Validating Inputs

Input validation checks external data—user submissions, API responses, files, database results—against type, range, format, and null constraints before processing. This prevents garbage data from causing bugs, corrupting state, or creating security vulnerabilities by rejecting or raising errors on invalid inputs.

Assertions catch programmer errors during development and should fail fast; error handling manages expected production anomalies gracefully. Use assertions for invariants that should never occur if code is correct, and error handling for untrusted external inputs that may legitimately be invalid.

Validate each field against its constraints: check email format, password strength, age range, required field presence. Raise specific exceptions (ValueError, TypeError) when validation fails, providing explicit error messages. This ensures only clean data enters downstream processing.

Validate all external inputs: user-submitted forms, API responses, database query results, file contents, and network data. Any data crossing a system boundary from an untrusted source requires validation to ensure data integrity and prevent processing corrupted or malicious input.

Yes. Input validation applies to any scale and context where external data enters your system. The two-level defense strategy—assertions for development, robust error handling for production—scales from simple functions to complex distributed systems.

Skipping validation risks data corruption, unexpected exceptions deep in processing logic, security vulnerabilities, and hard-to-debug failures. Invalid data propagates through your system, making root-cause analysis difficult and leaving your application vulnerable to garbage input.