Refactor pyreverse Association Logic

doc/whatsnew/fragments/9045.feature

@@ -0,0 +1,4 @@
+Enhanced pyreverse to properly distinguish between UML relationship types (association, aggregation, composition) based on object ownership semantics. Type annotations without assignment are now treated as associations, parameter assignments as aggregations, and object instantiation as compositions.
+
+Closes #9045
+Closes #9267

pylint/pyreverse/diagrams.py

@@ -226,6 +226,7 @@ def extract_relationships(self) -> None:
             obj.attrs = self.get_attrs(node)
             obj.methods = self.get_methods(node)
             obj.shape = "class"
+
             # inheritance link
             for par_node in node.ancestors(recurs=False):
                 try:
@@ -234,27 +235,41 @@ def extract_relationships(self) -> None:
                 except KeyError:
                     continue
 
-            # associations & aggregations links
-            for name, values in list(node.aggregations_type.items()):
+            # Track processed attributes to avoid duplicates
+            processed_attrs = set()
+
+            # Process in priority order: Composition > Aggregation > Association
+
+            # 1. Composition links (highest priority)
+            for name, values in list(node.compositions_type.items()):
+                if not self.show_attr(name):
+                    continue
                 for value in values:
-                    if not self.show_attr(name):
-                        continue
+                    self.assign_association_relationship(
+                        value, obj, name, "composition"
+                    )
+                    processed_attrs.add(name)
 
+            # 2. Aggregation links (medium priority)
+            for name, values in list(node.aggregations_type.items()):
+                if not self.show_attr(name) or name in processed_attrs:
+                    continue
+                for value in values:
                     self.assign_association_relationship(
                         value, obj, name, "aggregation"
                     )
+                    processed_attrs.add(name)
 
+            # 3. Association links (lowest priority)
             associations = node.associations_type.copy()
-
             for name, values in node.locals_type.items():
                 if name not in associations:
                     associations[name] = values
 
             for name, values in associations.items():
+                if not self.show_attr(name) or name in processed_attrs:
+                    continue
                 for value in values:
-                    if not self.show_attr(name):
-                        continue
-
                     self.assign_association_relationship(
                         value, obj, name, "association"
                     )

pylint/pyreverse/dot_printer.py

@@ -30,12 +30,18 @@ class HTMLLabels(Enum):
 # pylint: disable-next=consider-using-namedtuple-or-dataclass
 ARROWS: dict[EdgeType, dict[str, str]] = {
     EdgeType.INHERITS: {"arrowtail": "none", "arrowhead": "empty"},
-    EdgeType.ASSOCIATION: {
+    EdgeType.COMPOSITION: {
         "fontcolor": "green",
         "arrowtail": "none",
         "arrowhead": "diamond",
         "style": "solid",
     },
+    EdgeType.ASSOCIATION: {
+        "fontcolor": "green",
+        "arrowtail": "none",
+        "arrowhead": "vee",
+        "style": "solid",
+    },
     EdgeType.AGGREGATION: {
         "fontcolor": "green",
         "arrowtail": "none",