Topic 2: Using try, except, finally Blocks

Python for All: Discover, Learn, and Master Lesson 7: Exception Handling Topic 2: Using try, except, finally Blocks

1. Introduction

Python provides the try, except, finally constructs to facilitate exception handling. These blocks allow programmers to capture errors, handle them gracefully, and ensure certain operations are always carried out, regardless of errors.

2. The `try` Block

The primary objective of the try block is to execute code that might produce an exception. By placing this potentially problematic code inside a try block, we prepare ourselves to handle the error if it occurs.

Example:

python

try:
    result = 5 / 0
except:
    print("An error occurred.")

3. The `except` Block

When an exception occurs within the try block, the code inside the except block is executed. This block can catch specific exceptions or a generic exception.

Example:

Handling a specific exception:

python

try:
    result = 5 / 0
except ZeroDivisionError:
    print("Division by zero!")

Handling multiple exceptions:

python

try:
    # some code that might raise various exceptions
    number = int("invalid")
except (ZeroDivisionError, ValueError) as e:
    print(f"Caught an error: {e}")

Using multiple except blocks:

python

try:
    # some code
except ZeroDivisionError:
    print("Division by zero!")
except ValueError:
    print("Invalid value!")

4. The `else` Block

The else block, if present, is executed after the try block code successfully runs without any exceptions. It’s a good place to keep code that you want to execute if no exceptions are raised.

Example:

python

try:
    number = int("5")
except ValueError:
    print("Conversion failed.")
else:
    print(f"Conversion successful. Number is {number}.")

5. The `finally` Block

The finally block will always be executed, regardless of whether an exception was raised in the try block. It’s commonly used for cleanup actions, like closing files or releasing resources.

Example:

python

try:
    file = open("sample.txt", "r")
    # some file operations
except FileNotFoundError:
    print("File not found.")
finally:
    file.close()
    print("File closed.")

6. Raising Exceptions with `raise`

Sometimes, you might want to raise an exception explicitly, based on certain conditions. The raise keyword lets you trigger exceptions in your code.

Example:

python

age = -1
if age < 0:
    raise ValueError("Age cannot be negative.")

7. Assertions

Another mechanism for raising exceptions is using the assert statement. It’s a debugging aid that tests a condition. If the condition is True, it does nothing, but if it’s False, it raises an AssertionError with an optional error message.

Example:

python

age = -1
assert age >= 0, "Age cannot be negative."

8. Conclusion

try, except, finally, and the associated constructs offer a comprehensive mechanism to deal with exceptions in Python. They ensure that we can write code that’s resilient to unforeseen errors, thereby improving the stability and reliability of our applications. Proper use of these constructs is fundamental in creating user-friendly and robust software solutions.

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