dependency-management

name	dependency-management
description	Python dependency and environment management for multi-service or monorepo python backends. Use when: (1) adding, upgrading, or removing a Python package, (2) responding to Dependabot or security vulnerability alerts (GHSA/CVE), (3) creating a new service that needs its own requirements files, (4) debugging pip install failures or Docker build issues related to dependencies, (5) reviewing or auditing the dependency tree, (6) running pip-compile. Enforces the pip-compile locked-file workflow and tiered dependency hierarchy.
allowed-tools	Bash, Read, Write

Dependency Management

Core Rules

1. Never pip install <pkg> directly. All changes flow through .in → pip-compile → .txt.
2. Always commit both .in and .txt together. The .in is human intent; the .txt is the machine-verified lockfile.
3. One runtime per service. Each isolated service owns its own requirements.txt lockfile.

Repository Layout (Example)

src/
├── requirements-core.in          # Tier 1: shared baseline (fastapi, pydantic…)
├── requirements-core.txt         # Lockfile for core
├── services/
│   ├── auth_service/
│   │   ├── requirements.in       # Tier 2: inherits core + auth deps
│   │   └── requirements.txt
│   ├── payments_service/
│   │   ├── requirements.in
│   │   └── requirements.txt
│   └── database_service/
│       ├── requirements.in
│       └── requirements.txt

Tiered Hierarchy

Tier	Scope	File	Examples
1 – Core	Shared by >80% of services	requirements-core.in	fastapi, pydantic, httpx
2 – Specialized	Service-specific heavyweights	<service>/requirements.in	stripe, redis, asyncpg
3 – Dev tools	Never in production containers	requirements-dev.in	pytest, black, ruff

Each service .in file usually begins with -r ../../requirements-core.in to inherit the core dependencies.

Workflow: Adding or Upgrading a Package

1. Declare — Add or update the version constraint in the correct .in file.

   • If the package is needed by most services → requirements-core.in
   • If only one service → that service's .in
   • Security floor pins use >= syntax: cryptography>=46.0.5

2. Lock — Compile the lockfile:

   # Core
   pip-compile src/requirements-core.in \
     --output-file src/requirements-core.txt
   
   # Individual service (example: auth)
   pip-compile src/services/auth_service/requirements.in \
     --output-file src/services/auth_service/requirements.txt
   

   Because services inherit core via -r, recompiling a service also picks up core changes.

3. Sync — Install locally to verify:

   pip install -r src/services/<service>/requirements.txt
   

4. Verify — Rebuild the affected Docker/Podman container to confirm stable builds.

5. Commit — Stage and commit both .in and .txt files together.

Workflow: Responding to Dependabot / Security Alerts

1. Identify the affected package and fixed version from the advisory (GHSA/CVE).

2. Determine tier placement:

   • Check if the package is a direct dependency (appears in an .in file).
   • If it only appears in .txt files, it's transitive — pinned by something upstream.

3. For direct dependencies: Bump the version floor in the relevant .in file.

   # SECURITY PATCHES (Mon YYYY)
   package-name>=X.Y.Z
   

4. For transitive dependencies: Add a version floor pin in the appropriate .in file to force the resolver to pull the patched version, even though it's not a direct dependency.

5. Recompile all affected lockfiles. Since services inherit core, a core change means recompiling every service lockfile. Use this compilation order:

   # 1. Core first
   pip-compile src/requirements-core.in \
     --output-file src/requirements-core.txt
   
   # 2. Then each service
   for svc in auth_service payments_service database_service; do
     pip-compile "src/services/${svc}/requirements.in" \
       --output-file "src/services/${svc}/requirements.txt"
   done
   

6. Verify the patched version appears in all affected .txt files:

   grep -i "package-name" src/requirements-core.txt \
     src/services/*/requirements.txt
   

7. If no newer version exists (e.g., inherent design risk like pickle deserialization), document the advisory acknowledgement as a comment in the .in file and note mitigations.

Container / Dockerfile Constraints

• Dockerfiles only use COPY requirements.txt + RUN pip install -r requirements.txt.
• No RUN pip install <pkg> commands. No manual installs.
• Copy requirements.txt before source code to preserve Docker layer caching.

Common Pitfalls

• Forgetting to recompile downstream services after a core .in change.
• Pinning == instead of >= for security floors — use >= so pip-compile can resolve freely.
• Adding dev tools to production .in files — keep pytest, ruff, etc. in requirements-dev.in.
• Committing .txt without .in — always commit them as a pair.