Exception chaining

What is Exception Chaining?

Exception chaining is a mechanism that allows you to propagate an exception while preserving the context of the original exception. This is particularly useful when you want to add additional information to an error or when one exception leads to another.

Using raise with Exception Chaining

You can chain exceptions by using the from keyword with the raise statement. This keeps the original exception as the cause of the new exception, which helps in debugging by maintaining the complete traceback.

try:
    result = 10 / 0
except ZeroDivisionError as e:
    raise ValueError("An error occurred while performing division.") from e

In this example, if a ZeroDivisionError occurs, it will be caught and re-raised as a ValueError with the original ZeroDivisionError as its cause.

Why Use Exception Chaining?

  • Preserve Context: Retains the original exception information, making it easier to trace the root cause.
  • Add Context: Allows adding more context or a custom message to the new exception.
  • Enhanced Debugging: Provides a more detailed traceback, showing both the original and the new exceptions.

Example of Exception Chaining

Let’s consider a more detailed example:

def divide_numbers(a, b):
    try:
        return a / b
    except ZeroDivisionError as e:
        raise ValueError(f"Cannot divide {a} by zero.") from e

def process_numbers(num_list):
    results = []
    for num in num_list:
        try:
            result = divide_numbers(10, num)
            results.append(result)
        except ValueError as e:
            print(f"Error processing number {num}: {e}")
    return results

numbers = [2, 0, 5]
results = process_numbers(numbers)
print("Results:", results)

Output:

Error processing number 0: Cannot divide 10 by zero.
Results: [5.0, 2.0]

Inspecting the Chained Exceptions

When you chain exceptions, the traceback will show both the new exception and the original exception, making it easier to debug.

try:
    divide_numbers(10, 0)
except ValueError as e:
    print(f"Caught ValueError: {e}")
    print(f"Original exception: {e.__cause__}")

Output:

Caught ValueError: Cannot divide 10 by zero.
Original exception: division by zero

Best Practices for Exception Chaining

Preserve Original Exception:

Always use from to chain exceptions to maintain the original context.

try:
    # Code that may raise an exception
except SomeError as e:
    raise AnotherError("Additional context") from e
Add Meaningful Context:

When raising a new exception, provide a clear and informative message that explains the error.

try:
    # Code that may raise an exception
except SomeError as e:
    raise AnotherError("Failed due to some error") from e
Use Exception Chaining Judiciously:

While chaining exceptions is powerful, avoid overusing it as it can make the code harder to read if not used carefully.

Document the Chaining:

When chaining exceptions, it’s helpful to document why the exception was re-raised and what additional context is being provided.