String Slicing and String Methods in Python (Class 12 Computer Science)

Understand String Slicing and String Methods

String Slicing and String Methods: key facts

Slicing rules

• •s[a:b]: characters a to b - 1
• •s[a:]: from a to the end
• •s[:b]: from the start to b - 1
• •s[::k]: every k-th character
• •s[::-1]: reversed copy
• •s[a:b:-k]: from a leftwards, stopping before b

A slice never raises IndexError; it can be the empty string.

Methods returning a new string

lower() / upper()	change the case
capitalize()	first character upper, rest lower
title()	first letter of each word upper
strip(), lstrip(), rstrip()	remove spaces at both ends, left, right
replace(old, new[, n])	replace all (or first n)
sep.join(seq)	join strings with sep

s itself never changes: write s = s.upper() to keep the result.

Searching and counting

count(sub)	non-overlapping occurrences
find(sub)	first index, -1 if absent
index(sub)	first index, ValueError if absent
startswith(p), endswith(p)	True or False
sub in s	True or False

find and index accept a start position: 'banana'.find('n', 3) is 4.

Character tests (True/False)

isalpha()	letters only
isdigit()	digits only
isalnum()	letters or digits only
islower() / isupper()	all cased letters lower / upper
isspace()	whitespace only

All give False for an empty string; 'ABC1'.isupper() is True (digits are ignored).

split versus partition

split()	list of words, cut at whitespace
split(sep)	list, cut at every sep; adjacent seps give ''
partition(sep)	tuple (before, sep, after), cut at the first sep; if sep is absent: (s, '', '')

Brackets give it away: split answers use [ ], partition answers use ( ).

How to answer String Slicing and String Methods questions

1. 1Write the string with its indexes underneath (positive, and negative if the code uses them). Count spaces as characters.
2. 2For a slice, write the indexes visited: start, start + step, ... and stop before stop. For a negative step, start from the right if start is missing.
3. 3For each method, decide what it returns: a new string, a number, True/False, a list (split) or a tuple (partition). The original string is unchanged.
4. 4For split(sep), mark every occurrence of sep and write what lies between them, including empty strings.
5. 5In loops that build a string, track the result after every character; check which branch (if/elif/else) each character takes.
6. 6In programs, use split() to get words and len(), startswith(), isupper() and similar tests on each word.
7. 7Write the output exactly: lists with quotes around each string, tuples with round brackets, and no quotes when a plain string is printed.

String Slicing and String Methods 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 String Slicing and String Methods

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

• Start in, stop out

  In every slice, the start index is included and the stop index is left out, in either direction.

  Use it when: All slicing questions.

• find is friendly, index is strict

  find() shrugs and returns -1 when the text is missing; index() complains with ValueError.

  Use it when: MCQs on find versus index.

• Strings are carved in stone

  No method carves into the original; each one gives you a fresh copy you must catch with =.

  Use it when: Questions where s.upper() is called but s is printed.

• Square or round?

  split splits into a list [ ]; partition parts into exactly three ( ).

  Use it when: Writing the output of split() and partition().

String Slicing and String Methods: solved examples

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

s = 'Board Exam'
print(s[2:7], s[-4:], s[::3])

print('committee'.split('t'))
print('a-b-c'.partition('-'), 'a-b-c'.partition('#'))

code = 'Ab3#z9'
out = ''
for ch in code:
    if ch.isupper():
        out += ch.lower()
    elif ch.isdigit():
        out += str(int(ch) * 3)
    elif ch.isalpha():
        out += '*'
    else:
        out += '@'
print(out)

w = 'PROGRAMMING'
print(w[-3:2:-2], w[8:-12:-3], w[::-4])

Where students lose marks in String Slicing and String Methods

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

• ✗Mistake: Leaving out the empty strings in split(sep)

  Two separators next to each other have an empty string between them, and split(sep) keeps it.

  Wrong working

  'committee'.split('t') → ['commi', 'ee']

  Right working

  'committee'.split('t') → ['commi', '', 'ee']

  ✓Do this: Mark each separator and write what lies between every pair, even if it is nothing.

