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Improved static typing in dash.callback#3746

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odorikakeru:baker/typed-callback
Open

Improved static typing in dash.callback#3746
odorikakeru wants to merge 7 commits intoplotly:devfrom
odorikakeru:baker/typed-callback

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@odorikakeru odorikakeru commented Apr 16, 2026

This PR improves static typing for the callback decorator by preserving wrapped function signatures in the callback decorator type contract.

Based on this conversation thread: [Feature Request] A "@dash.typed_callback" that plays more nicely with MyPy than "@dash.callback" #3691 - The decision was made to not implement @dash.typed_callback and instead update @dash.callback based on feedback.

Compliance tests added (not included in integration test directory as type hints don't affect run time behaviour).

DISCLOSURE: Testing code was written/edited with AI assistance.

Contributor Checklist

  • I have broken down my PR scope into the following TODO tasks
    • Update callback decorator type signature
    • Add and validate callback decorator typing/runtime tests (some testing code written with AI assistance)
  • I have run the tests locally and they passed. (refer to testing section in contributing)
  • I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

  • I have added entry in the CHANGELOG.md
  • If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
    • this GitHub #PR number updates the dash docs
    • here is the show and tell thread in Plotly Dash community

@T4rk1n
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T4rk1n commented Apr 16, 2026

This is looking good.

The new typing test file is not running on ci, the command needs to be adapted here:

dash/.github/workflows/testing.yml

Line 139 in bf5a329

pytest tests/compliance/test_typing.py

Also need to run npm run format and fix the failing pylint:

************* Module dash._callback
dash/_callback.py:14:0: C0413: Import "import flask" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:16:0: C0413: Import "from .dependencies import handle_callback_args, handle_grouped_callback_args, Output, ClientsideFunction, Input" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:23:0: C0413: Import "from .development.base_component import ComponentRegistry" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:24:0: C0413: Import "from .exceptions import InvalidCallbackReturnValue, PreventUpdate, WildcardInLongCallback, MissingLongCallbackManagerError, BackgroundCallbackError, ImportedInsideCallbackError" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:33:0: C0413: Import "from ._grouping import flatten_grouping, make_grouping_by_index, grouping_len" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:38:0: C0413: Import "from ._utils import create_callback_id, stringify_id, to_json, coerce_to_list, AttributeDict, clean_property_name" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:47:0: C0413: Import "from . import _validate" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:48:0: C0413: Import "from .background_callback.managers import BaseBackgroundCallbackManager" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:49:0: C0413: Import "from ._callback_context import context_value" should be placed at the top of the module (wrong-import-position)
dash/_callback.py:50:0: C0413: Import "from ._no_update import NoUpdate" should be placed at the top of the module (wrong-import-position)

-----------------------------------
Your code has been rated at 9.98/10

Fix: test_callback_typing.py not running on ci
bbb91c7 
Fix: import order failing pylint
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odorikakeru commented Apr 17, 2026

Thanks for the pointers. I made a couple of small changes.

It seems that the linter didn't like my sys.version check in the imports, I took it out and replaced it with a try-except (I know this is more Pythonic, but I'm old fashioned and usually don't like letting expected behaviour raise errors). The advantage is we don't need to import sys anymore.

CircleCI is failing on black, but I'm not sure if that's unrelated.

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T4rk1n commented Apr 17, 2026

CircleCI is failing on black, but I'm not sure if that's unrelated.

Black is the Python formatter, just need to run npm run format and commit the change to fix the error.

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