Python Basics for Class 11 & 12: Important Concepts, Programs and PYQ Practice

1. What CBSE asks you to prepare

Class XI school examinations are set by schools, so the exact annual paper and marks distribution can vary. Class XII board papers follow the published CBSE curriculum and sample-paper pattern. Across the Python portions, students should be comfortable reading code, predicting output, correcting errors and writing a complete function or program from a situation.

What “PYQ” means on this page

The worked questions below are original practice questions based on recurring CBSE question patterns, with some tasks adapted in wording and data for learning. They are not presented as verbatim copies of every question from every board-paper set. The stack push/pop practice reflects the type of stack-function question found in the official CBSE Class XII Computer Science 2025 paper; use the linked original paper for exact wording. Class XI papers are generally school-set rather than one common nationwide CBSE board paper. For authentic paper wording, use the official CBSE sample papers, marking schemes and previous-year question-paper archive linked at the end.

2. Python fundamentals

2.1 A first program, comments and indentation

print("Hello, Python!")
# This is a comment
marks = 87
print("Marks =", marks)

print() displays output. A comment begins with # and is ignored by the interpreter. Indentation (spaces at the beginning of a line) defines a block in statements such as if, for, while and def. Use consistent indentation—normally four spaces.

2.2 Tokens, identifiers and keywords

• Keywords: reserved words such as if, else, for, while, def, return, True and None.
• Identifiers: names for variables and functions. They may contain letters, digits and underscores, but cannot begin with a digit. Python is case-sensitive.
• Literals: fixed values such as 25, 3.5, "CBSE" and True.
• Operators: symbols/words that perform operations, such as +, // and in.
• Punctuators: symbols such as parentheses, commas, colons and brackets.

2.3 Core data types

2.4 Input and type conversion

input() returns text (a string), even if the user types digits. Convert the value when arithmetic is required.

name = input("Enter name: ")
age = int(input("Enter age: "))
percentage = float(input("Enter percentage: "))
print(name, age, percentage)

Examples of explicit conversion: int("25") gives 25; float("4.5") gives 4.5; str(18) gives "18". Converting "4.5" directly to int raises a ValueError.

3. Operators and expressions

3.1 Division: a frequent output question

print(17 / 5)
print(17 // 5)
print(17 % 5)
print(2 ** 3 ** 2)

/ performs true division; // gives floor division; % gives the remainder. Exponentiation associates from right to left, so 2 ** 3 ** 2 means 2 ** (3 ** 2).

3.2 Trace the expression carefully

a = 7 + 3 * 2 ** 2
print(a)

Exponentiation is evaluated first: 2 ** 2 = 4; then multiplication: 3 * 4 = 12; then addition: 7 + 12 = 19. Parentheses can make the intended order clearer.

3A. Python modules: math, random and statistics

The current Class XI Computer Science syllabus lists the math, random and statistics modules. Import a module before using its functions, or import selected functions directly.

import math
import random
import statistics

print(math.sqrt(81))
print(math.ceil(4.2))
print(math.floor(4.8))
print(random.randint(1, 6))
print(statistics.mean([10, 20, 30]))

The random result can vary each time the program runs. randint(1, 6) can return either endpoint, including 1 and 6. Other syllabus-listed examples include math.pi, math.e, math.pow(), random.randrange(), statistics.median() and statistics.mode().

from math import sqrt, ceil
print(sqrt(49))
print(ceil(3.1))

4. Conditional statements and loops

4.1 if, if–else and if–elif–else

marks = 76
if marks >= 90:
    grade = "A1"
elif marks >= 75:
    grade = "A2"
elif marks >= 60:
    grade = "B"
else:
    grade = "C"
print(grade)

Conditions are checked from top to bottom. The first true branch runs; later branches are skipped. A colon and an indented block are required.

4.2 for loop and range()

for i in range(2, 8, 2):
    print(i, end=" ")

range(start, stop, step) includes the start (when reached) but excludes the stop. Therefore, range(2, 8, 2) produces 2, 4 and 6.

