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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.

@geetu040 geetu040 mentioned this pull request Dec 30, 2025
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codecov-commenter commented Dec 31, 2025

Codecov Report

❌ Patch coverage is 0% with 234 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.68%. Comparing base (99928f8) to head (43276d2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines Patch % Lines
openml/_api/clients/http.py 0.00% 95 Missing ⚠️
openml/_api/resources/tasks.py 0.00% 47 Missing ⚠️
openml/_api/config.py 0.00% 31 Missing ⚠️
openml/_api/runtime/core.py 0.00% 29 Missing ⚠️
openml/_api/resources/datasets.py 0.00% 9 Missing ⚠️
openml/_api/resources/base.py 0.00% 8 Missing ⚠️
openml/_api/runtime/fallback.py 0.00% 6 Missing ⚠️
openml/_api/__init__.py 0.00% 4 Missing ⚠️
openml/_api/resources/__init__.py 0.00% 3 Missing ⚠️
openml/_api/clients/__init__.py 0.00% 2 Missing ⚠️
Additional details and impacted files
@@            Coverage Diff             @@
##             main    #1576      +/-   ##
==========================================
- Coverage   52.75%   49.68%   -3.08%     
==========================================
  Files          36       46      +10     
  Lines        4333     4567     +234     
==========================================
- Hits         2286     2269      -17     
- Misses       2047     2298     +251     

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cache: CacheConfig


settings = Settings(
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I would move the settings to the individual classes. I think this design introduces too high coupling of the classes to this file. You cannot move the classes around, or add a new API version without making non-extensible changes to this file here - because APISettings will require a constructor change and new classes it accepts.

Instead, a better design is to apply the strategy pattern cleanly to the different API definitions - v1 and v2 - and move the config either to their __init__, or a set_config (or similar) method.

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Overall really great, I have a design suggestion related to the configs.

The config.py file and the coupling on it breaks an otherwise nice strategy pattern.

I recommend to follow the strategy pattern cleanly instead, and move the configs into the class instances, see above.

This will make the backend API much more extensible and cohesive.

key="...",
),
v2=APIConfig(
server="http://127.0.0.1:8001/",
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should this be hardcoded? I guess this is just for your local development

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this is hard-coded, they are the default values though the local endpoints will be replaced by remote server when deployed hopefully before merging this in main


if strict:
return v2

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In a previous commit the 'FallbackProxy' was used here. Do we still need this class?

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I removed this because of the ruff errors. I'll put them back and fix the pre-commit when the class is implemented.

@geetu040 geetu040 changed the title [ENH] Migration: set up core/base structure [ENH] V1 → V2 API Migration - core structure Jan 9, 2026
@geetu040 geetu040 marked this pull request as draft January 12, 2026 18:47
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Overall I agree with the suggested changes. This seems like a reasonable way to provide a unified interface for two different backends, and also separate out some concerns that were previously coupled or scattered more than they should (e.g., caching, configurations).

My main concern is with the change to caching behavior. I have a minor concern over the indirection APIContext introduces (perhaps I misunderstood its purpose), and the introduction of allowing Response return values.

In my comments you will also find some things that may already have been "wrong" in the old implementation. In that case, I think it simply makes sense to make the change now so I repeat it here for convenience.

from openml._api.config import APIConfig


class CacheMixin:
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The ttl should probably heavily depend on the path. If we do end up using caching at this level, we should use the Cache-Control HTTP Header response so the server can inform us how long to keep it in cache for (something that, I believe, neither servers do right now). A dataset search query can change if any dataset description changes (to either be now included or excluded), so caching probably shouldn't even be on by default for such type of queries. Dataset descriptions might change, but likely not very frequently. Dataset data files or computed qualities should (almost?) never change. This is the reason that the current implementation only caches description, features, qualities, and the dataset itself.

With this implementation, you also introduce some new issues:

  • What if the paths change, or even the query parameters? there is now dead cache. Do we now add cache cleanup routines? How does openml-python know what is no longer valid if they were responses with high TTL?
  • URLs may be (much) longer than the default max path of Windows (260 characters). If I'm not mistaken, this will lead to an issue unless you specifically work around it.
  • More of an implementation detail, but authenticated and unauthenticated requests are not differentiated. If a user accidentally makes an unauthenticated request, gets an error, and then authenticates they would still get an error.

tasks=TasksV1(v1_http),
)

if version == "v1":
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nit: supported versions should be encoded in an Enum. This helps function signatures (type checking, code completion) and reduces chance for erroneous input.

Comment on lines +1 to +8
from openml._api.runtime.core import APIContext


def set_api_version(version: str, *, strict: bool = False) -> None:
api_context.set_version(version=version, strict=strict)


api_context = APIContext()
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@PGijsbers PGijsbers Jan 15, 2026

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It's not clear what the function of the APIContext is here. Why do we need it and cannot just use backend directly? E.g.:

Suggested change
from openml._api.runtime.core import APIContext
def set_api_version(version: str, *, strict: bool = False) -> None:
api_context.set_version(version=version, strict=strict)
api_context = APIContext()
from openml._api.runtime.core import build_backend
_backend = build_backend("v1", strict=False)
def set_api_version(version: str, *, strict: bool = False) -> None:
global _backend
_backend = build_backend(version=version, strict=strict)
def backend() -> APIBackend:
return _backend

If it is just to avoid the pitfall where users assign the returned value to a local variable with a scope that is too long lived, then the same would apply if users would assign api_context.backend to a variable. We could instead extend the APIBackend class to allow updates to its attributes?

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I agree with you, it's not really useful, I am going to iterate over the design and will keep this in mind

server: str
base_url: str
key: str
timeout: int = 10 # seconds
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nit: Add a unit suffix (timeout_seconds) so the unit is clear without navigating to the source.

ps. I also considered typing it as datetime.timedelta but considering you probably only use it in seconds and there is a real risk of developers erroneously using datetime.timedelta.seconds instead of datetime.timedelta.total_seconds(), I think keeping it an integer is better.

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makes sense

class ConnectionConfig:
retries: int = 3
delay_method: DelayMethod = "human"
delay_time: int = 1 # seconds
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nit: here too, including the unit makes sense (delay_time_seconds)

@dataclass
class CacheConfig:
dir: str = "~/.openml/cache"
ttl: int = 60 * 60 * 24 * 7 # one week
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nit: Considering the TTL of the HTTP standard is already defined in seconds, maybe it is fine to exclude it in the variable name? Though as noted above there is a discussion to be had about having this as a cache level property in the first place.
For future reference, setting the value to timedelta(weeks=1).total_seconds() is preferred over the arithmetic+comment.


@dataclass
class CacheConfig:
dir: str = "~/.openml/cache"
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@PGijsbers PGijsbers Jan 15, 2026

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Default should continue to respect XDG_CACHE_HOME.

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6 participants