🧱 Python Fundamentals

In the last chapter you wrote your first print() program. Now it's time to learn Python's alphabet and grammar! 🔤 Just as English has letters, words and sentences, Python has a character set, tokens and statements. This chapter answers the BIG questions: What are the smallest pieces of a Python program? How do we name things correctly? What is a variable and how does it store a value? How do we take input from the user and print neat output? Let's build the foundation! 🧱

3.1 🏁 Introduction

Every language is built in layers. Python is too!

graph LR
    A["🔤 Characters\n(letters, digits,\nsymbols)"]
    B["🧩 Tokens\n(smallest meaningful\nunits)"]
    C["📝 Statements\n(one complete\ninstruction)"]
    D["📄 Program\n(many statements\ntogether)"]

    A --> B --> C --> D

    style A fill:#9E9E9E,color:#fff
    style B fill:#2196F3,color:#fff
    style C fill:#FF9800,color:#fff
    style D fill:#4CAF50,color:#fff

Analogy: In English, letters make words, words make sentences, and sentences make an essay. ✍️ In Python, characters make tokens, tokens make statements, and statements make a program!

3.2 🔤 Python Character Set

The character set is the list of all characters that Python recognises and allows in a program.

3.3 🧩 Tokens — The Building Blocks

A token is the smallest individual unit of a program — like a single word or punctuation mark in a sentence.

graph TD
    T["🧩 TOKENS"]
    K["🔑 Keywords\n(if, for, True)"]
    I["🏷️ Identifiers\n(names like marks, total)"]
    L["💎 Literals\n(fixed values: 10, 'Hi')"]
    O["➕ Operators\n(+, -, *, ==)"]
    P["📍 Punctuators\n(brackets, comma,\ncolon, #)"]

    T --> K
    T --> I
    T --> L
    T --> O
    T --> P

    style T fill:#9C27B0,color:#fff
    style K fill:#F44336,color:#fff
    style I fill:#2196F3,color:#fff
    style L fill:#4CAF50,color:#fff
    style O fill:#FF9800,color:#fff
    style P fill:#795548,color:#fff

Look at this one line and spot all five types of tokens:

   total  =  marks  +  5     # add grace marks
   ─────  ─  ─────  ─  ─     ─────────────────
     │    │    │    │  │            │
     │    │    │    │  │            └─ comment (# is a punctuator)
     │    │    │    │  └─ Literal (5)
     │    │    │    └─ Operator (+)
     │    │    └─ Identifier (marks)
     │    └─ Operator (=)
     └─ Identifier (total)

3.3.1 Keywords 🔑

Keywords are reserved words that have a special meaning in Python. You cannot use them as names for variables.

That's 35 keywords. You can see them yourself:

import keyword
print(keyword.kwlist)
print(len(keyword.kwlist))

Analogy: Keywords are like reserved seats 💺 in a bus — "For Ladies", "For Senior Citizens". They are set aside for a special purpose, so you can't use them for anything else!

3.3.2 Identifiers 🏷️

Identifiers are the names we give to variables, functions and other things in a program — like marks, total, student_name.

Valid or invalid? Let's check!

3.3.3 Literals 💎

Literals are fixed values written directly in the program — like 25, 3.14 or "Hello". They are also called constant values.

name = "Riya"          # string literal
age = 16               # integer literal
height = 5.4           # float literal
passed = True          # Boolean literal
result = None          # special literal None
print(name, age, height, passed, result)

Output:

Riya 16 5.4 True None

Triple quotes let a string go over several lines:

poem = """Twinkle twinkle
little star"""
print(poem)

Output:

Twinkle twinkle
little star

3.3.4 Operators ➕

Operators are symbols that perform an operation on values. The values they work on are called operands.

        10   +   5
        ──   ─   ─
        │    │   └─ operand
        │    └─ operator
        └─ operand

You will study each operator in detail in the Data Handling chapter.

3.3.5 Punctuators 📍

Punctuators are symbols used to organise the program — like commas and full stops in English.

' " # \ ( ) [ ] { } @ , : . =

Example: In print("Hi", name), the ( ) brackets, the " " quotes and the , comma are all punctuators.

3.4 📐 Barebones of a Python Program

A Python program is made of a few basic parts. Let's look at a small program:

Code Example: area.py

# Program to find the area of a rectangle
length = 12
breadth = 5
area = length * breadth
if area > 50:
    print("Big rectangle")
print("Area =", area)

Output:

Big rectangle
Area = 60

3.4.1 Comments 💬

• Single-line comment: starts with #.
• Multi-line comment: use # on each line, or write a triple-quoted string (''' ... ''') that is not stored anywhere — Python simply skips it.

