Exception Handling

When an error occurs, or exception as we call it, Python will normally stop and generate an error message. These exceptions can be handled using the try statement:

try:
    print(x)
except:
    print("An exception occurred")

• The try block lets you test a block of code for errors.
• The except block lets you handle the error.
• The else block lets you execute code when there is no error.
• The finally block lets you execute code, regardless of the result of the try- and except blocks.

Here is a more detailed example incorporating all these blocks:

try:
    print("Hello")
    x = 1 / 0
except ZeroDivisionError:
    print("Division by zero error")
else:
    print("Nothing went wrong")
finally:
    print("The 'try except' is finished")

In this example:

• The try block will execute the print statement and then attempt to divide by zero.
• The except block will catch the division by zero error and print an appropriate message.
• The else block will not execute because there was an error.
• The finally block will execute regardless of the result of the try and except blocks.

Basic Try-Except

try:
    print(x)  # x is not defined
except:
    print("An exception occurred")

Output:

An exception occurred

Catching Specific Exceptions

try:
    print(1 / 0)  # Division by zero
except ZeroDivisionError:
    print("Cannot divide by zero!")

Output:

Cannot divide by zero!

Multiple Exceptions

try:
    print(1 / 0)  # Division by zero
    print(x)  # x is not defined
except ZeroDivisionError:
    print("Cannot divide by zero!")
except NameError:
    print("Variable is not defined!")

Output:

Cannot divide by zero!

Using Else

try:
    print("Hello World!")
except:
    print("An exception occurred")
else:
    print("Nothing went wrong")

Output:

Hello World!
Nothing went wrong

Finally Block

try:
    print(x)  # x is not defined
except:
    print("An exception occurred")
finally:
    print("The 'try except' block is finished")

Output:

An exception occurred
The 'try except' block is finished

Full Example with All Blocks

try:
    print("Hello")
    x = 1 / 0  # This will raise a division by zero error
except ZeroDivisionError:
    print("Division by zero error")
else:
    print("Nothing went wrong")
finally:
    print("The 'try except' is finished")

Output:

Hello
Division by zero error
The 'try except' is finished

Example with Custom Exception

class CustomError(Exception):
    pass

try:
    raise CustomError("This is a custom error")
except CustomError as e:
    print(e)
finally:
    print("The 'try except' block is finished")

Output:

This is a custom error
The 'try except' block is finished

Examples as per above images

Multiple Exceptions

try:
    print(x)  # x is not defined
except NameError:
    print("Variable x is not defined")
except:
    print("Something else went wrong")

output:
Variable x is not defined

Try-Except-Finally

try:
    print(x)  # x is not defined
except:
    print("Something went wrong")
finally:
    print("The 'try except' is finished")

Output:
Something went wrong
The 'try except' is finished

Try-Except-Else

try:
    print("Hello")
except:
    print("Something went wrong")
else:
    print("Nothing went wrong")


output:
Hello
Nothing went wrong

Nested Try-Except-Finally for File Handling

try:
    f = open("demofile.txt")
    try:
        f.write("Lorum Ipsum")
    except:
        print("Something went wrong when writing to the file")
    finally:
        f.close()
except:
    print("Something went wrong when opening the file")

Output:
Something went wrong when writing to the file

Examples that illustrate how to raise exceptions :

Raising a Generic Exception

x = -1

if x < 0:
    raise Exception("Sorry, no numbers below zero")

Output:

Exception: Sorry, no numbers below zero

Raising a TypeError

x = "hello"

if not type(x) is int:
    raise TypeError("Only integers are allowed")

Output:

TypeError: Only integers are allowed