🐍 Chapter 5: Python Fundamentals

5.1 Introduction

Python is a high-level, interpreted, general-purpose programming language created by Guido van Rossum and first released in 1991. It is named after the British comedy series "Monty Python's Flying Circus" — not the snake! 🐍

5.2 Python Character Set

A character set is the set of valid characters that a language can recognize. A character is the smallest unit of a program.

5.3 Tokens

The smallest individual unit in a Python program is called a Token (or lexical unit). The Python interpreter breaks every line of code into tokens before processing it.

flowchart TD
    A[Python Tokens] --> B[Keywords]
    A --> C[Identifiers]
    A --> D[Literals]
    A --> E[Operators]
    A --> F[Punctuators]
    class A normalNode
    class B,C,D,E,F tryNode

5.3.1 Keywords

Keywords are reserved words that have special meaning to the Python interpreter. They cannot be used as variable names, function names, or any other identifier.

5.3.2 Identifiers (Names)

An identifier is the name given to entities like variables, functions, classes, modules, etc.

5.3.3 Literals / Values

A literal is a raw data value directly written in the source code.

Numeric literals can further be written in different number systems:

5.3.4 Operators

Operators are special symbols that carry out arithmetic or logical computation. The value(s) the operator acts on are called operands.

5.3.5 Punctuators

Punctuators are symbols used to organize sentence structure, indicate operations, and structure the program. Examples:

' " # \ ( ) [ ] { } @ , : . = ; += -= *= /=

5.4 Barebones of a Python Program

A minimal Python program has a very simple, indentation-based structure — no semicolons, no curly braces required.

# This is my first Python program
print("Hello, World!")

flowchart TD
    A[Start Program] --> B[Comment / Docstring]
    B --> C[Import Statements]
    C --> D[Statements with proper Indentation]
    D --> E[End Program]
    class A normalNode
    class B,C,D tryNode
    class E normalNode

5.5 Variables and Assignments

A variable is a named location in memory used to store a value that can change during program execution.

5.5.1 Creating a Variable

In Python, you don't need to declare a variable's type separately — a variable is created the moment you assign a value to it.

age = 17          # integer variable
name = "Riya"     # string variable
height = 5.4      # float variable

5.5.2 Multiple Assignments

Python allows assigning values to multiple variables in different flexible ways:

5.5.3 Variable Definition

A variable in Python is essentially a reference (name) bound to an object stored somewhere in memory. Python variables have:

x = 10
print(id(x))    # shows the memory address of the object 10
print(type(x))  # shows <class 'int'>

5.5.4 Dynamic Typing

Python is a dynamically typed language — the type of a variable is decided at runtime, based on the value assigned, and can change if reassigned.

x = 10        # x is int
x = "Hello"   # now x becomes str — perfectly valid!
x = 3.14      # now x becomes float

5.5.5 l-value and r-value ⬅️

In an assignment statement, the = sign does not mean "equal to". It means "evaluate the right side, then store the result in the name on the left".

          total   =   marks + 5
          ─────       ─────────
         l-value       r-value

a = 10          # a is the l-value, 10 is the r-value
b = a * 2       # b is the l-value, a * 2 is the r-value
a = a + b       # the r-value (10 + 20) is worked out first, then stored in a
print(a, b)

Output:

30 20

5.6 Simple Input and Output

5.6.1 Reading Numbers

The built-in input() function always returns a string, even if the user types a number. To use it as a number, you must explicitly type-cast it.

age = input("Enter your age: ")       # age is a string, e.g. "17"
age = int(input("Enter your age: "))  # age is now an integer 17
height = float(input("Enter height: "))  # for decimal numbers

5.6.2 Output Through print() Function

The print() function displays output on the screen. It has several useful optional parameters:

print("Hello", "World", sep="-", end="!\n")
# Output: Hello-World!

Modern f-strings are the CBSE-recommended way to combine text and variables:

name = "Amit"
marks = 95
print(f"{name} scored {marks} marks")

📝 Board Exam Practice Zone

1 Mark Questions

1. Name the creator of Python.
2. What will type(True) return?
3. Is input() output a str, int, or float by default?
4. What is the default value of sep in print()?
5. Which of these is a valid identifier: 2fruit, _fruit, for, fruit#1?

2 Mark Questions

1. Differentiate between Keywords and Identifiers with one example each.
2. Differentiate between / and // operators with example.
3. What is dynamic typing? Explain with an example.
4. Write the rules for naming a valid Python identifier.

3 Mark Questions

1. Write a program to swap two numbers without using a third variable.
2. Explain any three types of literals in Python with examples.
3. Predict the output:

a, b, c = 5, 10, 15
print(a, b, c, sep="*", end="#")

4 Mark / Output-Based Questions

1. Identify errors, if any, in the following code and rewrite it correctly:

1name = input(Enter name)
Age = int(input("Enter age"))
print(Name, "is", age, "years old")

2. Write a program that takes two numbers as input and prints their sum, difference, product, and floor division result, each on a separate line.