Fix MRO ordering for multiple inheritance in GraphQL and JSON Schema/OpenAPI

src/datamodel_code_generator/parser/base.py

@@ -485,6 +485,71 @@ def sort_data_models(  # noqa: PLR0912, PLR0915
     return unresolved_references, sorted_data_models, require_update_action_models
 
 
+def sort_base_classes_for_mro(sorted_data_models: SortedDataModels) -> None:
+    """Sort base classes in each model to ensure valid Python MRO.
+
+    When a class inherits from multiple base classes where some bases inherit
+    from others, Python's C3 linearization requires that child classes appear
+    before their parent classes in the inheritance list.
+
+    For example, if B inherits from A, then class C(A, B) is invalid but
+    class C(B, A) is valid.
+    """
+    for model in sorted_data_models.values():
+        base_classes = model.base_classes
+        if len(base_classes) <= 1:
+            continue
+
+        # Build set of base class paths for quick lookup
+        base_class_paths = {b.reference.path for b in base_classes if b.reference}
+
+        def get_ancestors(
+            ref_path: str,
+            base_class_paths: set[str] = base_class_paths,
+        ) -> set[str]:
+            """Get all ancestor paths that are in our base class list."""
+            ancestors: set[str] = set()
+            source_model = sorted_data_models.get(ref_path)
+            if source_model is None:  # pragma: no cover
+                return ancestors
+            to_visit = [
+                bc.reference.path
+                for bc in source_model.base_classes
+                if bc.reference and bc.reference.path in base_class_paths
+            ]
+            while to_visit:
+                parent_path = to_visit.pop()
+                if parent_path in ancestors:
+                    continue
+                ancestors.add(parent_path)
+                parent_model = sorted_data_models.get(parent_path)
+                if not parent_model:
+                    continue
+                to_visit.extend(
+                    bc.reference.path
+                    for bc in parent_model.base_classes
+                    if bc.reference and bc.reference.path in base_class_paths
+                )
+            return ancestors
+
+        # Build ancestor map for each base class
+        ancestor_map = {b.reference.path: get_ancestors(b.reference.path) for b in base_classes if b.reference}
+
+        def sort_key(
+            bc: BaseClassDataType,
+            ancestor_map: dict[str, set[str]] = ancestor_map,
+        ) -> int:
+            """Sort key: classes that are ancestors of others come later."""
+            if not bc.reference:
+                return 0
+            path = bc.reference.path
+            # Count how many other base classes have this one as an ancestor
+            return sum(1 for other_path in ancestor_map if path in ancestor_map.get(other_path, set()))
+
+        # Use stable sort to preserve original order for elements with equal keys
+        model.base_classes = sorted(base_classes, key=sort_key)
+
+
 def relative(
     current_module: str,
     reference: str,
@@ -3161,6 +3226,7 @@ def parse(  # noqa: PLR0913, PLR0914, PLR0917
         )
 
         _, sorted_data_models, require_update_action_models = sort_data_models(self.results)
+        sort_base_classes_for_mro(sorted_data_models)
 