# This is a single-line comment
'''
This is a multi-line comment
written with triple quotes
'''
print("Comments are ignored!")   # inline comment

Output:

Comments are ignored!

3.4.2 Indentation 📦

Indentation means the spaces at the beginning of a line. In Python, indentation is compulsory to show which statements belong to a block (for example, the statements under an if).

if area > 50:              ← line ending with a colon ( : )
    print("Big rectangle") ← indented → inside the if-block
print("Area =", area)      ← not indented → outside the block

Analogy: Indentation is like sub-points in your notebook. 📒 Points written a little to the right belong to the heading above them!

3.5 📦 Variables and Assignment

A variable is a named location in memory that stores a value. The value can change while the program runs — that's why it's called a variable!

marks = 85
print(marks)
marks = 92          # value changed
print(marks)

Output:

85
92

Analogy: A variable is like a labelled tiffin box 🍱. The label (marks) is the variable's name, and the food inside (85) is its value. You can empty the box and put in something new (92) — the label stays the same!

3.5.1 How Python Stores a Variable 🧠

In Python, a variable is a name that points to a value stored in memory:

    marks = 85                         marks = 92

    marks ──────► ┌────┐               marks ─┐   ┌────┐
                  │ 85 │                      │   │ 85 │  (no longer used)
                  └────┘                      │   └────┘
                                              │   ┌────┐
                                              └──►│ 92 │
                                                  └────┘

When we give a new value, Python does not change the old box — the name simply points to the new value.

3.5.2 l-value and r-value ⬅️

In an assignment statement, the = sign does not mean "equal to". It means "store the value on the right into the name on the left".

          total   =   marks + 5
          ─────       ─────────
          l-value      r-value
       (left side:    (right side: a value
        a name that    or expression that
        can store)     gives a value)

3.5.3 Multiple Assignments 🔁

1. Same value to many variables:

a = b = c = 0
print(a, b, c)

Output:

0 0 0

2. Different values to many variables in one line:

name, age, city = "Aman", 16, "Kolkata"
print(name, age, city)

Output:

Aman 16 Kolkata

3. Swapping two values — Python can swap in one line!

x = 5
y = 10
x, y = y, x
print("x =", x, "and y =", y)

Output:

x = 10 and y = 5

3.5.4 Dynamic Typing 🔄

In Python, the same variable can hold a value of any type, and the type can change later. This is called dynamic typing. The type() function tells us the type of a value.

x = 10
print(type(x))
x = "ten"
print(type(x))
x = 10.5
print(type(x))

Output:

<class 'int'>
<class 'str'>
<class 'float'>

3.6 ⌨️ Taking Input — the input() Function

The input() function lets the user type a value while the program is running. The message inside the brackets is shown as a prompt.

name = input("Enter your name: ")
print("Hello", name)

Sample run (the user types Priya):

Enter your name: Priya
Hello Priya

3.6.1 input() ALWAYS Gives a String! ⚠️

Whatever the user types — even a number — input() returns it as a string (text).

a = input("Enter first number: ")
b = input("Enter second number: ")
print("Sum =", a + b)

Sample run (the user types 10 and 20):

Enter first number: 10
Enter second number: 20
Sum = 1020

😲 1020, not 30! Because "10" + "20" joins two pieces of text.

Analogy: input() is like a postman 📮 who always brings you a letter. Even if someone writes the number "10" on it, it is still a piece of paper with writing — you have to convert it before you can do maths with it!

3.6.2 Converting Input with int() and float() 🔁

Code Example: add_numbers.py

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
print("Sum =", a + b)

Sample run (the user types 10 and 20):

Enter first number: 10
Enter second number: 20
Sum = 30

Code Example: simple_interest.py

p = float(input("Enter principal: "))
r = float(input("Enter rate: "))
t = float(input("Enter time in years: "))
si = p * r * t / 100
print("Simple Interest =", si)

Sample run (the user types 5000, 8 and 2):

Enter principal: 5000
Enter rate: 8
Enter time in years: 2
Simple Interest = 800.0

l = float(input("Enter length: "))
b = float(input("Enter breadth: "))
print("Area =", l * b)
print("Perimeter =", 2 * (l + b))

3.7 🖨️ More About print() — sep and end

When you print many items separated by commas, Python puts a space between them and moves to a new line at the end. We can change both!

