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layer-violation

Category: Module Structure
Severity: Configurable via architecture.rules[].severity
Triggered by: pyscn analyze, pyscn check --select deps

What it does

Flags an import statement when the source module's layer is not permitted to depend on the target module's layer, according to the [[architecture.rules]] you configured. Layers are assigned to modules by matching package name fragments defined in [[architecture.layers]].

Why is this a problem?

A layered architecture only pays off while the layers hold. A single shortcut from presentation into infrastructure is enough to:

  • Defeat testability. The presentation layer can now only be exercised with a real database / HTTP client behind it.
  • Create hidden coupling. Swapping the infrastructure implementation silently breaks UI code that was never supposed to know it existed.
  • Normalise the violation. Once one shortcut exists, the next one is easier to justify.

The rule is the automated enforcement of the architecture diagram you already drew in a design doc.

Example

Config:

[[architecture.layers]]
name = "presentation"
packages = ["api", "handlers"]

[[architecture.layers]]
name = "application"
packages = ["services", "usecases"]

[[architecture.layers]]
name = "infrastructure"
packages = ["repositories", "db"]

[[architecture.rules]]
from = "presentation"
allow = ["application"]
deny = ["infrastructure"]

Violating code:

# myapp/api/orders.py  (presentation)
from myapp.repositories.orders import OrderRepository   # ← forbidden

def list_orders():
    return OrderRepository().all()

presentation reaches past application straight into infrastructure.

Use instead

Route the call through the application layer:

# myapp/services/orders.py  (application)
from myapp.repositories.orders import OrderRepository

def list_orders():
    return OrderRepository().all()

# myapp/api/orders.py  (presentation)
from myapp.services.orders import list_orders

def get():
    return list_orders()

api now depends only on services, and the infrastructure is replaceable without touching the presentation layer.

Options

Option Default Description
[[architecture.layers]] — Defines layers and the package fragments that belong to each.
[[architecture.rules]] — from / allow / deny / optional severity per rule.
architecture.validate_layers true Set to false to disable this rule.
architecture.strict_mode true In strict mode, anything not explicitly allowed is denied.
architecture.fail_on_violations false Non-zero exit code when a violation is found.

With no layers configured, the analyzer runs in permissive mode and this rule produces no findings.

References