Python: list assignment out of range

Question

This module is part of a simple todo app I made with Python...

def deleteitem():
             showlist()
             get_item = int(raw_input( "\n Enter number of item to delete: \n"))
             f = open('todo.txt')
             lines = f.readlines()
             f.close()
             lines[get_item] = ""
             f = open('todo.txt','w')
             f.writelines(lines)
             f.close()
             showlist()

The number of lines in f obviously changes as items are added to the list... Problem here is that for example if a user enters '10' when there are only 9 lines in the file ( or anything else not in range) , it exits as expected with :

IndexError: list assignment index out of range

What can I add to the module so as to have it prompt user to enter an item within the range? I am assuming maybe a Try block ... Or is there a way to catch an exception.. I am guessing there is an easy way to do this...

Answer 1

Either catch IndexError when indexing or check the len() of the list beforehand.

Answer 2

First read the file, and then ask user in a loop, until the answer is acceptable:

while True:
    get_item = int(raw_input( "\n Enter number of item to delete: \n"))
    if get_item >=0 and get_item < len(lines):
        break

That will, of course, break when the file is empty and doesn't give any hint about the acceptable values to the user. But let's keep some exercise for you.

Answer 3

def deleteitem():
             showlist()
             get_item = int(raw_input( "\n Enter number of item to delete: \n"))
             f = open('todo.txt')
             lines = f.readlines()
             f.close()
             try:
                 lines[get_item] = ""
             except Exception,err: 
                 print err
             f = open('todo.txt','w')
             f.writelines(lines)
             f.close()
             showlist()

Answer 4

Judicious changes to your current code:

def deleteitem():
  showlist()

  with open("todo.txt") as f:
    lines = f.readlines()
  if len(lines) == 0:  # completely empty file
    return  # handle appropriately
  prompt = "Enter number to delete (1-%d), or 0 to abort: " % len(lines)
  while True:
    input = raw_input(prompt)
    try:
      input = int(input, 10)
    except ValueError:
      print "Invalid input."
    else:
      if 0 <= input <= len(lines):
        break
      print "Input out of range."
  if input == 0:
    return
  input -= 1  # adjust from [1,len] to [0,len)

  #del lines[input]  # if you want to remove that line completely
  lines[input] = "\n"  # or just make that line blank (what you had)

  with open("todo.txt", "w") as f:
    f.writelines(lines)

  showlist()

Answer 5

Try something like this:

def deleteitem():

showlist()
f = open('todo.txt')
lines = f.readlines()
f.close()
if len(lines) == 0:
    print "File is empty!"
    return False
print "File has %d items\n" % len(lines)
item = 0
while item < len(lines):
    item = raw_input( "\n Enter number of item to delete(0-%d): \n" % len(lines))
    item = int(item) # because of the width of the code
f = open('todo.txt','w')
f.write(lines[0:item-1])
f.write(lines[item::])
f.close()
showlist()

Answer 6

For what it's worth.... I'll put the code to my todo.py program here... It's just something I run from a Terminal in OS X to keep control of stuff I need to do at work...I am sure it's horribly un-pythonic,inefficient and everything else...but maybe it'll be of use to some one who stumbles across this thread :

from __future__ import with_statement
import sys
import os
import fileinput

os.system('clear')

print ("##############          TO DO LIST       ############")
print ("##############                           ############")

def showlist():
    os.system('clear')
    print ("############  Current To Do List  ######")
    print ("########################################")

    get_list = open('todo.txt')
    entire_list = get_list.readlines()
    for i in range (len(entire_list)):
        print i, entire_list[i]
    get_list.close()
    print ("########################################")
    print ("########################################")

def appendlist():
    print ("#######################################")
    print ("#######################################")


    addtolist = str( raw_input("Enter new item:  \n"))
    thelist = open('todo.txt', 'a')
    thelist.write(str(addtolist))
    thelist.write(str('\n'))
    thelist.close()  
    showlist()


def deleteitem():
    showlist()

        with open("todo.txt") as f:
            lines = f.readlines()
            if len(lines) == 0:  
                return  
        prompt = "Enter number to delete or '0' to abort: " 
        while True:
                input = raw_input(prompt)
                try:
                    input = int(input, 10)
                except ValueError:
                    print "Invalid input."
                else:
                    if 0 <= input <= len(lines):
                        break
                    print "Input out of range."
        if input == 0:
                  return

        lines[input] = "" 

            with open("todo.txt", "w") as f:
                f.writelines(lines)

        showlist()

while True:

    askme = raw_input("\nDo you want to:\n(S)ee list\n(A)ppend list\n(D)elte from list\n(Q)Quit?\n")
    print str('\n')

    if askme == "S":
        showlist()
    elif askme == "A":
        appendlist()
    elif askme == "D":
        deleteitem()

    elif askme == "Q":
        sys.exit()
    else: 
        print ("Try again?")

print ("#######################################")
print ("#######################################")