Revision of Python Topics from Class XI: CBSE Class 12 Computer Science

What Revision of Python Topics from Class XI is about

Weightage: This chapter is part of Unit 1, Computational Thinking and Programming - 2, which carries 40 of the 70 theory marks together with Functions, Exception Handling in Python, File Handling in Python and Stack.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 1 Computational Thinking and Programming - 2

Key concepts

Data types and mutability

NCERT section Class XI NCERT 5.7 (data types), 8.3, 9.2, 10.1

Every value in Python has a type. Numbers (int, float, complex), bool, str, tuple and None are immutable: an operation that seems to change them actually builds a new object. list and dict are mutable: they can be changed in place. Dictionary keys must be immutable, so a list can never be a key, while a tuple can. A tuple holding a list is still a tuple: you cannot replace its items, but the list inside can grow.

Worked example

Which of these statements raise an error? s = 'abc'; s[0] = 'A' (i), L = [1, 2]; L[0] = 5 (ii), t = (1, [2, 3]); t[1].append(4) (iii), d = {[1, 2]: 'x'} (iv)

Try it first, then show the solutionSolution

1. (i) A string is immutable, so item assignment fails: TypeError.
2. (ii) A list is mutable: L becomes [5, 2]. No error.
3. (iii) The tuple is not changed (it still holds the same list object); the list inside grows, t becomes (1, [2, 3, 4]). No error.
4. (iv) A list is unhashable, so it cannot be a key: TypeError.

Answer: (i) and (iv) raise TypeError; (ii) and (iii) run.

Arithmetic operators and precedence

NCERT section Class XI NCERT 5.8, 5.9

Precedence from highest to lowest: **, then unary -, then * / // %, then + -, then relational operators, then not, and, or. Operators of the same level work left to right, except **, which works right to left: 2 ** 3 ** 2 is 2 ** 9 = 512. / always gives a float (8 / 2 is 4.0). // gives the floor (rounds towards minus infinity), so -7 // 2 is -4, and % takes the sign of the divisor, so -7 % 2 is 1 and 7 % -2 is -1. The rule that ties them: a == (a // b) * b + a % b.

Worked example

Evaluate: (a) 10 - 4 * 2 // 3 + 5 % 3 (b) -7 // 2, -7 % 2 (c) 2 ** 3 ** 2

Try it first, then show the solutionSolution

1. (a) *, // and % first, left to right: 4 * 2 = 8, 8 // 3 = 2, 5 % 3 = 2. Then 10 - 2 + 2 = 10.
2. (b) -7 / 2 = -3.5; the floor of -3.5 is -4. Check: (-4) * 2 + r = -7 gives r = 1.
3. (c) Right to left: 3 2 = 9, then 2 9 = 512.

Answer: (a) 10 (b) -4 1 (c) 512

Relational and logical operators (short-circuit)

NCERT section Class XI NCERT 5.8.3

Relational operators can be chained: 1 < x < 5 means 1 < x and x < 5. The logical operators do not always give True or False: x or y returns x if x is truthy, else y; x and y returns x if x is falsy, else y. Falsy values are 0, 0.0, '', [], (), {}, None and False; everything else is truthy. Python stops evaluating as soon as the result is known (short-circuit), so 0 and 1/0 gives 0 without an error. not binds tighter than and, which binds tighter than or.

Worked example

Predict the output: print(0 or 'Hi', 3 and 0, '' or [] or 'x') and print(not 5 > 3 or 2 > 1 and 0)

Try it first, then show the solutionSolution

1. 0 or 'Hi': 0 is falsy, so the result is 'Hi'.
2. 3 and 0: 3 is truthy, so the result is the second operand, 0.
3. '' or [] or 'x': both '' and [] are falsy, so 'x'.
4. Second line: not 5 > 3 is False; 2 > 1 and 0 is True and 0, which is 0; False or 0 is 0.

Answer: Hi 0 x
0

Flow of control: if, for, while, break and continue

NCERT section Class XI NCERT 6.3-6.5

range(start, stop, step) stops before stop. break leaves the innermost loop at once; continue skips the rest of the current pass and goes to the next one. After a for loop ends (normally or by break), the loop variable keeps its last value. In a while loop, check where the variable changes: before or after the print decides the output.

Worked example

Predict the output:

x = 10
while x > 0:
    x -= 3
    if x % 2 == 0:
        continue
    print(x, end=' ')

Try it first, then show the solutionSolution

1. Pass 1: x = 7, odd, prints 7.
2. Pass 2: x = 4, even, continue (nothing printed).
3. Pass 3: x = 1, odd, prints 1.
4. Pass 4: x = -2, even, continue. Now x > 0 is False, so the loop ends.

