Python Solid Design

Python SOLID Design

Announce

Using the python-solid-design skill for OOP design/review.

Read order

1. Tier-A checklist below — apply on design and review.
2. Open the principle file that matches the smell:
   • srp.md — Single Responsibility
   • ocp.md — Open/Closed
   • lsp.md — Liskov Substitution
   • isp.md — Interface Segregation
   • dip.md — Dependency Inversion
3. examples.md — Violation / Compliant pairs when refactoring
4. references.md — definitions, smell map, Python idioms

When to apply

Apply when adding or changing classes/modules, especially:

• God-objects mixing domain, I/O, payment, and notifications
• Growing if/elif switches on type or mode
• Subclasses that raise, no-op, or change base contracts
• Fat interfaces forcing unused methods
• High-level policy that constructs concrete collaborators

Skip forcing SOLID onto tiny one-off scripts with no extension or test pressure.

Review workflow

1. Name each class’s responsibility and reasons to change.
2. Prefer new subtypes / strategies over editing closed, tested branches.
3. Check subtype contracts (inputs accepted, outputs promised, no surprise exceptions).
4. Split fat interfaces; clients depend only on methods they use.
5. Depend on abstractions; inject collaborators (constructor injection).
6. After structural refactors, format/lint with the project style skill when relevant.

Tier A — enforce on OOP edits

• S — One concept / one primary reason to change per class
• O — Extend behavior with new code; avoid modifying closed switches for every variant
• L — Subtypes must be safely substitutable for their base/Protocol
• I — Prefer small, role-specific interfaces over one kitchen-sink ABC
• D — High-level modules depend on abstractions, not concrete low-level types

Anti-overengineering

• SRP means one concept, not one method per class — avoid a fragmented sea of single-method types.
• Do not invent an ABC/Protocol for a single forever-stable implementation.
• Prefer composition when inheritance breaks substitutability.
• SOLID guides design under change pressure; clarity beats ceremony.

Additional resources

• references.md
• srp.md
• ocp.md
• lsp.md
• isp.md
• dip.md
• examples.md