python-best-practices

Python Best Practices

This skill applies modern Python mechanisms without adding annotation or packaging ceremony. It favors inference, precise structural contracts, explicit runtime validation, side-effect-free imports, and context-managed resource ownership.

References

Read as many linked references as are relevant to the current task before writing or reviewing Python.

• Let the checker infer obvious implementation results; add annotations only when inference cannot express the contract.
• Parse serialized input once at the controlled boundary so untrusted mappings do not spread inward.
• Depend on the smallest required behavior with protocols, not a vendor's concrete client.
• In Python 3.13+, express caller-relevant relationships with PEP 695 generics, avoiding module-level TypeVar declarations and needless generics.
• Make a new finite-state member a type-checking failure with exhaustive handling.
• Narrow optional values before use so the owning layer—not a cast or fabricated fallback—decides whether absence is preserved or rejected.
• Do not turn missing required data into a plausible value; preserve absence or fail at the owning boundary.
• Keep required runtime checks active under optimization; do not use assertions for validation.
• Catch only expected exceptions at the operation that raises them through narrow exception handling.
• Keep imports inert and consumer-facing names deliberate with explicit public exports.
• Prevent checkout-dependent imports by following the Python naming and src layout.
• Publish intentional inline types with py.typed, reserving third-party stubs for dependencies that lack complete inline types instead of adding markers merely to silence diagnostics.
• Make async cleanup inseparable from acquisition with async context managers.
• Preserve page boundaries and consumer control over remote traversal through async pagination.
• Give every independently buildable distribution its own declared metadata and direct dependencies.
• Make packaging errors visible by not mutating import paths.