Assertions vs. Exceptions: When to Use Each for Error Handling?

Distinguishing the Roles of Assertions and Exceptions for Error Handling

In software development, one often confronts the choice between using assertions and exceptions for error handling. While both mechanisms serve the purpose of flagging potential errors, their intended usage differs significantly.

Assertions: Crash Early to Detect Corruption

Assertions are primarily intended for checking conditions that should never occur under normal circumstances. Their goal is to help you catch program state corruption early on and crash the application immediately. Consider the following example:

assert x >= 0, 'x is less than zero'

This assertion checks that x is a non-negative value. If x is ever found to be negative, it raises an assertion error and crashes the program. Assertions are particularly valuable for detecting internal errors introduced by bugs or corrupted data.

Exceptions: Handling Recoverable Errors

Exceptions, on the other hand, are designed to handle errors that can potentially occur but are not inherently catastrophic. These errors can be gracefully recovered from, allowing the program to continue execution. For instance:

if x <p>This code raises a user-defined Exception if x is negative, providing a meaningful error message. Exceptions can be caught using try/except blocks, allowing you to handle the error and potentially recover.</p><p><strong>Setting Business Rules without Try/Except</strong></p><p>You may also want to enforce business rules that trigger errors unconditionally, regardless of the code's position. While assertions can check conditions at the function level, they do not propagate throughout the code. To achieve this desired behavior, you can define your own custom exceptions and raise them wherever the business rule is violated. For example:</p><pre class="brush:php;toolbar:false">class BusinessRuleViolationException(Exception):
    pass

def check_x_non_negative(x):
    if x <p>This ensures that whenever this function is called with a negative x, an exception is raised, providing a robust way to enforce your business rules.</p>

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