Introduction to Python: ISC Class 11 Computer Science Notes

What Introduction to Python is about

Weightage: Introduction to Python is topic 12 of the ISC Class XI Computer Science syllabus, in Section C - Python and Conceptual aspects. CISCE weights Section C at 15 of the 70 marks in Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Computer Science syllabus, examination year 2028, Class XI unit weightage

Key concepts

Python: features, installation, IDEs and modes

Features of Python:

1. High-level and easy to read: English-like syntax; indentation instead of braces.
2. Interpreted: there is no separate compile step for the programmer; the interpreter runs the program statement by statement, so a run-time error appears only when the faulty line is reached.
3. Dynamically typed: a variable's type is decided by the value it holds; no declarations.
4. Free and open source; cross-platform (Windows, Linux, macOS).
5. Large standard library and many extra packages.
6. Supports procedural and object oriented styles.

Installation: download from python.org and install (version 3.13 or above for the syllabus). The installer includes IDLE, Python's own simple IDE.

IDEs and editors: IDLE (simple, built in), PyCharm (full-featured), VS Code (editor with a Python extension), Spyder (scientific, variable explorer), Jupyter (notebooks: code and output in cells, popular for data analysis).

Modes:

• Interactive mode: type a statement at the >>> prompt and see the result immediately; good for testing small ideas; work is not saved.
• Script mode: write a program in a .py file, save it and run the whole file; used for real programs.

Worked example

State one advantage each of interactive and script mode. [2 marks]

Try it first, then show the solutionSolution

1. Interactive: immediate result of each statement; useful for testing and learning.
2. Script: the program is saved in a .py file and can be edited and run again.

Answer: Interactive mode gives instant feedback; script mode saves complete programs for reuse.

Character set, tokens, variables, l-value and r-value, comments

Character set: letters (A-Z, a-z), digits (0-9), special symbols (+ − * / ( ) [ ] { } : , . ' " # etc.), whitespace, and any Unicode character (Python 3 source is UTF-8).

Tokens (smallest units of a program):

1. Keywords: reserved words with special meaning: if, elif, else, for, while, break, continue, True, False, None, and, or, not, in, is, def, return, import.
2. Identifiers: names of variables and functions: letters, digits and underscore; cannot start with a digit; not a keyword; case-sensitive (Marks ≠ marks).
3. Literals: fixed values: 25, 3.14, 3+4j, 'Riya', True, None.
4. Operators: + − * / // % ** == != and or not in is ...
5. Punctuators: ( ) [ ] { } , : . ' " # = and so on.

Variables: names that refer to objects; created by assignment (marks = 87); the type comes from the value and can change (marks = 'A' later is allowed).

l-value and r-value: in x = x + 1, the r-value (right side) is evaluated to a value; the l-value (left side) must be a name (or element) that can receive a value. 5 = x is an error.

Multiple assignment: a, b = 17, 5; swapping: a, b = b, a.

Comments: # starts a single-line comment; triple-quoted strings are often used for multi-line notes (docstrings).

Worked example

Which of these are valid identifiers: total_2, 2total, for, _count, Total? [1 mark]

Try it first, then show the solutionSolution

1. 2total starts with a digit; for is a keyword.

Answer: Valid: total_2, _count and Total.

Data types; mutable and immutable types

Category	Type	Example
Number	int (any size)	16, −3, 10**20
	float	1.62, 2.5e3
	complex	3 + 4j (z.real, z.imag)
Boolean	bool	True, False
Sequence	str	'Riya'
	list	[78, 85, 92]
	tuple	(3, 4)
None	NoneType	None (no value)
Mapping	dict	{'name': 'Riya', 'class': 11}

type(x) tells the type of a value.

Mutable types can be changed in place after creation: list, dict (and set).

Immutable types cannot be changed in place; an 'update' creates a new object: int, float, complex, bool, str, tuple (and None).

marks[0] = 80 works on a list; name[0] = 'P' on a string raises TypeError.

