Python Modernization Analyzer

"The best time to modernize was five years ago. The second best time is now." -- Adapted from software engineering practice

"Big bang rewrites fail. Incremental migration succeeds." -- Martin Fowler, Refactoring

Core Philosophy

Legacy Python codebases accumulate technical debt in predictable patterns: Python 2 syntax that 2to3 can partially fix, synchronous I/O that blocks under load, monolithic Flask/Django apps that resist scaling, requirements.txt files with unpinned dependencies, and bare dictionaries where Pydantic models would provide safety.

This skill assesses, quantifies, and plans — it does NOT perform the migration. The output is a prioritized modernization plan with risk scores, effort estimates, and a recommended migration sequence. The plan is the deliverable; execution is a separate task.

Non-Negotiable Constraints:

1. Assess before acting — never recommend a migration path without evidence from the codebase.
2. Incremental over big-bang — every recommendation must be achievable in phases, not a single rewrite.
3. Preserve business logic unchanged — the goal is modernization, not reimplementation.
4. Dependencies are the real blockers — a framework migration blocked by an unmaintained dependency is not viable.
5. Every recommendation must cite evidence — grep -r "from flask" is evidence; "I think this is Flask" is not.

What this skill is NOT: a migration execution tool (it produces a plan, not code); a code-quality review (use python-architecture-checklist); a security review (use python-security-review).

The 10 domain principles and the knowledge-base lookup queries that ground every decision live in references/domain-principles.md.

Workflow

Phase 1: SCAN

Inventory the codebase: Python version(s); framework (Flask/Django/FastAPI/bare/none); packaging (requirements.txt/setup.py/setup.cfg/pyproject.toml); test framework and coverage; async usage; ORM/database access pattern; deployment method. Full detection command set (Python 2 markers, framework greps, packaging, coverage, async, 2to3 -l, pyupgrade --dry-run) is in references/scan-commands.md.

Phase 2: ASSESS

Score each migration path by effort, risk, and blocker potential. The migration-path catalog (tool

• assessment method) is in references/scan-commands.md; scoring dimensions and the full risk table are in references/migration-risk-matrix.md.

Phase 3: PLAN

Produce a prioritized, phased plan, each phase leaving the app in a working state: 0. Prerequisites — tests, CI/CD, Docker (if missing)

1. Low-risk wins — packaging, type hints, linting
2. Framework-independent — Python version upgrade, dependency updates
3. Architecture — service layer extraction, ORM adoption
4. Framework — Flask/Django → FastAPI (if applicable)
5. Async — sync → async migration (if applicable)
6. Auth — authentication modernization (last, highest risk)

Phase 4: REPORT

Deliver the plan with evidence, risk scores, and effort estimates. Use the risk scoring table in references/migration-risk-matrix.md and the deliverable layout in ## Output Template below.

Apply the domain principles throughout (references/domain-principles.md), and follow the AI discipline rules in references/discipline-and-recovery.md at every phase.

State Block

<python-modernization-state>
  phase: SCAN | ASSESS | PLAN | REPORT | COMPLETE
  python_version: [2.x / 3.x / unknown]
  framework: flask | django | fastapi | bare | unknown
  packaging: requirements_txt | setup_py | pyproject_toml | unknown
  test_coverage: none | low | medium | high | unknown
  async_usage: none | partial | full
  migration_paths_identified: 0
  blockers_identified: 0
  last_action: [description]
  next_action: [description]
</python-modernization-state>

Output Template

The full Modernization Assessment Summary (Markdown — header block, migration-paths table, blockers table, phased plan) is in references/assessment-summary-template.md. Before/after compatibility examples for Python 2→3, sync→async, and Flask→FastAPI are in references/compatibility-patterns.md.

AI Discipline

Evidence before recommendation, quantify before scoring, and incremental plans only — with worked WRONG/RIGHT examples, the 10-row anti-pattern catalog, and error-recovery procedures (tools missing, unknown dependency compatibility, zero test coverage) — are in references/discipline-and-recovery.md.

Integration with Other Skills