4.3 while loop, break and continue

n = 1
while n <= 5:
    if n == 3:
        n += 1
        continue
    print(n, end=" ")
    n += 1

continue skips the remaining statements in the current iteration. break exits the loop. In a while loop, update the control variable so that the loop can finish.

4.4 Classic programming questions

n = int(input("Enter a non-negative integer: "))
fact = 1
for i in range(1, n + 1):
    fact *= i
print("Factorial =", fact)

n = int(input("Enter an integer: "))
if n < 2:
    print("Not prime")
else:
    is_prime = True
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            is_prime = False
            break
    print("Prime" if is_prime else "Not prime")

n = int(input("Enter a positive integer: "))
total = 0
while n > 0:
    total += n % 10
    n //= 10
print(total)

5. Strings: indexing, slicing and methods

A string is an immutable sequence of characters. Indexing starts at 0 from the left and at −1 from the right. A slice follows text[start:stop:step]; the stop index is excluded.

s = "COMPUTER"
print(s[0], s[-1])
print(s[1:5])
print(s[::-1])
print("put" in s.lower())

The final line converts the string to lowercase before checking membership.

5.1 Useful string methods

5.2 Exam-style output: partition()

print("Amrit Mahotsav @ 75".partition(" "))

partition() splits at the first occurrence of the separator and returns a tuple containing the part before it, the separator itself and the part after it. If the separator is absent, the original string is the first item and the other two items are empty strings.

6. Lists

A list is an ordered, mutable collection. It can contain mixed data types and nested lists. Lists are widely used in Class XI questions and to implement a stack in Class XII.

marks = [72, 88, 65, 91]
marks.append(79)
marks.insert(1, 80)
marks.remove(65)
print(marks)
print(marks[1:4])
print(sum(marks), max(marks), min(marks))

6.1 List methods you should know

6.2 append() versus extend()

a = [1, 2]
a.append([3, 4])
print(a)

b = [1, 2]
b.extend([3, 4])
print(b)

6.3 Linear search and frequency count

numbers = [4, 7, 2, 7, 9]
target = 7
if target in numbers:
    print("Found at index", numbers.index(target))
else:
    print("Not found")

print("Frequency =", numbers.count(target))

For a question that asks you to write linear search yourself, use a loop and compare each element with the target rather than relying on index().

numbers = [4, 7, 2, 7, 9]
target = int(input("Search for: "))
position = -1
for i in range(len(numbers)):
    if numbers[i] == target:
        position = i
        break
if position == -1:
    print("Not found")
else:
    print("Found at index", position)

6.4 A subtle mutability question

a = [10, 20, 30]
b = a
b[0] = 99
print(a)

b = a does not make a separate list; both names refer to the same list. Use b = a.copy() or b = a[:] for a shallow copy.

7. Tuples

A tuple is an ordered, immutable sequence. It supports indexing, slicing, membership, iteration and methods such as count() and index().

t = (10, 20, 10, 40)
print(t[1])
print(t.count(10))
print(t.index(40))
print(t[1:3])
single = (5,)
print(type(single))

The comma creates a single-item tuple: (5,). (5) is simply the integer 5 inside parentheses.

Nested list and tuple: a classic misconception

a = [1, 2, (3, 4)]
a[2][1] = 5

This raises a TypeError. The outer list is mutable, but the item at index 2 is a tuple, and tuple elements cannot be reassigned. A list containing a tuple does not make the tuple itself mutable.

8. Dictionaries

A dictionary stores key–value pairs. Keys must be hashable and unique; assigning a value to an existing key updates that value. Dictionaries are mutable.

marks = {"Aarav": 88, "Diya": 94}
marks["Kabir"] = 81
marks["Aarav"] = 90
print(marks["Diya"])
print(marks.get("Riya", "Not found"))
print(list(marks.keys()))
print(list(marks.values()))
print(list(marks.items()))

8.1 Important dictionary methods

8.2 Count each character using a dictionary

This is a useful long-answer pattern because it combines a loop, a condition and dictionary updates.

text = "banana"
freq = {}
for ch in text:
    if ch in freq:
        freq[ch] += 1
    else:
        freq[ch] = 1
print(freq)

Shorter equivalent: freq[ch] = freq.get(ch, 0) + 1 inside the loop. If a question asks for a case-insensitive count, process text.lower() first.