# Data types, mutability and type conversion in Python
age = 16                      # int
height = 1.62                 # float
z = 3 + 4j                    # complex
ok = True                     # bool
name = "Riya"                 # str (immutable sequence)
marks = [78, 85, 92]          # list (mutable sequence)
point = (3, 4)                # tuple (immutable sequence)
info = {"name": "Riya", "class": 11}   # dict (mapping)
nothing = None                # NoneType

for v in (age, height, z, ok, name, marks, point, info, nothing):
    print(type(v).__name__, v)

print("abs(z) =", abs(z), " z.real =", z.real, " z.imag =", z.imag)

marks[0] = 80                 # lists can be changed in place
print("marks after change:", marks)
try:
    name[0] = "P"             # strings cannot
except TypeError as e:
    print("TypeError:", e)

print(int("42") + 8, float("2.5") * 2, str(7) + "7", int(9.99), round(2.675, 2))
print(3 + 2.5, type(3 + 2.5).__name__, True + 1)   # implicit conversion

Output:

int 16
float 1.62
complex (3+4j)
bool True
str Riya
list [78, 85, 92]
tuple (3, 4)
dict {'name': 'Riya', 'class': 11}
NoneType None
abs(z) = 5.0  z.real = 3.0  z.imag = 4.0
marks after change: [80, 85, 92]
TypeError: 'str' object does not support item assignment
50 5.0 77 9 2.67
5.5 float 2

(The TypeError is caught here only so that the program can show the message and continue.)

Worked example

After a = [5, 6]; b = a; b.append(7), what are a and b? Would the same happen with a tuple? [2 marks]

Try it first, then show the solutionSolution

1. b refers to the same list as a; append changes that list in place: both show [5, 6, 7].
2. A tuple is immutable and has no append; it cannot be changed in place.

Answer: a = b = [5, 6, 7]; a tuple could not be changed this way because it is immutable.

Console input and output; errors

Output: print(value1, value2, ..., sep=' ', end='\n'). sep sets the separator (default a space); end sets what follows (default a new line): print(x, end=' ') keeps output on one line.

Input: input(prompt) always returns a string; convert it: n = int(input('Enter a number: ')); r = float(input()).

Errors:

1. Syntax errors: the code breaks Python's grammar (missing colon after if, wrong indentation, unmatched brackets); the program does not start running.
2. Run-time errors (exceptions): occur while running: ZeroDivisionError (10 / 0), ValueError (int('abc')), TypeError ('5' + 5), NameError (using an undefined name), IndexError (list index out of range).
3. Logical errors: the program runs but gives wrong results (using / instead of //, wrong condition); found only by testing.

Worked example

Classify: (i) if x > 5 print(x); (ii) print(int('12a')); (iii) avg = a + b / 2 meant as an average. [2 marks]

Try it first, then show the solutionSolution

1. (i) Missing colon: syntax error.
2. (ii) ValueError at run time; (iii) runs but divides only b: logical error (should be (a + b) / 2).

Answer: (i) Syntax; (ii) run-time (ValueError); (iii) logical.

Operators, expressions, precedence and type conversion

Arithmetic: + − * (power) / (true division, always float) // (floor division, rounds down**) % (remainder, sign of the divisor).

Relational: < <= > >= == !=. Logical: and, or, not (they short-circuit and actually return one of the operands: 0 and 5 gives 0; 7 or 0 gives 7).

Assignment: =; augmented: += −= *= /= //= %= **=.

Identity: is, is not: same object? Membership: in, not in: is the value present in a sequence?

Unary (−x, not x), binary (a + b) and ternary (x if condition else y) operators.

Precedence (high to low): → unary −, + → * / // % → + − → relational and identity and membership → not → and → or. is right-associative (2 3 2 = 2 ** 9 = 512); most others are left to right.

Type conversion: implicit (int + float gives float; True counts as 1) and explicit with int(), float(), str(), bool(): int(9.99) = 9 (truncates); int('42') = 42.

# Operators and expressions in Python
a, b = 17, 5
print(a + b, a - b, a * b, a / b, a // b, a % b, a ** 2)
print(-17 // 5, -17 % 5)            # floor division rounds down
print(2 ** 3 ** 2)                  # ** is right-associative: 2 ** 9
print(10 - 4 - 3, 2 + 3 * 4)        # left to right; * before +

x = 10
x += 5; x *= 2; x //= 4             # augmented assignment
print("x =", x)

print(a > b and b > 0, a < b or b == 5, not a == 17)
print(0 and 5, 7 or 0, "" or "default")   # and/or return an operand

L1 = [1, 2]
L2 = [1, 2]
L3 = L1
print(L1 == L2, L1 is L2, L1 is L3)       # equality vs identity
print(3 in L1, "y" in "Python", 9 not in L1)

big = 50 if a > 10 else 0                 # conditional (ternary) expression
print("big =", big)

Output:

22 12 85 3.4 3 2 289
-4 3
512
3 14
x = 7
True True False
0 7 default
True False True
False True True
big = 50

Worked example

Evaluate: 7 // 2, 7 / 2, 7 % 2, 2 ** −1 and −7 // 2. [2 marks]

Try it first, then show the solutionSolution

1. 7 // 2 = 3; 7 / 2 = 3.5; 7 % 2 = 1; 2 ** −1 = 0.5.
2. −7 // 2 = −4 (floor of −3.5 rounds down).

