageBox

root = Tkinter.Tk(className="To Do List py experiment")
textPad = ScrolledText(root, width=100, height=30)
task_list = tk.Listbox(root, width=50, height=6)


def open_task():
    fin = tkFileDialog.askopenfile(mode='r',title='Select a Task File')
    if fin is not None:
        task_list = fin.readlines()
    for item in task_list:
            task_list.insert(tk.END, item)
    fin.close()


def exit():
    if tkMessageBox.askokcancel("Quit", "Do you really want to quit?"):
    root.destroy()

def about():
    label = tkMessageBox.showinfo("About", "To do List py experiment")


def new_task():
    task_list.insert(tk.END, input.get())


def delete_item():
    """
    delete a selected line from the listbox
    """
    try:
    # get selected line index
    index = task_list.curselection()[0]
    task_list.delete(index)
    except IndexError:
    pass


def get_list(event):
    """
    function to read the listbox selection
    and put the result in an entry widget
    """
    # get selected line index
    index = task_list.curselection()[0]
    # get the line's text
    seltext = task_list.get(index)
    # delete previous text in input
    input.delete(0, 50)
    # now display the selected text
    input.insert(0, seltext)

def set_list(event):
    """
    insert an edited line from the entry widget
    back into the listbox
    """
    try:
    index = task_list.curselection()[0]
    # delete old listbox line
    task_list.delete(index)
    except IndexError:
    index = tk.END
    # insert edited item back into task_list at index
    task_list.insert(index, input.get())


def save_tasks():
    """
    save the current listbox contents to a file
    """
    # get a list of listbox lines
    temp_list = list(task_list.get(0, tk.END))
    # add a trailing newline char to each line
    temp_list = [task + '\n' for task in temp_list]
    # give the file a different name

    if temp_list is not None:
    fout = tkFileDialog.asksaveasfile(mode='w')
    fout.writelines(temp_list)
    fout.close()

menu = Menu(root)
root.config(menu=menu)

filemenu = Menu(menu)
menu.add_cascade(label="File", menu=filemenu)
filemenu.add_command(label="Open Task File", command=open_task)
filemenu.add_command(label="Save", command=save_tasks)
filemenu.add_separator()
filemenu.add_command(label="Exit", command=exit)


helpmenu = Menu(menu)
menu.add_cascade(label="Help", menu=helpmenu)
helpmenu.add_command(label="About ", command=about)

# create the listbox (note that size is in characters)
#task_list = tk.Listbox(root, width=50, height=6)
task_list.grid(row=0, column=0)

# create a vertical scrollbar to the right of the listbox
yscroll = tk.Scrollbar(command=task_list.yview, orient=tk.VERTICAL)
yscroll.grid(row=0, column=1, sticky=tk.N+tk.S)
task_list.configure(yscrollcommand=yscroll.set)

#task_list.bind("<<ListboxSelect>>", printer)

# use entry widget to display/edit selection
input = tk.Entry(root, width=50)
input.insert(0, 'Write Your Task here')
input.grid(row=1, column=0)
# pressing the enter key will update edited line
input.bind('<Return>', set_list)

#This button is used to add tasks
button_add_task = tk.Button(root, text='Add entry text to listbox', command=new_task)
button_add_task.grid(row=2, column=0, sticky=tk.E)

#This Button is used to call the delete function
button_delete = tk.Button(root, text='Delete selected Task     ', command=delete_item)
button_delete.grid(row=3, column=0, sticky=tk.E)

# left mouse click on a list item to display selection
task_list.bind('<ButtonRelease-1>', get_list)

root.mainloop()

tata