Answer: 7 1

Strings: indexing, slicing and methods

NCERT section Class XI NCERT 8.2-8.7

A string of length n has indices 0 to n-1 and -1 to -n. A slice s[a:b:c] starts at a, stops before b, steps by c; a missing a or b means "from the end in the direction of the step", and a slice never raises IndexError (s[10:] on a short string is just ''). Methods return new strings and never change s: upper(), lower(), title(), capitalize(), strip(), replace(old, new[, count]), split([sep]), sep.join(list), partition(sep). Searching: find() returns -1 when the text is absent, but index() raises ValueError. count() counts non-overlapping occurrences. Tests: isalpha(), isdigit(), isalnum(), isupper(), islower(), isspace(), startswith(), endswith().

Worked example

If s = 'Board Exam 2027', find s[2:8], s[-4:], s[::-2], s.split('a') and s.find('a').

Try it first, then show the solutionSolution

1. Index the characters: B0 o1 a2 r3 d4 (space)5 E6 x7 a8 m9 (space)10 2(11) 0(12) 2(13) 7(14).
2. s[2:8] takes indices 2 to 7: 'ard Ex'. s[-4:] is the last four characters: '2027'.
3. s[::-2] starts at index 14 and goes back two at a time: 14, 12, 10, 8, 6, 4, 2, 0, giving '70 aEdaB'.
4. split('a') cuts at each 'a' and drops it: ['Bo', 'rd Ex', 'm 2027']. find('a') gives the first index, 2.

Answer: 'ard Ex', '2027', '70 aEdaB', ['Bo', 'rd Ex', 'm 2027'], 2

Lists: methods, slicing and aliasing

NCERT section Class XI NCERT 9.2-9.7

append(x) adds one item at the end; extend(seq) adds every item of seq; insert(i, x) puts x before index i; pop() removes and returns the last item (or the item at an index); remove(x) deletes the first x (ValueError if absent); sort() sorts in place and returns None, while sorted(L) returns a new list. Aliasing: M = L gives a second name for the same list, so a change through M shows in L. A copy (L[:], list(L) or L.copy()) is a separate list. L = L + [4] builds a new list and rebinds L, but L += [4] changes the list in place, so an alias sees it.

Worked example

Predict the output:

L = [10, 20, 30, 40]
M = L
M.append(50)
N = L[:]
N.append(60)
print(L, N)
print(L.pop(), L.pop(0), L)

Try it first, then show the solutionSolution

1. M is an alias of L, so appending 50 through M changes L: [10, 20, 30, 40, 50].
2. N is a copy of that list; appending 60 changes only N: [10, 20, 30, 40, 50, 60].
3. Arguments are evaluated left to right: L.pop() returns 50, then L.pop(0) returns 10, leaving [20, 30, 40].

Answer: [10, 20, 30, 40, 50] [10, 20, 30, 40, 50, 60]
50 10 [20, 30, 40]

Tuples and dictionaries

NCERT section Class XI NCERT 10.1-10.6

A tuple is an ordered, immutable sequence. A one-item tuple needs a comma: (5,) is a tuple, (5) is just the int 5. Tuples support indexing, slicing, +, *, in, len(), count() and index(), and unpacking (a, b = b, a swaps). A dictionary maps unique, immutable keys to values. Assigning to an existing key replaces its value; to a new key adds a pair. d[k] raises KeyError for a missing key but d.get(k) returns None (or a default you give). Iterating over a dictionary gives its keys in insertion order. Useful methods: keys(), values(), items(), update(), pop(k), popitem(), setdefault(k, v), dict.fromkeys(keys, v), clear(), copy().

Worked example

Predict the output:

D = {'A': 1, 'B': 2, 'C': 3}
L = []
for k in D:
    D[k] = D[k] * 2
    L.append(k * D[k])
print(D)
print(L)

Try it first, then show the solutionSolution

1. k = 'A': D['A'] becomes 2; 'A' * 2 = 'AA' is appended.
2. k = 'B': D['B'] becomes 4; 'BBBB' is appended.
3. k = 'C': D['C'] becomes 6; 'CCCCCC' is appended. Changing values (not keys) while looping is allowed.