Answer: 3, 3.5, 1, 0.5, −4.

Flow of control: indentation, if-elif-else, for, range(), while

Indentation (usually 4 spaces) marks a block: every statement in a block must be indented equally; a colon (:) ends the line that starts a block (if, elif, else, for, while). Wrong indentation is a syntax error (IndentationError).

Sequential flow: statements run one after another.

Conditional flow: if; if-else; if-elif-else (the first true condition's block runs).

Iterative flow:

• for variable in sequence: repeats for each item: for ch in 'Python', for x in [3, 5].
• range(stop), range(start, stop), range(start, stop, step): numbers from start up to but not including stop: range(2, 11, 3) gives 2, 5, 8.
• while condition: repeats while the condition is true.
• break exits the loop; continue skips to the next iteration.
• Nested loops: a loop inside a loop; print(x, end=' ') keeps a row on one line.

# Flow of control: input, if-elif-else, for with range(), while, break, continue, nested loops
n = int(input("Enter marks: "))
print()
if n >= 90:
    grade = "A"
elif n >= 75:
    grade = "B"
elif n >= 40:
    grade = "C"
else:
    grade = "Fail"
print("Grade:", grade)

print(list(range(5)), list(range(2, 11, 3)), list(range(10, 0, -4)))

total = 0
for i in range(1, 11):
    if i % 3 == 0:
        continue                 # skip multiples of 3
    total += i
print("sum of 1..10 without multiples of 3 =", total)

num, digits = 90817, 0
while num > 0:
    digits += 1
    num //= 10
print("digits =", digits)

for k in range(101, 200):
    if k % 7 == 0 and k % 9 == 0:
        print("first above 100 divisible by 7 and 9:", k)
        break

for row in range(1, 4):          # nested loops
    for col in range(1, row + 1):
        print(col, end=" ")
    print()

Output (input 82):

Enter marks: 
Grade: B
[0, 1, 2, 3, 4] [2, 5, 8] [10, 6, 2]
sum of 1..10 without multiples of 3 = 37
digits = 5
first above 100 divisible by 7 and 9: 126
1 
1 2 
1 2 3

Worked example

What does list(range(10, 0, −4)) give, and how many times does for i in range(1, 20, 4) run? [2 marks]

Try it first, then show the solutionSolution

1. range(10, 0, −4): 10, 6, 2 (stops before 0).
2. range(1, 20, 4): 1, 5, 9, 13, 17 → 5 times.

Answer: [10, 6, 2]; 5 times.

Introduction to Python 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

• Python Flow of Control: if-elif-else, for, range() and while
• Python: Tokens, Variables, Data Types, Operators, Precedence and Type Conversion

Introduction to Python: syntax and key facts

Division operators

/ true division (float); // floor division (rounds down); % remainder; a == (a // b) * b + a % b

range()

range(start, stop, step): start, start + step, ... up to but not including stop

Conditional expression

value_if_true if condition else value_if_false

Precedence (short)

** > unary − > * / // % > + − > comparisons, in, is > not > and > or

Conversions

int(x), float(x), str(x), bool(x), complex(a, b); input() returns str

Introduction to Python: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Introduction to Python

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

Tricks to remember Introduction to Python

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

• Slash vs double slash

  / always gives a float; // floors down to the integer below.

  Use it when: Division operators.

• Mutable: list and dict change; the rest stay

  'Lists and dictionaries are diaries: you can rewrite them.'

  Use it when: Mutability.

• range stops short

  range(1, 5) is 1, 2, 3, 4: the stop value is never included.

  Use it when: Loops.

• is = same object; == = same value

  Twins look the same (==) but are different people (is).

  Use it when: Identity vs equality.

Introduction to Python: exam-level solved questions

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

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 2 + 1 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.

Where students lose marks in Introduction to Python

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

• ✗Mistake: Forgetting that input() returns a string

  Arithmetic on strings repeats or joins text, or raises TypeError.

  ✓Do this: Wrap with int() or float().

  Usually costs: 1 mark.

• ✗Mistake: Expecting range(1, 5) to include 5

  The stop value is excluded.

  ✓Do this: Use range(1, 6) for 1 to 5.

  Usually costs: 1 mark.

• ✗Mistake: Treating // as truncation for negatives

  // is floor division: −7 // 2 = −4.

  ✓Do this: Round down, not towards zero.

  Usually costs: 1 mark.

