~/Getting Started with Python

8.1 Introduction to Python

Python was born at Centrum Wiskunde & Informatica (CWI) in the Netherlands in December 1989, when Guido van Rossum was looking for a "hobby project" over the Christmas break. The language was named after the British comedy group Monty Python's Flying Circus — not the snake (although the snake logo is now the official mascot).

8.2 Features of Python

Python stays popular because it checks almost every box a modern language should. The nine features below are the ones CBSE examines most often.

8.3 Your First Program — "Hello, World!"

A long tradition: every programming lesson begins with a program that prints a greeting. In Python it is just one line.

print("Hello, World!")

Save the file as hello.py, open a terminal in that folder, and run:

python hello.py

Expected output:

Hello, World!

8.4 Execution Modes — Interactive vs. Script

Python can be used in two complementary ways.

8.4.1 Comparison

8.5 Python Character Set

The character set of a language is the collection of symbols it understands. Python uses the full Unicode character set, but the characters you will actually type in your code fall into four groups:

8.6 Python Tokens

When Python reads your source code, the very first job of the interpreter is to chop the text into small meaningful pieces called tokens. A token is the smallest unit of a program that has meaning to the compiler. Python has five kinds of tokens:

8.6.1 Keywords

Keywords are reserved words that have a fixed meaning in Python. You cannot use them as the name of a variable, function or class. Python 3.13 has 35 keywords; you do not need to memorise all of them, but you will meet every one during Class XI & XII.

import keyword
print(keyword.kwlist)

8.6.2 Identifiers

An identifier is a name that you give to something in your program — a variable, a function, a class, a module. Identifiers follow strict rules:

8.6.3 Literals

A literal is a fixed value written directly in the source code. Python supports five groups:

8.6.4 Operators

An operator is a symbol that tells Python to perform some action on one or two values (called operands). The full treatment of operators comes in Chapter 10; here is a preview:

• Arithmetic — + − * / // % **
• Relational — == != < > <= >=
• Logical — and, or, not
• Assignment — = and augmented forms like +=, -=, *=
• Identity — is, is not
• Membership — in, not in

8.6.5 Punctuators

Punctuators are symbols used to structure a program. They do not act on data directly; they tell Python where things start and end.

8.7 Variables

name = "Ananya"      # string
age  = 16            # integer
pi   = 3.14159       # float
can_vote = False    # boolean

print(name, age, pi, can_vote)

Output: Ananya 16 3.14159 False

x = 10           # x is an int
x = "hello"      # now x is a string — perfectly legal

8.7.1 How an assignment works — the Box Model

When you write age = 16, Python does three things:

1. Creates an integer object 16 in memory.
2. Creates a name age.
3. Makes age refer to the object.

8.8 L-Value and R-Value

Every assignment statement in Python (or any C-family language) has two sides separated by =:

• L-value (left-hand value) — the location where the data will be stored. It must be something that can hold a value — a variable name, a list element, etc.
• R-value (right-hand value) — the value that will be placed there. It can be a literal, an expression, or another variable.

marks = 85
# L-value: marks  (a variable — can hold a value)
# R-value: 85     (the value being stored)

area = length * breadth
# L-value: area
# R-value: length * breadth  (an expression, evaluated first)

10 = marks          # ❌ 10 is not a location
marks + 5 = 100      # ❌ an expression cannot be an L-value

8.9 Comments

A comment is text inside the source file that Python ignores. Comments exist for human readers — to explain why a piece of code exists, a non-obvious shortcut, or a reminder to fix something later.

8.9.1 Single-line comments (#)

Anything from # to the end of the line is a comment.

# Compute the GST-included price
price = base * 1.18      # 18% GST

8.9.2 Multi-line "comments" (triple-quoted strings)

Python has no formal multi-line comment syntax. The common trick is to use a triple-quoted string, which Python will evaluate and immediately discard if it isn't assigned to anything.

"""
This function computes the area of
a rectangle given its length and breadth.
Returns a float rounded to 2 decimal places.
"""
def area(length, breadth):
    return round(length * breadth, 2)

8.9.3 Why write comments?

• Non-obvious reasoning — explain why the code does what it does, not what (well-named variables already tell that).
• Constants that have meaning — TAX_RATE = 0.18 # 18% GST for India (2026).
• TODO reminders — # TODO: handle negative inputs.
• Disable a line temporarily during debugging — a popular "comment it out" trick.

# ❌ WHAT — useless, the code already says this
i = i + 1              # add 1 to i

# ✓ WHY — valuable, captures the intent
i = i + 1              # skip duplicate row — see issue #42