Class 9 AI Chapter - Python Topic - Operator

 

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Chapter - Introduction to Python 
Other Topics

• Python Features
• Token Operators 
• Control Statements 
• Operator, Errors 
• Functions 

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1. Syntax Error

Definition:
Occurs when the code breaks the grammar rules of Python. These errors are detected before the program runs.

Example:

# Missing colon (:) after the if statement
x = 10
if x > 5
    print("x is greater than 5")

❌Error:

SyntaxError: expected ':'

Why it happens:

Python expects a colon : after if x > 5, but it's missing.

2. Logical Error

Definition:
The program runs without crashing, but the output is incorrect due to a mistake in logic.

Example:

# Incorrect formula for average
def average(a, b):
    return a + b / 2  # Wrong logic: should use parentheses

print(average(10, 2))  # Output: 11.0 (Incorrect)

Correct Version:

def average(a, b):
    return (a + b) / 2  # Correct formula

print(average(10, 2))  # Output: 6.0

❌ No error message, but the result is logically wrong.

3. Runtime Error (Exception)

Definition:
Occurs while the program is running, often due to invalid operations.

Example:

a = int(input("Enter a number: "))
print(10 / a)

 Input: 0

❌ Error:

ZeroDivisionError: division by zero

🔍 Why it happens:

Dividing by zero is not allowed in math, so Python throws an exception.

Fix with Try-Except:

try:
    a = int(input("Enter a number: "))
    print(10 / a)
except ZeroDivisionError:
    print("You can't divide by zero!")

Summary Table:

Error Type	When It Occurs	What Happens	Example
Syntax Error	Before execution	Program won't run	Missing colon, wrong indent
Logical Error	During execution	Wrong output, no crash	Wrong formula or condition
Runtime Error	During execution	Program crashes (unless handled)	Division by zero, bad input

🔹List in Python

A list is a built-in data structure in Python that allows you to store a collection of items in a single variable.

Key Features:

• Ordered (items maintain their order)
• Mutable (can be changed after creation)
• Can store different types (int, float, string, etc.)
• Allows duplicate elements

🔹Create a List in Python

✅ Syntax:

my_list = [item1, item2, item3, ...]

✅ Examples:

# A list of numbers
numbers = [1, 2, 3, 4, 5]

# A list of strings
fruits = ["apple", "banana", "cherry"]

# A mixed list
mixed = [10, "hello", 3.14, True]

✅ Empty list:

empty_list = []

🔹 Access Elements of a List

Use indexing to access elements.

✅ Indexing starts from 0.

✅ Example:

fruits = ["apple", "banana", "cherry"]

print(fruits[0])  # Output: apple
print(fruits[1])  # Output: banana
print(fruits[2])  # Output: cherry

🔸 Negative Indexing:

Access elements from the end of the list.

print(fruits[-1])  # Output: cherry
print(fruits[-2])  # Output: banana

🔹Slicing a List

You can get a sub-list using slicing:

fruits = ["apple", "banana", "cherry", "date", "fig"]

print(fruits[1:4])  # Output: ['banana', 'cherry', 'date']
print(fruits[:3])   # Output: ['apple', 'banana', 'cherry']
print(fruits[::2])  # Output: ['apple', 'cherry', 'fig']

✅ Summary:

Operation	Example	Output
Create list	colors = ["red", "blue"]	["red", "blue"]
Access by index	colors[0]	"red"
Access last item	colors[-1]	"blue"
Slice list	colors[0:2]	["red", "blue"]

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🔹 1. Using + Operator with Lists

The + operator is used to concatenate (join) two lists.

✅ Example:

list1 = [1, 2, 3]
list2 = [4, 5]

result = list1 + list2
print(result)

🔸 Output:

[1, 2, 3, 4, 5]

📝 It creates a new list that combines the two.

🔹 2. Using * Operator with Lists

The * operator is used to repeat the list multiple times.

✅ Example:

list1 = ["a", "b"]
result = list1 * 3
print(result)

🔸 Output:

['a', 'b', 'a', 'b', 'a', 'b']

📝 Useful when you want to create repeated patterns.

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🔹 3 Traversing a List in Python

Traversing a list means going through each element in the list one by one — usually using a loop — so you can read, process, or modify each item.

🔹 Why Traverse a List?

You might want to:

• Print all elements
• Find a specific item
• Apply an operation to each item
• Count something, or filter values

🔹 Common Ways to Traverse a List

✅ 1. Accessing Each Value Only

fruits = ['apple', 'banana', 'cherry']

for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

✅ 2. Accessing Each Value and Its Index (using enumerate)

for index, value in enumerate(fruits):
    print(f"Index: {index}, Value: {value}")

Output:

Index: 0, Value: apple
Index: 1, Value: banana
Index: 2, Value: cherry

🔹 Extra: Start Index from 1 Instead of 0

for index, value in enumerate(fruits, start=1):
    print(f"Index: {index}, Value: {value}")

Output:

Index: 1, Value: apple
Index: 2, Value: banana
Index: 3, Value: cherry

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