• ✗Mistake: Missing colons or inconsistent indentation

  Both are syntax errors in Python.

  ✓Do this: End headers with a colon; indent each block consistently (4 spaces).

  Usually costs: 1-2 marks.

Practice questions on Introduction to Python

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 · MCQExam level

   Which operator gives the floor quotient in Python?
   (a) /
   (b) //
   (c) %
   (d) **

   Hint

   Double slash.

   Answer

   (b) //.

2. Q2 · 1 mark · Very short answerExam level

   Name two IDEs for Python.

   Hint

   Built in or popular.

   Answer

   IDLE and PyCharm (also VS Code, Spyder, Jupyter).

3. Q3 · 2 marks · Short answerExam level

   List the five kinds of tokens in Python with one example each.

   Hint

   Keyword, identifier, literal, operator, punctuator.

   Answer

   Keyword (while), identifier (total), literal (3.14), operator (//), punctuator (:).

4. Q4 · 2 marks · Short answerHarder exam level

   Give the output: print(7 // 2, 7 / 2, 7 % 2, 2 ** -1, -7 // 2)

   Hint

   Floor for negatives.

   Answer

   3 3.5 1 0.5 -4

5. Q5 · 2 marks · Short answerHarder exam level

   Give the output of print('ab' * 3, 'x' in 'xyz', [1, 2] + [3]) and of print(type(10 / 2).__name__, type(10 // 2).__name__).

   Hint

   Repetition, membership, concatenation.

   Answer

   ababab True [1, 2, 3]; float int.

6. Q6 · 2 marks · Short answerHOTS / competency

   What are the values of not 0, bool('') and bool('False')? Explain the last one.

   Hint

   Empty vs non-empty.

   Answer

   True, False, True. Any non-empty string is truthy, so bool('False') is True; only the empty string is False.

7. Q7 · 5 marks · Short answerHarder exam level

   Write a Python program to find the factorial of a number entered by the user using a for loop, and give the output for 5.

   Hint

   range(1, n + 1).

   Answer

   n = int(input('Enter n: '))
   fact = 1
   for k in range(1, n + 1):
   fact *= k
   print('Factorial =', fact)
   For 5: 1 × 2 × 3 × 4 × 5 = 120.

8. Q8 · 5 marks · Short answerHarder exam level

   Distinguish between mutable and immutable data types with two examples each, and explain what happens on a = [5, 6]; b = a; b.append(7).

   Hint

   In-place change; shared reference.

   Answer

   Mutable types can be changed in place (list, dict); immutable types cannot (str, tuple, also int, float). In the example, b refers to the same list object as a, so after b.append(7) both a and b are [5, 6, 7].

9. Q9 · 10 marks · Long answerHOTS / competency

   Write a Python program that checks whether a number is an Armstrong number (sum of cubes of digits for three-digit numbers) using a while loop, and test it with 371.

   Hint

   % 10 and // 10.

   Answer

   num = int(input('Enter a 3-digit number: '))
   t = num
   s = 0
   while t > 0:
   d = t % 10
   s += d ** 3
   t //= 10
   if s == num:
   print(num, 'is an Armstrong number')
   else:
   print(num, 'is not an Armstrong number')
   For 371: 1 + 343 + 27 = 371, so it is an Armstrong number.

10. Q10 · 2 marks · Short answerHOTS / competency

    Why does round(2.675, 2) give 2.67 in Python, not 2.68?

    Hint

    Binary floating point.

    Answer

    2.675 cannot be stored exactly in binary; the stored value is slightly less than 2.675 (about 2.67499999...), so rounding to two places gives 2.67, as the data-types program printed.

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.Python file extension?Show answer

   .py.

2. 2.Two modes of Python?Show answer

   Interactive and script mode.

3. 3.Type returned by input()?Show answer

   str.

4. 4.Mutable types?Show answer

   list and dict.

5. 5.Membership operators?Show answer

   in, not in.

6. 6.range(2, 11, 3) gives?Show answer

   2, 5, 8.

Quick revision checklist

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

• ☐Python: interpreted, dynamically typed, indentation-based, open source; IDLE, PyCharm, VS Code, Spyder, Jupyter.
• ☐Tokens: keywords, identifiers, literals, operators, punctuators.
• ☐Types: int, float, complex, bool, str, list, tuple, None, dict; list and dict mutable.
• ☐input() → str; print(sep, end); errors: syntax, run-time, logical.
• ☐/ float, // floor, % remainder, ** power (right-assoc.); is identity; in membership.
• ☐if-elif-else; for x in range(start, stop, step); while; break; continue.

Introduction to Python: frequently asked questions