Python List Methods Class 11: append, extend, insert, remove, pop and sort with Output Tracing

Understand List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11)

L = [3, 1, 2]
L.append(9)
L.insert(0, 7)
print(L)          # [7, 3, 1, 2, 9]
print(L.pop(), L) # 9 [7, 3, 1, 2]
L.sort()
print(L)          # [1, 2, 3, 7]

List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11): key facts

Add items

• •append(x) adds one item
• •extend(seq) adds each item
• •insert(i, x) adds at index i

All three change the list in place and return None.

Remove items

• •remove(x) by value, first match
• •pop(i) by index, returns item
• •del L[i] by index, no return

remove raises ValueError; pop raises IndexError.

Order items

• •sort() in place
• •sort(reverse=True) descending
• •sorted(L) new list
• •reverse() in place

sort() and reverse() return None.

Search items

• •count(x) number of times
• •index(x) first position
• •x in L True or False

index raises ValueError if x is absent.

How to answer List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11) questions

1. 1Write the list as it is before the first method call.
2. 2After each statement write the full new list; insert shifts later items to the right.
3. 3For pop and remove note which item goes and what is returned.
4. 4Remember that sort(), reverse(), append(), extend(), insert() and remove() return None.
5. 5Write the printed line exactly, including brackets, commas and quotes of strings.

List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11) in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11)

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Append one, extend many

  append puts the whole argument in as one item; extend opens the argument and adds its pieces.

  Use it when: Predicting len() after adding a list or string.

• None means in place

  If a method changes the list itself, it returns None, so never write L = L.sort().

  Use it when: sort, reverse, append, extend, insert, remove.

• Pop returns, remove does not

  pop gives back the item it removes; remove just deletes by value.

  Use it when: Questions that print the result of pop.

List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11): solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

L = [1, 2, 3]
L.append([4, 5])
print(len(L))

L = [3, 1, 2]
M = L.sort()
print(L, M)

L = [10, 20, 30, 40, 50]
L.insert(2, 25)
x = L.pop()
L.remove(20)
L.extend([5, 5])
print(L.count(5), L.index(30), x)
print(sorted(L))
print(L)

L = [2, 2, 3, 2]
for x in L:
    if x == 2:
        L.remove(x)
print(L)

Where students lose marks in List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11)

The slips examiners see most often, and what to write instead.

• ✗Mistake: Assigning the result of sort()

  Expecting sort to return the sorted list.

  Wrong working

  L = L.sort(); print(L[0]) raises TypeError because L is None

  Right working

  L.sort(); print(L[0]) or M = sorted(L)

  ✓Do this: In-place methods return None.

  Usually costs: 1 to 2 marks

• ✗Mistake: Using extend like append

  Not seeing that extend loops over its argument.

  Wrong working

  L = [1]; L.extend(5) # TypeError: int is not iterable

  Right working

  L.append(5) or L.extend([5])

  ✓Do this: extend needs a list, tuple or string.

  Usually costs: 1 mark

• ✗Mistake: Assuming remove deletes by position

  Confusing remove(x) with pop(i).

  Wrong working

  L = [5, 6, 7]; L.remove(1) expecting 6 to go; it raises ValueError

  Right working

  L.pop(1) removes 6; L.remove(6) also removes 6

  ✓Do this: remove takes a value; pop takes an index.

  Usually costs: 1 mark

Practice questions on List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11)

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which list method removes an item and also returns it?
   (a) remove()
   (b) pop()
   (c) clear()
   (d) delete()

   Hint

   The return value matters.

   Answer

   (b) pop()

2. Q2 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): For a list L, the statement print(L.sort()) prints None.
   Reason (R): sort() arranges the items of L in place and returns nothing useful.
   (a) Both A and R are true, and R is the correct explanation of A.
   (b) Both A and R are true, but R is not the correct explanation of A.
   (c) A is true, but R is false.
   (d) A is false, but R is true.

   Hint

   Return value of in-place methods.

   Answer

   (a) Both A and R are true, and R is the correct explanation of A.

3. Q3 · 3 marks · Short answerHarder board level

   Give the output and the length printed:

   L = [4, 8, 15]
   L.extend("ab")
   L.append("cd")
   print(L, len(L))

   (3 marks)

   Hint

   extend splits the string into characters.

   Answer

   extend adds 'a' and 'b' as two items; append adds 'cd' as one. Output: [4, 8, 15, 'a', 'b', 'cd'] 6

4. Q4 · 4 marks · Short answerHOTS / competency

   Find the output of the code and name the error that stops the program.

   L = [7, 8, 9]
   print(L.pop(1), L)
   print(L.pop(5))

   (4 marks)

   Hint

   The second pop uses a bad index.

   Answer

   First print: 8 [7, 9]. The second pop(5) on a list of length 2 raises IndexError: pop index out of range, so nothing more is printed.

5. Q5 · 4 marks · Short answerHarder board level

   Trace this code and give its output:

   L = [5, 3, 8, 3]
   L.reverse()
   L.remove(3)
   L.insert(len(L), 9)
   L.insert(-1, 7)
   print(L, L.index(7), max(L) - min(L))

   (4 marks)

   Hint

   insert(-1, x) places x before the last item.

   Answer

   reverse: [3, 8, 3, 5]; remove(3): [8, 3, 5]; insert at end: [8, 3, 5, 9]; insert(-1, 7): [8, 3, 5, 7, 9]. index(7) = 3; max 9 - min 3 = 6. Output: [8, 3, 5, 7, 9] 3 6

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does append() add?Show answer

   Its argument as one single item at the end.

2. 2.What does sort() return?Show answer

   None; it sorts the list in place.

3. 3.Which error does remove() raise for a missing value?Show answer

   ValueError.

4. 4.How is pop() different from del?Show answer

   pop returns the removed item; del does not.

5. 5.Which gives a new sorted list?Show answer

   sorted(L).

List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11): frequently asked questions