Python Import-Time Side Effects

Rule

No implicit state or side effects at import time.

Allowed Exceptions

Framework-standard registration mechanisms are permitted when required by the framework. Examples include:

• SQLAlchemy ORM model declarations (including Table definitions and declarative class registration in Base.metadata)
• FastAPI router definitions (APIRouter() creation and decorator-based route registration)

These are allowed only because the frameworks require import-time registration. All other side effects remain forbidden.

Forbidden Examples

• Engine or session creation
• Settings loading or environment mutation
• Network calls
• File system writes
• Global mutable state initialization