Python Exceptions

What Are Python Exceptions?

In Python, an exception is an error that disrupts the normal flow of a program. When something goes wrong during execution—like dividing by zero or trying to access an index that doesn’t exist—Python stops and raises an exception.

Knowing how and why they occur is the first step in building resilient, bug-free Python applications. Later, you'll learn how to catch and handle them, but first, we must learn to recognize them.

When Does Python Raise an Exception?

Python raises an exception during runtime when it encounters an issue that it doesn’t know how to handle. This stops the execution of the program and displays an error message (also called a traceback).

Basic Example: Division by Zero

x = 5
y = 0
result = x / y
print(result)

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    result = x / y
ZeroDivisionError: division by zero

Python gives us a traceback—a report of where the error occurred. In this case, it tells us exactly which line failed and what kind of error it was: ZeroDivisionError.

Another Example: Accessing a Non-Existent Index

fruits = ["apple", "banana", "cherry"]
print(fruits[5])

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print(fruits[5])
IndexError: list index out of range

Here, the list only has 3 elements, indexed 0 through 2. Trying to access index 5 raises an IndexError.

Common Built-in Exceptions in Python

Exception	Description
ZeroDivisionError	Raised when dividing by zero
TypeError	Raised when an operation is applied to an object of inappropriate type
NameError	Raised when a variable is not defined
IndexError	Raised when a sequence index is out of range
KeyError	Raised when a dictionary key isn’t found
ValueError	Raised when a function receives an argument of right type but inappropriate value
AttributeError	Raised when an invalid attribute reference is made
ImportError	Raised when a module or name cannot be found during import

Best Practices for Beginners

• Don’t ignore exceptions; read and understand them.
• Use print statements or debugging tools to inspect values before failure points.
• Always know the type and shape of your data before using it.
• Refer to the traceback line numbers to locate and fix errors quickly.

Summary

Python exceptions are runtime errors that occur when something goes wrong while executing your code. In the next section, we’ll explore how to catch and handle these exceptions using try and except.