8.3 Traverse a dictionary

salary = {"Aman": 25000, "Riya": 30000}
for name, amount in salary.items():
    print(name, amount)

Use for key in d to iterate over keys or for key, value in d.items() to access both components.

9. Functions: parameters, return values and scope

A function is a named, reusable block of code. Define it with def. Parameters receive inputs; return sends a result back to the caller. print() displays a value but does not return it as the function's result.

def area_rectangle(length, breadth):
    return length * breadth

answer = area_rectangle(8, 5)
print(answer)

9.1 print() versus return

def first():
    print(5)

def second():
    return 5

x = first()
y = second()
print(x, y)

The first function displays 5 and then implicitly returns None. The second returns 5 to the caller.

9.2 Default and positional arguments

def greet(name, message="Welcome"):
    print(message, name)

greet("Asha")
greet("Asha", "Good morning")

9.3 Local and global variables

x = 10

def show():
    x = 4
    print(x)

show()
print(x)

The assignment inside show() creates a local variable. It does not change the global x. Avoid using global unless the question specifically requires it.

9.4 A function to return the largest of three numbers

def largest(a, b, c):
    if a >= b and a >= c:
        return a
    elif b >= a and b >= c:
        return b
    else:
        return c

print(largest(12, 27, 19))

9.5 Recursion: understand the base case

def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

print(factorial(4))

Each recursive call reduces the problem. The base case stops the calls. Whether recursion is part of your examinable syllabus depends on the prescribed course/session; learn it if your teacher or syllabus includes it.

10. Text file handling (Class XII Computer Science)

File questions often give a scenario and ask you to count, display, search or process words, lines or characters. Learn the difference between reading the whole file, reading one line and traversing line by line.

Prefer with open(...): it closes the file automatically when the block ends.

10.1 Read and display a file

with open("Notes.txt", "r", encoding="utf-8") as f:
    data = f.read()
    print(data)

Common reading methods:

• read() reads the remaining content as one string.
• readline() reads one line.
• readlines() returns a list of lines.
• for line in f: processes the file one line at a time.

10.1A File pointer: seek() and tell()

tell() returns the current position of the file pointer. seek(position) moves the pointer to a specified position.

with open("Notes.txt", "r", encoding="utf-8") as f:
    print(f.tell())
    first = f.read(5)
    print(first)
    print(f.tell())
    f.seek(0)
    print(f.readline())

The initial pointer position is normally 0. After reading, the position advances; seek(0) returns it to the beginning.

10.2 Count words in a text file

def count_words():
    with open("Notes.txt", "r", encoding="utf-8") as f:
        text = f.read()
    words = text.split()
    print("Word count =", len(words))

count_words()

split() without an argument separates on runs of whitespace, so it handles multiple spaces and line breaks better than splitting on one literal space.

10.3 Count lines that contain a particular word

def count_python_lines():
    count = 0
    with open("Prog.txt", "r", encoding="utf-8") as f:
        for line in f:
            if "python" in line.lower():
                count += 1
    print("Matching lines =", count)

This counts matching lines, not every occurrence of the word. To count occurrences, split into words or use a suitable counting method and state how punctuation is handled.

10.4 Count vowels, uppercase letters and words

def analyse_file():
    vowels = 0
    uppercase = 0
    words = 0
    with open("Story.txt", "r", encoding="utf-8") as f:
        for line in f:
            words += len(line.split())
            for ch in line:
                if ch.lower() in "aeiou":
                    vowels += 1
                if ch.isupper():
                    uppercase += 1
    print("Vowels:", vowels)
    print("Uppercase:", uppercase)
    print("Words:", words)

Keep the counter initialisation outside the loop. If the task asks for words beginning with a particular letter, inspect each word after split().

