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.
raise
with Exception ChainingYou 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.
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]
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
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
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
While chaining exceptions is powerful, avoid overusing it as it can make the code harder to read if not used carefully.
When chaining exceptions, it’s helpful to document why the exception was re-raised and what additional context is being provided.