Lecture 3: Python Keywords, Identifiers, and I/O

Python Keywords

What are Keywords?

Python keywords are reserved words with fixed meanings that define Python's syntax. They cannot be used as variable names, function names, or any other identifiers.

Fun Fact

All keywords in Python are in lowercase except True, False, and None!

Keyword Categories

Category	Keywords
Value Keywords	True, False, None
Operator Keywords	and, or, not, is, in
Control Flow	if, else, elif, for, while, break, continue, pass
Exception Handling	try, except, finally, raise, assert
Function & Class	def, return, lambda, yield, class
Import & Module	import, from, as
Scope Management	global, nonlocal
Async Programming	async, await

View All Keywords

You can programmatically get all Python keywords:

import keyword
print(keyword.kwlist)

Python Identifiers

What are Identifiers?

Identifiers are user-defined names for variables, functions, classes, modules, etc. They help us identify and reference different elements in our code.

Rules for Naming Identifiers

✅ Can contain letters (A-Z, a-z), digits (0-9), and underscores (_)
✅ Must start with a letter or underscore (not a digit)
✅ Cannot be a reserved Python keyword
✅ Cannot contain spaces or special characters
✅ Case sensitive (name, Name, and NAME are different)

Valid vs Invalid Identifiers

✅ Valid Identifiers

name
_count
user_name
User1
calculate_sum

❌ Invalid Identifiers

2var (starts with digit)
my-name (contains hyphen)
class (keyword)
user@name (special char)
var name (contains space)

Best Practices

🎯 Use descriptive names (student_name vs s)
🐍 Follow Python conventions (snake_case for variables)
📏 Keep names reasonable in length

Input and Output Operations

Taking User Input

The input() function reads user input from the keyboard:

# Basic input
name = input("Enter your name: ")
print("Hello, " + name + "!")

Try It Yourself!

Output will appear here...

Type Conversion

By default, input() returns a string. Convert to other types:

# Convert to integer
age = int(input("Enter your age: "))

# Convert to float
price = float(input("Enter price: "))

# Convert to boolean
is_student = input("Are you a student? (y/n): ") == 'y'

Multiple Inputs

Take multiple values in one line using split():

# Take two inputs
x, y = input("Enter two numbers: ").split()
print("First:", x, "Second:", y)

# Convert to integers
a, b = map(int, input("Enter two numbers: ").split())

Advanced Output Formatting

# f-strings (Python 3.6+)
name = "Alice"
age = 25
print(f"{name} is {age} years old.")

# format() method
print("Hello, {}. You are {}.".format(name, age))

# String formatting with specifiers
pi = 3.14159
print("Pi = {:.2f}".format(pi))

Practical Examples

Example 1: Logical Operators

# Using and, or, not operators
print(True and True)   # True
print(True or False)   # True
print(not False)      # True

Example 2: Control Flow

# Using if, elif, else
grade = 85

if grade >= 90:
    print("A grade")
elif grade >= 80:
    print("B grade")
elif grade >= 70:
    print("C grade")
else:
    print("Need improvement")

Example 3: Loops with break and continue

for i in range(1, 11):
    if i == 5:
        continue  # Skip 5
    if i == 8:
        break     # Stop at 8
    print(i)

Example 4: Function Definition

def calculate_area(length, width):
    """Calculate rectangle area"""
    return length * width

# Call the function
area = calculate_area(5, 3)
print(f"Area: {area}")

Example 5: Exception Handling

def safe_divide(a, b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        return "Cannot divide by zero!"
    finally:
        print("Division attempt completed")

print(safe_divide(10, 2))
print(safe_divide(10, 0))

Test Your Knowledge

Question 1: Which of these is NOT a Python keyword?

if

variable

for

while

Question 2: Which identifier is valid?

2name

my-name

_count

class

Question 3: What does input() return by default?

Integer

String

Float

Boolean

Question 4: Which is the correct way to convert input to integer?

int input()

int(input())

input(int)

input().int()