10.5 Write to a file

with open("Result.txt", "w", encoding="utf-8") as f:
    f.write("Python is useful.\n")
    f.write("Practise every day.\n")

write() does not add a newline automatically. Include \n where a line break is needed. writelines() writes each string from an iterable, but it does not insert newline characters automatically.

11. CSV and binary file basics (Class XII Computer Science)

11.1 CSV files

CSV (comma-separated values) stores tabular records as text. Python's csv module handles quoted values and commas more reliably than manually splitting every line.

import csv

with open("students.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["RollNo", "Name", "Marks"])
    writer.writerow([1, "Aarav", 88])
    writer.writerow([2, "Diya", 94])

import csv

with open("students.csv", "r", newline="", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

Rows read by csv.reader are lists of strings. Convert numeric fields with int() or float() before calculations. If the syllabus specifically teaches csv.writer and csv.reader, practise their exact syntax.

11.2 Exception handling: try, except and finally

The current Class XII Computer Science syllabus includes handling exceptions. A try block contains code that may fail; except handles a specified error; finally runs whether an exception occurs or not.

try:
    number = int(input("Enter a number: "))
    print(100 / number)
except ValueError:
    print("Please enter an integer.")
except ZeroDivisionError:
    print("Cannot divide by zero.")
finally:
    print("Program finished.")

Use specific exceptions where possible. Avoid a broad except: unless the question explicitly calls for it.

11.3 File modes at a glance

11.4 Binary files and pickle

In CBSE Computer Science, binary-file programs commonly use the pickle module to write and retrieve Python records. The key operations are pickle.dump() to write and pickle.load() to read.

import pickle

record = [101, "Meera", 91]
with open("student.dat", "wb") as f:
    pickle.dump(record, f)

import pickle

try:
    with open("student.dat", "rb") as f:
        while True:
            record = pickle.load(f)
            print(record)
except EOFError:
    pass

EOFError signals that no more pickled objects remain. In a question requiring search or update, read each record, test the specified field and display or write the matching record as requested. When rewriting a binary file, take care not to lose records that do not match the update condition.

import pickle

def search_student(roll_to_find):
    found = False
    try:
        with open("student.dat", "rb") as f:
            while True:
                rec = pickle.load(f)
                if rec[0] == roll_to_find:
                    print(rec)
                    found = True
    except EOFError:
        pass
    if not found:
        print("Record not found")

12. Stack using a Python list

A stack follows LIFO—Last In, First Out. The most recently pushed item is the first one popped. A Python list is commonly used to implement a stack.

• Push: insert an item at the top, usually with append().
• Pop: remove and return the top item, usually with pop().
• Underflow: attempt to pop from an empty stack.
• Display/peek: inspect items or the top without deleting them, according to the question.

stack = []
stack.append("Red")
stack.append("Blue")
stack.append("Green")
print(stack)
print("Popped:", stack.pop())
print(stack)

12.1 Exam-ready push, pop and display functions

def push_item(stack, item):
    stack.append(item)

def pop_item(stack):
    if len(stack) == 0:
        print("Stack Underflow")
        return None
    return stack.pop()

def display_stack(stack):
    if len(stack) == 0:
        print("Empty Stack")
    else:
        for item in reversed(stack):
            print(item)

Use the exact function names and empty-stack messages stated in the question. If the task says display all items without deleting them, do not call pop() during display.

12.2 Push the last five elements of a list

def push_trail(N, myStack):
    for item in N[-5:]:
        myStack.append(item)

N = [1, 2, 3, 4, 5, 6, 7]
myStack = []
push_trail(N, myStack)
print(myStack)

N[-5:] selects the last five elements in their original order. This is a concise solution when the question guarantees that the list contains at least five elements.

13. Common errors and how to avoid them

Find the mistake

for i in range(1, 5)
    print(i)

Correction: place a colon after range(1, 5).

age = input("Age: ")
print(age + 1)

Correction: use age = int(input("Age: ")) if an integer age is intended.

