How Can I Manually Raise Exceptions in Python?

How do I manually raise (throw) an exception in Python?

Problem Statement

How can I raise an exception in Python that can be caught later using an except block?

Solution

Steps to Raise an Exception:

1. Use the Specific Exception Class: Identify the most appropriate exception class that semantically represents the error.
2. Create an Exception Instance: Instantiate the Exception class with a meaningful message using the constructor syntax: raise ExceptionClass(message).
3. Pass Additional Arguments: You can optionally pass additional arguments to the exception constructor, accessible via the args attribute.

Best Practices

Raising Exceptions

• Raise Specific Exceptions: Avoid raising generic Exception objects. Choose the most specific subclass that accurately describes the issue.
• Provide Meaningful Messages: Include a clear and concise error message in your exception instances.
• Preserve Stack Traces: Use raise ValueError('Error message') to preserve stack traces in captured exceptions.

Catching Exceptions

• Use Specific Catches: Handle errors by catching specific exceptions that may get raised. Avoid catching generic Exception if possible.
• Re-raise Exceptions: If you encounter an error within an except clause that you want to handle at a higher level, use raise with no arguments to re-raise it while preserving the stack trace.

Exception Chaining

• Python 3 Enhancement: In Python 3, you can use raise RuntimeError('specific message') from error to chain exceptions, preserving tracebacks and optionally changing the exception type.

Cautions

• Avoid Modifying Tracebacks: Manipulating exception tracebacks using sys.exc_info() is discouraged due to potential errors and Python version compatibility issues.
• Deprecation Warning: Avoid using raise ValueError, 'message' (Python 2) and raise 'message' (Python 2.4 and lower) as they can raise unintended exceptions.

Example Usage

def raise_custom_exception():
    raise CustomException("This custom exception was intentionally raised.")

Custom Exception Classes

In situations where no built-in exception class fits the specific error scenario, you can create custom exception classes by subclassing the appropriate base exception.

class MyAppError(Exception):
    def __init__(self, message):
        super().__init__(message)

The above is the detailed content of How Can I Manually Raise Exceptions in Python?. For more information, please follow other related articles on the PHP Chinese website!