Answer: {'A': 2, 'B': 4, 'C': 6}
['AA', 'BBBB', 'CCCCCC']

Modules: math, random and statistics

NCERT section Class XI NCERT 7.5 (modules)

Import with import math (then math.sqrt(16)) or from math import sqrt. math: sqrt() and pow() return floats, ceil() and floor() return ints, pi is a constant. random: random() gives a float in [0, 1); randint(a, b) gives an int from a to b including b; randrange(a, b) excludes b (like range). statistics: mean(), median() (middle of the sorted data, or the average of the two middle values) and mode(). In possible-output questions, list the smallest and largest value each random call can take, then test every option against those limits.

Worked example

What possible outputs can this code give? Also find the maximum value of start and the minimum value of end.

import random
A = ['RED', 'BLUE', 'GREEN', 'PINK']
start = random.randint(1, 2)
end = random.randint(2, 3)
for i in range(start, end + 1):
    print(A[i], end='#')

(a) RED#BLUE# (b) BLUE#GREEN#PINK# (c) GREEN#PINK# (d) PINK#

Try it first, then show the solutionSolution

1. start can be 1 or 2; end can be 2 or 3. So printing begins at A[1] or A[2] and ends at A[2] or A[3].
2. (a) starts at A[0]: impossible. (d) starts at A[3]: impossible.
3. (b) is start = 1, end = 3. (c) is start = 2, end = 3. Both possible.
4. Maximum start = 2, minimum end = 2.

Answer: (b) and (c); maximum start = 2, minimum end = 2.

Revision of Python Topics from Class XI in real life

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

Topics in this chapter

• List Methods and Operations
• Dictionary Methods and Operations
• String Slicing and String Methods

Revision of Python Topics from Class XI: syntax and key facts

Operator precedence (high to low)

** > unary - > * / // % > + - > relational (< <= > >= == !=, in, is) > not > and > or

Same level: left to right, except **, which is right to left.

Floor division and remainder

a == (a // b) * b + a % b; // rounds down (towards minus infinity); % takes the sign of b

-7 // 2 = -4 and -7 % 2 = 1; 7 // -2 = -4 and 7 % -2 = -1.

Slicing

seq[start:stop:step]: from start up to stop - 1; negative step walks backwards; seq[::-1] reverses

Out-of-range slices give an empty result, never an error; out-of-range indexing raises IndexError.

range()

range(stop), range(start, stop), range(start, stop, step): stop is never included

range(10, 0, -3) gives 10, 7, 4, 1.

random functions

randint(a, b)	a to b inclusive
randrange(a, b)	a to b - 1
random()	0.0 up to but not including 1.0

int(random.random() * n) gives 0 to n - 1.

find() versus index()

s.find(x) returns -1 if x is absent; s.index(x) raises ValueError

Lists have index() but no find().

Falsy values

0, 0.0, '', [], (), {}, None, False

x or y returns the first truthy operand (or the last one); x and y returns the first falsy operand (or the last one).

How the board asks Revision of Python Topics from Class XI

The question patterns to expect, and the method that works for each.

Tricks to remember Revision of Python Topics from Class XI

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

• PEMDAS for Python: "Power, Unary, Multiply, Add, Compare, Not, And, Or"

  Read it as "Please Use My Advice, Children Never Act Odd".

  Use it when: Order of evaluation in any expression question.

• Floor means floor

  // always rounds towards the floor (down the number line), never towards zero: -3.5 goes to -4.

  Use it when: Negative floor division and remainder questions.

• find is forgiving, index is strict

  find() quietly says -1; index() shouts ValueError.

  Use it when: Error-spotting and output questions on strings.

• Comma makes the tuple

  It is the comma, not the bracket, that makes a tuple: t = 5, is a tuple; (5) is not.

  Use it when: type() and single-item tuple questions.

• OR finds the first True, AND finds the first False

  or returns the first truthy value it meets; and returns the first falsy value it meets; if none, the last operand.

  Use it when: Output questions with and/or between non-Boolean values.

• sort() says nothing

  L.sort() sorts in place and returns None, so print(L.sort()) prints None; sorted(L) hands you a new list.

  Use it when: Output and error questions on lists.

