refactor: mark child_elements abstract in Element, so subtypes have to be expicit

Author: yt-ms

src/structurizr/mixin/childless_mixin.py

@@ -0,0 +1,30 @@
+# Copyright (c) 2020
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+"""Provide a mixin that indicates an element type does not have children."""
+
+
+from typing import Iterable
+
+from ..model.element import Element
+
+
+class ChildlessMixin:
+    """Define a mixin for childless element types."""
+
+    @property
+    def child_elements(self) -> Iterable[Element]:
+        """Return child elements (from `Element.children`)."""
+        return []

src/structurizr/model/component.py

@@ -20,6 +20,7 @@
 
 from pydantic import Field
 
+from ..mixin.childless_mixin import ChildlessMixin
 from .code_element import CodeElement, CodeElementIO
 from .static_structure_element import StaticStructureElement, StaticStructureElementIO
 from .tags import Tags
@@ -58,7 +59,7 @@ class ComponentIO(StaticStructureElementIO):
     size: Optional[int] = None
 
 
-class Component(StaticStructureElement):
+class Component(ChildlessMixin, StaticStructureElement):
     """
     Represent a component.
 

src/structurizr/model/element.py

@@ -16,7 +16,7 @@
 """Provide a superclass for all model elements."""
 
 
-from abc import ABC
+from abc import ABC, abstractmethod
 from typing import Iterable, Iterator, List, Optional
 
 from pydantic import Field, HttpUrl
@@ -83,12 +83,10 @@ def __repr__(self):
         return f"{type(self).__name__}(id={self.id}, name={self.name})"
 
     @property
+    @abstractmethod
     def child_elements(self) -> Iterable["Element"]:
-        """Return the elements that are children of this one.
-
-        Subclasses should override this to provide their specific children.
-        """
-        return []
+        """Return the elements that are children of this one."""
+        ...  # pragma: no cover
 
     def get_relationships(self) -> Iterator[Relationship]:
         """Return a Iterator over all relationships involving this element."""

src/structurizr/model/infrastructure_node.py

@@ -15,6 +15,7 @@
 
 from typing import TYPE_CHECKING, Optional
 
+from ..mixin.childless_mixin import ChildlessMixin
 from .deployment_element import DeploymentElement, DeploymentElementIO
 from .tags import Tags
 
@@ -40,7 +41,7 @@ class InfrastructureNodeIO(DeploymentElementIO):
     technology: Optional[str] = ""
 
 
-class InfrastructureNode(DeploymentElement):
+class InfrastructureNode(ChildlessMixin, DeploymentElement):
     """
     Represent an infrastructure node.
 

src/structurizr/model/person.py

@@ -20,6 +20,7 @@
 
 from pydantic import Field
 
+from ..mixin.childless_mixin import ChildlessMixin
 from .location import Location
 from .relationship import Relationship
 from .static_structure_element import StaticStructureElement, StaticStructureElementIO
@@ -43,7 +44,7 @@ class PersonIO(StaticStructureElementIO):
     )
 
 
-class Person(StaticStructureElement):
+class Person(ChildlessMixin, StaticStructureElement):
     """
     Represent a person in the C4 model.
 

src/structurizr/model/static_structure_element_instance.py

@@ -21,6 +21,7 @@
 
 from pydantic import Field
 
+from ..mixin.childless_mixin import ChildlessMixin
 from .deployment_element import DeploymentElement, DeploymentElementIO
 from .http_health_check import HTTPHealthCheck, HTTPHealthCheckIO
 from .static_structure_element import StaticStructureElement
@@ -41,7 +42,7 @@ class StaticStructureElementInstanceIO(DeploymentElementIO, ABC):
     health_checks: List[HTTPHealthCheckIO] = Field(default=(), alias="healthChecks")
 
 
-class StaticStructureElementInstance(DeploymentElement, ABC):
+class StaticStructureElementInstance(ChildlessMixin, DeploymentElement, ABC):
     """Define a superclass for all deployment instances."""
 
     def __init__(

tests/unit/model/test_element.py

@@ -18,10 +18,11 @@
 
 import pytest
 
+from structurizr.mixin.childless_mixin import ChildlessMixin
 from structurizr.model.element import Element
 
 
-class ConcreteElement(Element):
+class ConcreteElement(ChildlessMixin, Element):
     """Implement a concrete `Element` class for testing purposes."""
 
     pass