14. Solved CBSE PYQ-style practice set

These questions mirror the formats that appear in school and board-style Computer Science papers: MCQs, output prediction, short code tracing, function-writing, file handling and stack operations. They are rewritten practice items, not a claim that each line appeared verbatim in an official paper.

x = 7 * 8 / 5 // 2
print(x)

s = "This is Computer Science"
print(s.find("i"))

a = [2, 4, 6]
print(a.append(8))
print(a)

total = 0
for i in range(1, 6):
    if i % 2 == 0:
        total += i
print(total)

word = "Mississippi"
print(word.count("ss"))
print(word.replace("ss", "X"))

d = {"A": 2, "B": 3}
d["A"] = d["A"] + 4
d["C"] = 5
print(len(d), d["A"])

def count_vowels(text):
    count = 0
    for ch in text.lower():
        if ch in "aeiou":
            count += 1
    return count

print(count_vowels("Computer Science"))

numbers = [12, 18, 9, 21, 15]
if len(numbers) > 0:
    print("Maximum =", max(numbers))
    print("Minimum =", min(numbers))
    print("Average =", sum(numbers) / len(numbers))
else:
    print("List is empty")

def even_numbers(values):
    result = []
    for value in values:
        if value % 2 == 0:
            result.append(value)
    return result

print(even_numbers([3, 8, 11, 14, 20]))

def count_python():
    count = 0
    with open("Prog.txt", "r", encoding="utf-8") as f:
        words = f.read().lower().split()
    for word in words:
        word = word.strip(".,!?;:()[]{}\"'")
        if word == "python":
            count += 1
    print("Python occurs", count, "times.")

def pop_one(myStack):
    if len(myStack) == 0:
        print("Stack Underflow")
        return None
    return myStack.pop()

value = input("Enter a string: ")
D = {}
for ch in value:
    if ch in D:
        D[ch] += 1
    else:
        D[ch] = 1
print(D)

import csv

with open("students.csv", "r", newline="", encoding="utf-8") as f:
    reader = csv.reader(f)
    header = next(reader, None)
    for row in reader:
        if row and float(row[2]) >= 80:
            print(row)

import pickle

def display_above_90():
    try:
        with open("student.dat", "rb") as f:
            while True:
                rec = pickle.load(f)
                if rec[2] > 90:
                    print(rec)
    except EOFError:
        pass

t = (1, 2, 3)
t[0] = 9

15. Last-minute revision checklist

• Know the difference between /, //, % and **.
• Practise string slicing, negative indexing, find(), count(), split() and partition().
• Know which list methods mutate the list and which built-in functions return a new result.
• Remember: strings and tuples are immutable; lists and dictionaries are mutable.
• Trace nested loops and range() boundaries without skipping iterations.
• Write functions with correct parameters, indentation and a suitable return.
• For text files, distinguish read(), readline(), readlines() and line-by-line processing.
• For CSV, use csv.reader/csv.writer and convert numeric strings before arithmetic.
• For binary files, know pickle.dump(), pickle.load(), wb, rb and EOFError.
• For stacks, remember LIFO, push with append(), pop with pop(), and check for underflow.
• Read the question carefully: preserve the requested function name, filename, field order and exact message.
• Practise official sample papers and marking schemes under timed conditions.

16. Official CBSE resources and further practice

Use the official resources below to confirm the topics for your academic session and to practise with authentic question papers and marking schemes.

• CBSE Curriculum 2026–27 — check the current Class XI/XII Computer Science or Informatics Practices syllabus for your subject.
• Official Computer Science syllabus PDF (2026–27) — includes Class XI Python and Class XII functions, exception handling, file handling and stacks.
• CBSE Class XII Sample Question Papers and Marking Schemes 2026–27 — check this page for the current session’s official sample materials.
• CBSE Class XII Sample Papers and Marking Schemes 2025–26 — useful for additional practice with the previous session’s sample paper.
• Official CBSE Class XII Computer Science board paper (2025, Code 91) — includes Python stack and function-writing questions.
• CBSE Previous-Year Question Papers — official question-paper archive; select the examination year and subject.
• CBSE Marking Schemes — use these to understand expected answer structure and mark allocation.
• CBSE Computer Science Learning Framework — supporting learning outcomes for topics such as file handling and stacks.

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