Revision of Python Topics from Class XI: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

s = 'cbSE2027'
r = ''
for ch in s:
    if ch.isupper():
        r += ch.lower()
    elif ch.islower():
        r += ch.upper()
    elif ch.isdigit():
        r += str(int(ch) + 1)
print(r)

import random
P = [3, 6, 9, 12, 15]
N = random.randint(1, 3)
for i in range(N, 0, -1):
    print(P[i], end='@')

a = [1, 2, 3]
b = a
a = a + [4]
print(a, b)
c = [1, 2, 3]
e = c
c += [4]
print(c, e)

P = [3, 8, 1, 6]
P[1:3] = [0]
print(P, P * 2)
st = 'Python'
print(st[1:4] + st[-1] * 2, len(st[::3]))

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Num = int(input('Enter number')
For i in range(Num):
    if i%2=0:
        print(i)
    else
        print('odd')

Where students lose marks in Revision of Python Topics from Class XI

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

• ✗Mistake: Treating L = L + [x] and L += [x] as the same when an alias exists

  + builds a new list and rebinds only L, while += extends the existing list, which the alias also sees.

  Wrong working

  a = [1, 2, 3]; b = a; a = a + [4]
  print(b) gives [1, 2, 3, 4]

  Right working

  a = a + [4] makes a new list, so b still refers to the old one
  print(b) gives [1, 2, 3]

  ✓Do this: Ask: does this statement change the list object or make a new one? Only in-place changes (append, extend, +=, item assignment) show through an alias.

  Usually costs: 1 mark in an output question

• ✗Mistake: Writing the output of print() for a list or string without its exact format

  print() shows lists with square brackets, commas and quotes around strings, and a tuple with round brackets; a plain string prints without quotes.

  Wrong working

  print(['AA', 'BB']) gives AA BB

  Right working

  print(['AA', 'BB']) gives ['AA', 'BB']

  ✓Do this: Copy the punctuation print() would show, including quotes inside lists and dictionaries, and respect end= and sep=.

  Usually costs: Half to 1 mark

• ✗Mistake: Taking randint's upper limit as excluded

  Students mix up randint(a, b), which includes b, with randrange(a, b), which excludes b.

  Wrong working

  randint(1, 3) can give 1 or 2

  Right working

  randint(1, 3) can give 1, 2 or 3

  ✓Do this: Write the full list of values beside each random call before checking the options.

  Usually costs: The whole 2-mark possible-output question

• ✗Mistake: Rounding -7 // 2 towards zero

  In maths class -3.5 is often truncated to -3, but Python's floor division goes down to -4.

  Wrong working

  -7 // 2 = -3 and -7 % 2 = -1

  Right working

  -7 // 2 = -4 and -7 % 2 = 1 (because -4 × 2 + 1 = -7)

  ✓Do this: Check every // and % answer with a == (a // b) * b + a % b.

  Usually costs: 1 mark

• ✗Mistake: Correcting only the obvious error in an error-finding question

  The question usually hides three or four errors: a missing colon, = for ==, a capital letter in a keyword, a missing bracket.

  ✓Do this: Scan each line separately for each kind of error, and underline every change in the rewritten code: the marking scheme gives half a mark per correction.

  Usually costs: Half a mark per missed error

NCERT exercise guide: Revision of Python Topics from Class XI

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

1
1 2
1 2 3
1 2 3 4

Practice questions on Revision of Python Topics from Class XI

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 · Very short answerHarder board level

   Evaluate print(17 % -5, -17 // 5, 2 ** -1).

   Hint

   % takes the sign of the divisor; ** with a negative power gives a float.

   Answer

   -3 -4 0.5 (17 // -5 = -4 and -4 × -5 + (-3) = 17)

2. Q2 · 1 mark · MCQBoard level

   Which of these is a valid dictionary? (a) {[1]: 'a'} (b) {(1, 2): 'a'} (c) {{1: 2}: 'a'} (d) {1: 'a', [2]: 'b'}

   Hint

   Keys must be immutable.

   Answer

   (b) {(1, 2): 'a'}; a tuple can be a key, a list or dictionary cannot.

3. Q3 · 2 marks · Short answerBoard level

   Predict the output:

   T = (10, 20, 30, 40)
   print(T[1:3], T.index(30), 20 in T, len(T * 2))

   Hint

   Slicing a tuple gives a tuple.

   Answer

   (20, 30) 2 True 8

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output:

   n = 0
   for i in range(1, 10, 3):
       if i == 7:
           break
       n += i
   print(n, i)

   Hint

   range gives 1, 4, 7.

   Answer

   5 7 (n = 1 + 4; the loop breaks when i is 7, and i keeps that value)

5. Q5 · 2 marks · Short answerHOTS / competency

   Predict the output:

   d = {'Ravi': 78, 'Meena': 90}
   print(d.setdefault('Kiran', 60), d.setdefault('Ravi', 50))
   print(d)

   Hint

   setdefault() adds the key only if it is missing and returns the value now stored.

