how is del(item) different from list.remove(item) in python list

Question

I wanted to delete words from a list having length greater than 3. I used del (item) but it didn't worked. Here is the code:

lst=['XDA-OT','hi','loc','yeah']
for i in lst:
    if len(i)>3:
        del i

And the output:

lst
['XDA-OT', 'hi', 'loc', 'yeah']

Now I used remove() from Python List functions and I got the desired result.

Here is the code for the same:

lst=['XDA-OT','hi','loc','yeah']
for i in lst:
    if len(i)>3:
        lst.remove(i)

OUTPUT:

print(lst)
['hi', 'loc']

Also I have doubt regarding usage of index of list to delete a element from list but didn't understand how to frame the question.

I get IndexError whenever I delete a element using index. CODE:

lst=['XDA-OT','hi','loc','yeah']
for i in range(len(lst)):
    if len(lst[i])>3:
        del lst[i]

OUTPUT ERROR:

IndexError                                
<ipython-input-35-617282928840> in <module>()
      1 for i in range(len(lst)):
----> 2     if len(lst[i])>3:
      3         del lst[i]

IndexError: list index out of range

Same using remove() function. CODE:

lst=['XDA-OT','hi','loc','yeah']
for i in range(len(lst)):
    if len(lst[i])>3:
        lst.remove(lst[i])

OUTPUT ERROR:

IndexError                                
<ipython-input-39-1d824ca5b061> in <module>()
      1 for i in range(len(lst)):
----> 2     if len(lst[i])>3:
      3         lst.remove(lst[i])

IndexError: list index out of range

Answer 1

As a general rule, I would avoid modifying an object I'm iterating on. This could lead to very odd behaviors.

Have you considered a solutions with list comprehension? It seems to me like the most pythonic implementation in this case.

lst = ['XDA-OT', 'hi', 'loc', 'yeah']
lst = [itm for itm in lst if len(itm) <= 3]

Answer 2

You are having an index error because you are modifying the list as you go. The for loop receive a range(4) but in the end lst[4] does not exist anymore as you have deleted some items.

As for the question. del is a general function that delete an object whereas lst.remove() is a function of the list class. So in your case you can achieve what you want using both.

I would recommend that you go with the approach of @b3by as it makes usage of list comprehension which is faster in python than for loops as you where doing.

Answer 3

del deletes the desired element from wherever it is being held.

If used on a local variable, it'll delete the local variable so it cannot be accessed again, unless it's recreated (same thing applies on a global scope)

Same with list elements (del lst[2]) or dict items (del dct['MyKey'])

---

In your specific case, you are effectively deleting the local variable i, but not the element it refers to.

If you want to delete the specific element in the list, you don't have to store it on a separate variable:

 del lst[i]

However, I'd recommend to go with the second approach and use list.remove (or list.pop for that matter) as it's way more explicit and avoids undesired side effects.

---

As for your second question, you are getting an IndexError because on every iteration you reduce the size of the list (as you are removing elements from it), so you'll get to a point where your i is higher than the length of the list itself.

Answer 4

With a del statement you would have to specify for the index. For example

del list[0]

Simply deleting a string value from the list will not work with the del function. However, the .remove() method will work in this situation. As the argument in .remove() simply pass the list item that you want to remove from the list. That can work in a for loop like so:

lst=['XDA-OT','hi','loc','yeah']
for i in lst:
   if len(i)>3:
      lst.remove(i)

print(lst)

And here is your output:

['hi', 'loc']