Crimes with Python's Pattern Matching (2022)
2 days ago
- #Python
- #Abstract Base Classes
- #Pattern Matching
- Python's `__subclasshook__` in Abstract Base Classes (ABCs) allows defining custom subclass checks, enabling unconventional type matching.
- Python 3.10's pattern matching interacts with ABCs via `isinstance()`, allowing ABCs to influence match cases based on dynamic conditions.
- Example: `NotIterable` ABC hijacks pattern matching by checking for the absence of `__iter__`, demonstrating unexpected behavior.
- Pattern matching can destructure object fields, and ABCs can dynamically match objects with specific attributes (e.g., `DistanceMetric`).
- Combinators like `Not` and `And` can be created at runtime to compose complex matching logic, though syntax constraints exist.
- Side effects in `__subclasshook__` (e.g., caching, user prompts) are possible but discouraged due to unpredictability.
- Warning: Misusing `__subclasshook__` for pattern matching is considered dark magic and not recommended for production code.