Chapter 5: Getting Started with Python

CBSE Unit: Unit 2, Python Programming (45 marks) Marks Weightage: ~8-10 marks (foundational Python, heavily tested) Priority: CRITICAL, basics of Python, every concept tested

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Key Concepts

5.1 Features of Python

• High-level, free, and open source language
• Interpreted (executed line by line by interpreter)
• Case-sensitive (NUMBER and number are different)
• Portable and platform independent (runs on any OS), Rich library of predefined functions
• Uses indentation for blocks (not braces like C/Java)

5.2 Execution Modes

• Interactive mode: Type on >>> prompt, executes immediately; cannot save for reuse
• Script mode: Write in a file (.py), save and execute; reusable

5.3 Python Keywords (Reserved Words)

Cannot be used as identifiers. Examples: False, True, None, and, or, not, if, elif, else, for, while, break, continue, pass, def, return, class, import, from, as, try, except, finally, raise, with, yield, lambda, global, nonlocal, del, in, is, assert

5.4 Identifiers, Names given to variables, functions, classes, etc., Rules:

• Must start with letter (A-Z/a-z) or underscore (_)
• Can contain letters, digits (0-9), and underscores
• Cannot start with a digit
• Cannot be a keyword
• Case-sensitive
• No special characters (!, @, #, $, %, etc.) except underscore

5.5 Comments

• Single-line: # This is a comment
• Multi-line: Use # on each line OR triple quotes ('''comment''' or """comment"""), Comments are ignored by interpreter

5.6 Data Types

Type	Description	Example
int	Integer (no size limit in Python 3)	25, -10, 0
float	Decimal numbers	3.14, -2.5, 1.0
complex	Complex numbers	3+5j
bool	Boolean (True/False)	True, False
str	String (sequence of characters)	'Hello', "World"
list	Ordered, mutable sequence	[1, 2, 3]
tuple	Ordered, immutable sequence	(1, 2, 3)
dict	Key-value pairs	{'a': 1, 'b': 2}
None	Null/absence of value	None

• Mutable: Can be changed after creation (list, dict)
• Immutable: Cannot be changed (int, float, str, tuple, bool)
• type() function returns data type of a variable

5.7 Variables, Name that refers to a value stored in memory, Created by assignment statement: x = 10

• Dynamic typing: type determined at runtime (no declaration needed), Multiple assignment: a, b, c = 10, 20, 30 or a = b = c = 0

5.8 Operators

Arithmetic Operators

Operator	Operation	Example	Result
+	Addition	5 + 3	8
-	Subtraction	5 - 3	2
*	Multiplication	5 * 3	15
/	Division (float)	7 / 2	3.5
//	Floor Division (integer)	7 // 2	3
%	Modulus (remainder)	7 % 2	1
**	Exponentiation	2 ** 3	8

Relational (Comparison) Operators

==, !=, <, >, <=, >= - return True or False

Logical Operators

Operator	Description
and	True if BOTH are True
or	True if at least ONE is True
not	Reverses the boolean value

Assignment Operators

=, +=, -=, *=, /=, //=, %=, **=

Identity Operators

is, is not - check if two variables refer to same object

Membership Operators

in, not in - check if value exists in sequence

Operator Precedence (High to Low)

** > ~, +, - (unary) > *, /, //, % > +, - > relational > not > and > or

5.9 Expressions, Combination of values, variables, operators that evaluates to a value

• Arithmetic expression: a + b * c
• Relational expression: a > b
• Logical expression: a > b and c < d

5.10 Input and Output

Input

name = input("Enter your name: ")     # Always returns STRING
num = int(input("Enter number: "))     # Convert to int
price = float(input("Enter price: "))  # Convert to float

• input() always returns a string, must typecast for numeric operations

Output

print("Hello", "World")           # Separated by space (default sep=' ')
print("Hello", end="")            # No newline at end (default end='\n')
print("Name:", name, "Age:", age)  # Multiple values
print("Sum =", a + b)             # Expression in print

5.11 Type Conversion

Implicit (Automatic), Python automatically converts smaller type to larger: int + float = float

Explicit (Type Casting)

int()     # Convert to integer: int(3.7) = 3, int("25") = 25
float()   # Convert to float: float(5) = 5.0, float("3.5") = 3.5
str()     # Convert to string: str(25) = "25"
bool()    # Convert to boolean: bool(0) = False, bool(1) = True

5.12 Debugging

• Syntax Error: Violation of language rules (detected before execution)
• Logical Error: Program runs but gives wrong output
• Runtime Error: Error during execution (e.g., division by zero, wrong type)

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Common Board Exam Question Patterns

1. Find output of given Python code (2-4 marks): Most common question type
2. Identify errors in code (1-2 marks)
3. Data types and type() function (1-2 marks)
4. Operator precedence questions (2 marks)
5. Difference between / and //, = and ==, is and == (2 marks)
6. input() and type conversion (2 marks)
7. Keywords vs Identifiers (1-2 marks)
8. Write a Python program for simple calculation (3-4 marks)

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Key Points Students Miss

1. input() ALWAYS returns a string, must convert for arithmetic
2. / gives float result (7/2 = 3.5); // gives integer floor division (7//2 = 3)
3. ** has right-to-left associativity: 2**3**2 = 2**(3**2) = 2**9 = 512
4. Python is case-sensitive: True is boolean, true is an error
5. Indentation is mandatory in Python (not optional like in C/Java)
6. bool(0) = False, bool("") = False, bool([]) = False; any non-zero/non-empty = True
7. int(3.7) = 3 (truncates, does NOT round)
8. None is a valid value and data type (NoneType)
9. Multiple assignment: a, b = b, a swaps values without temp variable
10. Comments are ignored by interpreter but essential for code readability