Master Python Exception Handling: try, except, else, and finally Explained

Master Python Exception Handling: try, except, else, and finally Explained

This guide walks you through Python’s exception handling constructs with practical code snippets and best‑practice tips.

Video: Python Exception Handling (try..except..finally)

Exceptions in Python

Python raises built‑in exceptions whenever a runtime error occurs. The interpreter halts the current block of code and propagates the exception up the call stack until a handler is found. If no handler exists, the program terminates and prints a traceback.

Consider the following call chain: A calls B, which calls C. If C raises an exception that isn’t handled locally, the exception bubbles up to B and then to A. Unhandled exceptions result in an abrupt program crash.

Catching Exceptions in Python

To intercept an exception, place the potentially unsafe code inside a try block. The except clause contains the logic that executes when an exception occurs.

Here’s a concise example that demonstrates the flow:

# Import the sys module to inspect the exception type
import sys

random_list = ['a', 0, 2]

for entry in random_list:
    try:
        print("The entry is", entry)
        r = 1 / int(entry)
        break
    except:
        print("Oops!", sys.exc_info()[0], "occurred.")
        print("Next entry.")
        print()
print("The reciprocal of", entry, "is", r)

Output

The entry is a
Oops! <class 'ValueError'> occurred.
Next entry.

The entry is 0
Oops! <class 'ZeroDivisionError'> occurred.
Next entry.

The entry is 2
The reciprocal of 2 is 0.5

The code iterates over random_list. If the conversion to int or division fails, the except block handles the error, logs it, and moves to the next item. When a successful conversion occurs, the loop breaks and the reciprocal is printed.

Because all built‑in exceptions inherit from Exception, you can catch them generically:

# Import the sys module to inspect the exception type
import sys

random_list = ['a', 0, 2]

for entry in random_list:
    try:
        print("The entry is", entry)
        r = 1 / int(entry)
        break
    except Exception as e:
        print("Oops!", e.__class__, "occurred.")
        print("Next entry.")
        print()
print("The reciprocal of", entry, "is", r)

The output is identical, but the handler now references the specific exception class via e.__class__.

Catching Specific Exceptions in Python

Broadly catching all exceptions can mask bugs and make debugging difficult. Instead, list only the exception types you expect.

A try block may contain multiple except clauses; only the first matching clause executes. Example:

try:
    # code that may raise an error
    pass
except ValueError:
    # handle ValueError
    pass
except (TypeError, ZeroDivisionError):
    # handle either TypeError or ZeroDivisionError
    pass
except Exception:
    # fallback for any other exception
    pass

Raising Exceptions in Python

Beyond automatic exceptions, developers can raise errors explicitly using the raise statement. Optionally, pass a message to clarify the reason.

raise KeyboardInterrupt
# Traceback: KeyboardInterrupt

raise MemoryError("This is an argument")
# Traceback: MemoryError: This is an argument

try:
    a = int(input("Enter a positive integer: "))
    if a <= 0:
        raise ValueError("That is not a positive number!")
except ValueError as ve:
    print(ve)

Python try with else Clause

The optional else clause runs only when the try block completes without raising an exception. This pattern keeps success‑path code separate from error handling.

Example: printing the reciprocal of an even number.

# program to print the reciprocal of even numbers

try:
    num = int(input("Enter a number: "))
    assert num % 2 == 0
except:
    print("Not an even number!")
else:
    reciprocal = 1 / num
    print(reciprocal)

Output when entering an odd number:

Enter a number: 1
Not an even number!

Output when entering an even number:

Enter a number: 4
0.25

Note that if num is zero, ZeroDivisionError is raised in the else block and is not caught by the preceding except clause.

Python try…finally

The finally clause guarantees that cleanup code runs regardless of whether an exception occurred. This is essential when working with external resources such as files, network connections, or GUIs.

Typical usage with file handling:

try:
    f = open("test.txt", encoding='utf-8')
    # perform file operations
finally:
    f.close()

The file is always closed, even if an error arises during processing.