  Usually costs: The full mark of a 1-mark MCQ.

• ✗Mistake: Expecting a method to change the string

  Strings are immutable; methods return new strings.

  Wrong working

  s = 'cbse'
  s.upper()
  print(s) # CBSE

  Right working

  s = 'cbse'
  s = s.upper()
  print(s) # CBSE

  ✓Do this: Store or print the returned value.

  Usually costs: 1 mark.

• ✗Mistake: Starting a negative-step slice at the wrong end

  With a negative step and no start, the slice begins at the last character.

  Wrong working

  'PYTHON'[:2:-1] → 'PY'

  Right working

  'PYTHON'[:2:-1] → 'NOH' (indexes 5, 4, 3)

  ✓Do this: For a negative step, default start = last index and move left until just before stop.

  Usually costs: 1 mark.

• ✗Mistake: Writing split() output without quotes or brackets

  print() of a list shows the list form, with square brackets and quotes around each string.

  Wrong working

  print('a b'.split()) → a b

  Right working

  print('a b'.split()) → ['a', 'b']

  ✓Do this: Show the exact printed form of lists and tuples.

  Usually costs: Half to 1 mark.

Practice questions on String Slicing and String Methods

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 string method returns -1 when the substring is not found?
   (a) index() (b) find() (c) count() (d) split()

   Hint

   One is friendly, one is strict.

   Answer

   (b) find()

2. Q2 · 1 mark · Very short answerBoard level

   Write the output:

   print('Hello World'.lower().count('o'), 'abc' * 2)

   Hint

   Count after lowering; * repeats.

   Answer

   2 abcabc

3. Q3 · 1 mark · MCQBoard level

   What is 'Python'[1:4]?
   (a) 'Pyt' (b) 'yth' (c) 'ytho' (d) 'thon'

   Hint

   Start in, stop out.

   Answer

   (b) 'yth'

4. Q4 · 2 marks · Short answerHarder board level

   Write the output:

   print(' cbse 2026 '.strip().title(), '-'.join(['a', 'b']))
   print('mississippi'.replace('ss', 'SS', 1), 'mississippi'.count('ss'))

   Hint

   The third argument of replace limits the number of replacements.

   Answer

   Cbse 2026 a-b
   miSSissippi 2

5. Q5 · 3 marks · Short answerHarder board level

   Write a function mask_vowels(s) that returns a new string in which every vowel (upper or lower case) of s is replaced by '*'. Show the result for 'Education'.

   Hint

   Strings are immutable: build a new one.

   Answer

   def mask_vowels(s):
       res = ''
       for ch in s:
           if ch.lower() in 'aeiou':
               res += '*'
           else:
               res += ch
       return res
   print(mask_vowels('Education'))

   Output: *d*c*t**n

6. Q6 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): After s = 'cbse' and s.upper(), print(s) displays CBSE.
   Reason (R): Strings in Python are immutable.
   (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

   Was the result of upper() stored?

   Answer

   (d) A is false (it prints cbse, because upper() returns a new string that is not stored) but R is true.

7. Q7 · 2 marks · Short answerHOTS / competency

   Predict what happens, line by line:

   s = 'banana'
   print(s.find('n', 3), s.index('a', 2))
   print(s.find('x'))
   print(s.index('x'))

   Hint

   The second argument is where the search starts.

   Answer

   4 3
   -1
   then ValueError: substring not found (index() raises an error where find() returns -1).

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 is 'abcdef'[::-2]?Show answer

   'fdb'.

2. 2.What does 'a,b,,c'.split(',') return?Show answer

   ['a', 'b', '', 'c'].

3. 3.What does partition() return if the separator is missing?Show answer

   A tuple of the whole string and two empty strings, e.g. ('abc', '', '').

4. 4.What is the difference between capitalize() and title()?Show answer

   capitalize() makes only the first character of the string upper case (the rest lower); title() capitalises the first letter of every word.

5. 5.What does ''.isalpha() return?Show answer

   False.

6. 6.What happens on s[0] = 'X' for a string s?Show answer

   TypeError: 'str' object does not support item assignment.

String Slicing and String Methods: frequently asked questions