         (
             module_models,

tests/data/expected/main/graphql/interface_mro.py

@@ -0,0 +1,121 @@
+# generated by datamodel-codegen:
+#   filename:  interface_mro.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Annotated, Literal
+
+from pydantic import BaseModel, Field
+from typing_extensions import TypeAliasType
+
+Boolean = TypeAliasType("Boolean", bool)
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+ID = TypeAliasType("ID", str)
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+String = TypeAliasType("String", str)
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class BaseInterface(BaseModel):
+    base_field: Annotated[String | None, Field(alias='baseField')] = None
+    typename__: Annotated[
+        Literal['BaseInterface'] | None, Field(alias='__typename')
+    ] = 'BaseInterface'
+
+
+class Entity(BaseModel):
+    id: ID
+    typename__: Annotated[Literal['Entity'] | None, Field(alias='__typename')] = (
+        'Entity'
+    )
+
+
+class GrandParent(BaseModel):
+    grand_field: Annotated[String | None, Field(alias='grandField')] = None
+    typename__: Annotated[Literal['GrandParent'] | None, Field(alias='__typename')] = (
+        'GrandParent'
+    )
+
+
+class LeftBranch(BaseInterface):
+    base_field: Annotated[String | None, Field(alias='baseField')] = None
+    left_field: Annotated[String | None, Field(alias='leftField')] = None
+    typename__: Annotated[Literal['LeftBranch'] | None, Field(alias='__typename')] = (
+        'LeftBranch'
+    )
+
+
+class Node(BaseModel):
+    id: ID
+    typename__: Annotated[Literal['Node'] | None, Field(alias='__typename')] = 'Node'
+
+
+class Notification(Node):
+    id: ID
+    read_at: Annotated[String | None, Field(alias='readAt')] = None
+    typename__: Annotated[Literal['Notification'] | None, Field(alias='__typename')] = (
+        'Notification'
+    )
+
+
+class Parent(GrandParent):
+    grand_field: Annotated[String | None, Field(alias='grandField')] = None
+    parent_field: Annotated[String | None, Field(alias='parentField')] = None
+    typename__: Annotated[Literal['Parent'] | None, Field(alias='__typename')] = (
+        'Parent'
+    )
+
+
+class RightBranch(BaseInterface):
+    base_field: Annotated[String | None, Field(alias='baseField')] = None
+    right_field: Annotated[String | None, Field(alias='rightField')] = None
+    typename__: Annotated[Literal['RightBranch'] | None, Field(alias='__typename')] = (
+        'RightBranch'
+    )
+
+
+class CustomerNeedNotification(Entity, Notification, Node):
+    customer: String | None = None
+    id: ID
+    read_at: Annotated[String | None, Field(alias='readAt')] = None
+    typename__: Annotated[
+        Literal['CustomerNeedNotification'] | None, Field(alias='__typename')
+    ] = 'CustomerNeedNotification'
+
+
+class DiamondType(LeftBranch, RightBranch, BaseInterface):
+    base_field: Annotated[String | None, Field(alias='baseField')] = None
+    diamond_field: Annotated[String | None, Field(alias='diamondField')] = None
+    left_field: Annotated[String | None, Field(alias='leftField')] = None
+    right_field: Annotated[String | None, Field(alias='rightField')] = None
+    typename__: Annotated[Literal['DiamondType'] | None, Field(alias='__typename')] = (
+        'DiamondType'
+    )
+
+
+class Child(Parent, GrandParent):
+    child_field: Annotated[String | None, Field(alias='childField')] = None
+    grand_field: Annotated[String | None, Field(alias='grandField')] = None
+    parent_field: Annotated[String | None, Field(alias='parentField')] = None
+    typename__: Annotated[Literal['Child'] | None, Field(alias='__typename')] = 'Child'
+
+
+class LeafType(Child, Parent, GrandParent):
+    child_field: Annotated[String | None, Field(alias='childField')] = None
+    grand_field: Annotated[String | None, Field(alias='grandField')] = None
+    leaf_field: Annotated[String | None, Field(alias='leafField')] = None
+    parent_field: Annotated[String | None, Field(alias='parentField')] = None
+    typename__: Annotated[Literal['LeafType'] | None, Field(alias='__typename')] = (
+        'LeafType'
+    )

