[ENH] V1 → V2 API Migration - core structure (#1576)

Towards #1575

This PR sets up the core folder and file structure along with base
scaffolding for the API v1 → v2 migration.

It includes:

* Skeleton for the HTTP client, backend, and API context
* Abstract resource interfaces and versioned stubs (`*V1`, `*V2`)
* Minimal wiring to allow future version switching and fallback support

No functional endpoints are migrated yet. This PR establishes a stable
foundation for subsequent migration and refactor work.

---------

Co-authored-by: Satvik Mishra <112589278+satvshr@users.noreply.github.com>
Co-authored-by: Franz Király <fkiraly@gcos.ai>
Co-authored-by: Simon Blanke <simon.blanke@yahoo.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Matthias Feurer <lists@matthiasfeurer.de>
Co-authored-by: Pieter Gijsbers <p.gijsbers@tue.nl>

Author: 7 people

openml/__init__.py

@@ -35,6 +35,7 @@
     utils,
 )
 from .__version__ import __version__
+from ._api import _backend
 from .datasets import OpenMLDataFeature, OpenMLDataset
 from .evaluations import OpenMLEvaluation
 from .flows import OpenMLFlow
@@ -116,6 +117,7 @@ def populate_cache(
     "OpenMLTask",
     "__version__",
     "_api_calls",
+    "_backend",
     "config",
     "datasets",
     "evaluations",

openml/_api/__init__.py

@@ -0,0 +1,85 @@
+from .clients import (
+    HTTPCache,
+    HTTPClient,
+    MinIOClient,
+)
+from .resources import (
+    API_REGISTRY,
+    DatasetAPI,
+    DatasetV1API,
+    DatasetV2API,
+    EstimationProcedureAPI,
+    EstimationProcedureV1API,
+    EstimationProcedureV2API,
+    EvaluationAPI,
+    EvaluationMeasureAPI,
+    EvaluationMeasureV1API,
+    EvaluationMeasureV2API,
+    EvaluationV1API,
+    EvaluationV2API,
+    FallbackProxy,
+    FlowAPI,
+    FlowV1API,
+    FlowV2API,
+    ResourceAPI,
+    ResourceV1API,
+    ResourceV2API,
+    RunAPI,
+    RunV1API,
+    RunV2API,
+    SetupAPI,
+    SetupV1API,
+    SetupV2API,
+    StudyAPI,
+    StudyV1API,
+    StudyV2API,
+    TaskAPI,
+    TaskV1API,
+    TaskV2API,
+)
+from .setup import (
+    APIBackend,
+    APIBackendBuilder,
+    _backend,
+)
+
+__all__ = [
+    "API_REGISTRY",
+    "APIBackend",
+    "APIBackendBuilder",
+    "DatasetAPI",
+    "DatasetV1API",
+    "DatasetV2API",
+    "EstimationProcedureAPI",
+    "EstimationProcedureV1API",
+    "EstimationProcedureV2API",
+    "EvaluationAPI",
+    "EvaluationMeasureAPI",
+    "EvaluationMeasureV1API",
+    "EvaluationMeasureV2API",
+    "EvaluationV1API",
+    "EvaluationV2API",
+    "FallbackProxy",
+    "FlowAPI",
+    "FlowV1API",
+    "FlowV2API",
+    "HTTPCache",
+    "HTTPClient",
+    "MinIOClient",
+    "ResourceAPI",
+    "ResourceV1API",
+    "ResourceV2API",
+    "RunAPI",
+    "RunV1API",
+    "RunV2API",
+    "SetupAPI",
+    "SetupV1API",
+    "SetupV2API",
+    "StudyAPI",
+    "StudyV1API",
+    "StudyV2API",
+    "TaskAPI",
+    "TaskV1API",
+    "TaskV2API",
+    "_backend",
+]

openml/_api/clients/__init__.py

@@ -0,0 +1,8 @@
+from .http import HTTPCache, HTTPClient
+from .minio import MinIOClient
+
+__all__ = [
+    "HTTPCache",
+    "HTTPClient",
+    "MinIOClient",
+]