Python Try-Except

Python Try-Except

When an exception occurs, and if we do not provide any checks or mechanisms to handle that exception, it can crash our Python application.

Python provides the try-except block to handle errors gracefully, preventing the program from crashing due to exceptions.

Syntax

try:
    # Code that may raise an exception
except ExceptionType:
    # Code to handle the exception

When an error occurs in the try block, Python jumps to the corresponding except block to handle it instead of terminating execution.

How It Works

1. The code inside the try block runs normally.
2. If an exception occurs, execution stops and jumps to the except block.
3. If an ExceptionType is specified, the except block runs only for that specific type of exception.
4. The program does not crash and continues executing subsequent code.

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Examples

1. Handling Division by Zero

We attempt to divide a number by zero, which raises a ZeroDivisionError. Using try-except, we handle this error gracefully.

try:
    result = 10 / 0  # This will raise a ZeroDivisionError
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Output:

Error: Division by zero is not allowed.

Instead of crashing, Python executes the except block and prints an error message.

2. Catching Multiple Exceptions

We can catch multiple exceptions using multiple except blocks or by specifying multiple exception types in a single block.

try:
    num = int("abc")  # This will raise a ValueError
except ValueError:
    print("Error: Invalid number format.")
except ZeroDivisionError:
    print("Error: Division by zero is not allowed.")

Output:

Error: Invalid number format.

Since int("abc") raises a ValueError, the corresponding except block executes.

3. Using a Generic Exception

When we don’t know what kind of exception might occur, we can use a generic except block without specifying an exception type.

try:
    x = 1 / 0  # This raises ZeroDivisionError
except Exception as e:
    print("An error occurred:", e)

Output:

An error occurred: division by zero

The Exception class catches all types of exceptions, and the error message is displayed dynamically.

4. Using else with try-except

The else block executes only if no exceptions occur.

try:
    num = int("10")  # This will not cause an error
    print("Conversion successful:", num)
except ValueError:
    print("Error: Invalid input.")
else:
    print("No exceptions occurred.")

Output:

Conversion successful: 10
No exceptions occurred.

Since there was no error, the else block executes.

5. Using finally Block

The finally block executes regardless of whether an exception occurs or not.

try:
    print("Trying to open a file...")
    f = open("nonexistent.txt")  # This will raise an error
except FileNotFoundError:
    print("Error: File not found.")
finally:
    print("Closing resources...")

Output:

Trying to open a file...
Error: File not found.
Closing resources...

The finally block is always executed, making it useful for cleanup tasks like closing files or releasing memory.

6. Raising an Exception Using raise

We can manually raise exceptions using the raise statement.

try:
    age = -5
    if age < 0:
        raise ValueError("Age cannot be negative!")
except ValueError as e:
    print("Error:", e)

Output:

Error: Age cannot be negative!

By using raise, we enforce constraints and prevent invalid data from being processed.