tests/data/expected/main/graphql/union_class_name_prefix.py

@@ -0,0 +1,67 @@
+# generated by datamodel-codegen:
+#   filename:  union_class_name_prefix.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Annotated, Literal, Union
+
+from pydantic import BaseModel, Field
+from typing_extensions import TypeAliasType
+
+FooBoolean = TypeAliasType("FooBoolean", bool)
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+FooID = TypeAliasType("FooID", str)
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+FooInt = TypeAliasType("FooInt", int)
+"""
+The `Int` scalar type represents non-fractional signed whole numeric values. Int can represent values between -(2^31) and 2^31 - 1.
+"""
+
+
+FooString = TypeAliasType("FooString", str)
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class FooIResource(BaseModel):
+    id: FooID
+    typename__: Annotated[Literal['IResource'] | None, Field(alias='__typename')] = (
+        'IResource'
+    )
+
+
+class FooCar(FooIResource):
+    id: FooID
+    passenger_capacity: Annotated[FooInt, Field(alias='passengerCapacity')]
+    typename__: Annotated[Literal['Car'] | None, Field(alias='__typename')] = 'Car'
+
+
+class FooEmployee(FooIResource):
+    first_name: Annotated[FooString | None, Field(alias='firstName')] = None
+    id: FooID
+    last_name: Annotated[FooString | None, Field(alias='lastName')] = None
+    typename__: Annotated[Literal['Employee'] | None, Field(alias='__typename')] = (
+        'Employee'
+    )
+
+
+FooResource = TypeAliasType(
+    "FooResource",
+    Union[
+        'FooCar',
+        'FooEmployee',
+    ],
+)
+
+
+FooTechnicalResource = TypeAliasType("FooTechnicalResource", FooCar)

tests/data/expected/main/jsonschema/allof_mro.py

@@ -0,0 +1,31 @@
+# generated by datamodel-codegen:
+#   filename:  allof_mro.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+
+class BaseInterface(BaseModel):
+    id: str | None = None
+
+
+class LeftBranch(BaseInterface):
+    left_value: str | None = None
+
+
+class RightBranch(BaseInterface):
+    right_value: str | None = None
+
+
+class DiamondType(LeftBranch, RightBranch, BaseInterface):
+    """
+    Diamond inheritance pattern - inherits from both LeftBranch and RightBranch which both inherit from BaseInterface
+    """
+
+    diamond_value: str | None = None
+
+
+class Model(DiamondType):
+    pass

tests/data/expected/main/jsonschema/unknown_format.py

@@ -0,0 +1,11 @@
+# generated by datamodel-codegen:
+#   filename:  unknown_format.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+
+class Model(BaseModel):
+    data: str | None = None

tests/data/graphql/interface_mro.graphql

@@ -0,0 +1,61 @@
+interface Entity {
+    id: ID!
+}
+
+interface Node {
+    id: ID!
+}
+
+interface Notification implements Node {
+    id: ID!
+    readAt: String
+}
+
+type CustomerNeedNotification implements Entity & Node & Notification {
+    id: ID!
+    readAt: String
+    customer: String
+}
+
+interface BaseInterface {
+    baseField: String
+}
+
+interface LeftBranch implements BaseInterface {
+    baseField: String
+    leftField: String
+}
+
+interface RightBranch implements BaseInterface {
+    baseField: String
+    rightField: String
+}
+
+type DiamondType implements BaseInterface & LeftBranch & RightBranch {
+    baseField: String
+    leftField: String
+    rightField: String
+    diamondField: String
+}
+
+interface GrandParent {
+    grandField: String
+}
+
+interface Parent implements GrandParent {
+    grandField: String
+    parentField: String
+}
+
+interface Child implements GrandParent & Parent {
+    grandField: String
+    parentField: String
+    childField: String
+}
+
+type LeafType implements GrandParent & Parent & Child {
+    grandField: String
+    parentField: String
+    childField: String
+    leafField: String
+}

tests/data/graphql/union_class_name_prefix.graphql

@@ -0,0 +1,18 @@
+interface IResource {
+    id: ID!
+}
+
+type Employee implements IResource {
+    id: ID!
+    firstName: String
+    lastName: String
+}
+
+type Car implements IResource {
+    id: ID!
+    passengerCapacity: Int!
+}
+
+union Resource = Employee | Car
+
+union TechnicalResource = Car