   Answer

   60 78
   {'Ravi': 78, 'Meena': 90, 'Kiran': 60}

6. Q6 · 3 marks · Short answerBoard level

   Write a program that reads a list of integers and builds a new list holding the squares of only the even numbers, in the same order. Show the result for [3, 4, 7, 10].

   Hint

   Loop, test x % 2 == 0, append x * x.

   Answer

   L = [3, 4, 7, 10]
   sq = []
   for x in L:
       if x % 2 == 0:
           sq.append(x * x)
   print(sq)

   Output: [16, 100]

7. Q7 · 2 marks · Short answerHOTS / competency

   What possible outputs can this code give? (a) 30#40# (b) 10#20# (c) 20#30#40# (d) 40#

   import random
   L = [10, 20, 30, 40]
   k = random.randrange(1, 3)
   for i in range(k, 4):
       print(L[i], end='#')

   Hint

   randrange(1, 3) gives 1 or 2.

   Answer

   k = 1 prints 20#30#40#; k = 2 prints 30#40#. So (a) and (c).

8. Q8 · 2 marks · Short answerHarder board level

   Rewrite the code after correcting the errors:

   def = {'a': 1, 'b': 2}
   for k in def.keys
       Print(k, def[k])

   Hint

   A keyword cannot be a name; methods need brackets.

   Answer

   d = {'a': 1, 'b': 2}
   for k in d.keys():
       print(k, d[k])

   Corrections: def is a keyword so it is renamed d (both uses); keys needs (); colon after the for line; Print should be print.

9. Q9 · 3 marks · Short answerHarder board level

   Write a program that reads a string and prints it with every vowel replaced by '*'. For 'Computer', what is printed?

   Hint

   Build a new string; strings cannot be changed in place.

   Answer

   s = 'Computer'
   r = ''
   for ch in s:
       if ch in 'aeiouAEIOU':
           r += '*'
       else:
           r += ch
   print(r)

   Output: C*mp*t*r

10. Q10 · 2 marks · Short answerHOTS / competency

    Predict the output:

    x, y = 5, 9
    x, y = y, x + y
    print(x, y)
    import statistics
    print(statistics.median([3, 1, 4, 1, 5]), statistics.mode([1, 2, 2, 3]))

    Hint

    The right side is evaluated fully before assignment; the median needs the sorted list.

    Answer

    9 14 (x + y uses the old x = 5)
    3 2 (sorted list 1, 1, 3, 4, 5 has middle value 3; 2 occurs most often)

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 2 ** 3 ** 2 give, and why?Show answer

   512, because ** works right to left: 3 2 = 9, then 2 9.

2. 2.Name three immutable and two mutable types.Show answer

   Immutable: int, float, str, tuple, bool. Mutable: list, dict.

3. 3.What is the difference between s.find('x') and s.index('x') when 'x' is not in s?Show answer

   find() returns -1; index() raises ValueError.

4. 4.How do you write a tuple with one element 7?Show answer

   (7,) or 7,; (7) is just the integer 7.

5. 5.What does d.get('k', 0) return if 'k' is not a key?Show answer

   0, the default given; without a default it returns None. d['k'] would raise KeyError.

6. 6.What does print([1, 2, 3].sort()) print?Show answer

   None, because sort() sorts in place and returns None.

7. 7.What values can random.randrange(2, 8, 2) give?Show answer

   2, 4 or 6 (like range(2, 8, 2), 8 is excluded).

8. 8.Give the output of 'hello world'.title() and 'hello world'.capitalize().Show answer

   'Hello World' and 'Hello world'.

9. 9.What is the value of -17 % 5?Show answer

   3, because -17 // 5 = -4 and -4 × 5 + 3 = -17.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Immutable: int, float, str, tuple, bool, None. Mutable: list, dict.
• ☐** is right-associative; / always returns a float; // floors; % takes the divisor's sign.
• ☐or returns the first truthy operand, and the first falsy one; evaluation stops early.
• ☐range(a, b, c) never includes b; the loop variable keeps its last value after the loop.
• ☐Slices never raise IndexError; indexing past the end does.
• ☐find() gives -1, index() raises ValueError; split() with no argument splits on any whitespace.
• ☐append() adds one item, extend() adds each item; sort() returns None; sorted() returns a new list.
• ☐M = L is an alias; L[:], list(L) and L.copy() are copies.
• ☐(5,) is a tuple; (5) is an int. Dictionary keys must be immutable.
• ☐randint(a, b) includes b; randrange(a, b) excludes b.

Revision of Python Topics from Class XI: frequently asked questions