Using sep:

print("10", "05", "2026", sep="-")
print("A", "B", "C", sep="")
print("Name", "Riya", sep=" : ")

Output:

10-05-2026
ABC
Name : Riya

Using end:

print("Hello", end=" ")
print("World")
print("Loading", end="...")
print("Done!")

Output:

Hello World
Loading...Done!

Using both together:

print(1, 2, 3, sep="+", end=" = 6\n")

Output:

1+2+3 = 6

⚠️ Common Errors and Misconceptions

🔑 Quick Revision — Exam Ready!

• Character set → letters, digits, special symbols, whitespaces, Unicode
• Token → smallest unit of a program; 5 types → Keywords, Identifiers, Literals, Operators, Punctuators (KILOP)
• Keywords → 35 reserved words; only True, False, None start with a capital
• Identifier rules → letters, digits, underscore; not starting with a digit; not a keyword; no spaces or symbols; case sensitive
• Literals → String, Numeric (int, float, complex), Boolean (True, False), Special (None)
• Operators act on operands; Punctuators organise code (( ) [ ] { } , : # ' ")
• Comments → # for one line; triple quotes for many lines
• Indentation → compulsory for blocks; usually 4 spaces
• Variable → a named memory location whose value can change
• l-value → left of = (a name); r-value → right of = (a value or expression)
• Multiple assignment → a = b = 0 and a, b = 1, 2; swap → a, b = b, a
• Dynamic typing → same variable can hold different types; check with type()
• input() → always returns a string; convert with int() or float()
• print() → sep = between items (default space); end = at the end (default newline)

🎯 Sample Exam Questions

Q1: Very Short Answer [1 mark each]

a) What is the smallest individual unit in a Python program called? Answer: Token

b) Which of these is a keyword: Print, none, None, If? Answer: None

c) What type of value does input() return? Answer: A string (str)

d) What is the default value of sep in print()? Answer: A single space ' '

e) Name the function used to find the type of a value. Answer: type()

f) Is _total_2 a valid identifier? Answer: Yes

Q2: Short Answer [2 marks]

Identify the type of each token: if, marks, "Delhi", >=, 3.14, :

Answer:

Q3: Output Based [2 marks]

Write the output:

x, y = 4, 9
x, y = y, x
print(x, y, sep="#")
print("Sum", end=": ")
print(x + y)

Answer:

9#4
Sum: 13

Q4: Output Based [2 marks]

If the user enters 5 and then 7, what will this program print? Why?

a = input("A: ")
b = input("B: ")
print(a + b)
print(int(a) + int(b))

Answer:

A: 5
B: 7
57
12

input() returns strings, so a + b joins "5" and "7" to give 57. After converting with int(), the numbers are added to give 12.

Q5: Error Finding [2 marks]

Find and correct the errors:

1st num = int(input("Enter number: ")
print("Square =" 1st num * 1st num)

Answer:

num1 = int(input("Enter number: "))
print("Square =", num1 * num1)

Errors: the identifier had a digit at the start and a space, the closing bracket of int(...) was missing, and the comma between the items in print() was missing.

Q6: Program [3 marks]

Write a program to input the marks of a student in three subjects and print the total and the average.

Answer:

m1 = float(input("Enter marks 1: "))
m2 = float(input("Enter marks 2: "))
m3 = float(input("Enter marks 3: "))
total = m1 + m2 + m3
print("Total =", total)
print("Average =", total / 3)

Sample run (the user types 80, 90 and 70):

Enter marks 1: 80
Enter marks 2: 90
Enter marks 3: 70
Total = 240.0
Average = 80.0

✏️ Practice Problems

1. What is a token? Name its five types with one example of each.
2. Which of these are invalid identifiers, and why? Roll_No, roll-no, rollno1, 1rollno, global, _roll
3. Give one example each of a string, integer, float, Boolean and None literal.
4. Write the output: print("Good", "Morning", sep="***")
5. Write the output: print("A", end="") followed by print("B")
6. Write a program to input a number and print its square and cube.
7. Write a program to input temperature in Celsius and print it in Fahrenheit (F = C × 9 / 5 + 32).
8. Write a program to input two numbers and swap them. Print the values before and after swapping.
9. Why does print(input("A: ") * 2) print 55 when the user types 5? How would you print 10 instead?
10. What is indentation? What error do you get if a statement is indented without a reason?
11. Explain dynamic typing with a small program that uses type().
12. Write a program to input the price of an item and the quantity bought, and print the bill amount as Bill amount = Rs <amount>.