Fix callbacks double-registered when app file is loaded by import string#3883
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Fix callbacks double-registered when app file is loaded by import string#3883T4rk1n wants to merge 1 commit into
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When the app module runs as a script, a server that loads it by import
string (e.g. uvicorn.run("app:server", reload=True)) executes the same
file twice in the worker process: multiprocessing spawn re-runs it as
__mp_main__, then the import string imports it again under its real
name. Both passes run the @callback decorators, duplicating every spec
in _dash-dependencies and triggering "Duplicate callback outputs"
errors in the renderer.
Dash() now pre-registers the running main module in sys.modules under
its canonical import name (only when that name resolves to the same
file and isn't already imported), so the second import reuses the
already-executed module instead of re-executing the file.
Fixes #3818
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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camdecoster
approved these changes
Jul 13, 2026
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I left a couple of suggestions. The CHANGELOG entry is missing the PR number.
| ): | ||
| sys.modules[import_name] = module | ||
| except (ImportError, ValueError, OSError): | ||
| pass |
Contributor
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Should a warning be logged here?
| ## [Unreleased] | ||
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| ## Fixed | ||
| - [](https://github.com/plotly/dash/pull/) Fix callbacks being registered twice when running the app file as a script with a server that loads it by import string, e.g. `uvicorn.run("app:server", reload=True)` with `backend="fastapi"`. The spawned worker re-executes the main module as `__mp_main__` and the import string then executed the same file a second time, duplicating every callback in `_dash-dependencies` and triggering `Duplicate callback outputs` errors in the renderer. `Dash()` now pre-registers the running main module in `sys.modules` under its canonical import name so the second import reuses it instead of re-executing the file. Fixes [#3818](https://github.com/plotly/dash/issues/3818). |
Contributor
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This entry is pretty verbose. I'd suggest trimming it a bit.
Suggested change
| - [](https://github.com/plotly/dash/pull/) Fix callbacks being registered twice when running the app file as a script with a server that loads it by import string, e.g. `uvicorn.run("app:server", reload=True)` with `backend="fastapi"`. The spawned worker re-executes the main module as `__mp_main__` and the import string then executed the same file a second time, duplicating every callback in `_dash-dependencies` and triggering `Duplicate callback outputs` errors in the renderer. `Dash()` now pre-registers the running main module in `sys.modules` under its canonical import name so the second import reuses it instead of re-executing the file. Fixes [#3818](https://github.com/plotly/dash/issues/3818). | |
| - [3883](https://github.com/plotly/dash/pull/3883) Fix callbacks being registered twice when running the app file as a script with a server that loads it by import string, e.g. `uvicorn.run("app:server", reload=True)` with `backend="fastapi"`. The spawned worker re-executes the main module as `__mp_main__` and the import string then executed the same file a second time, duplicating every callback in `_dash-dependencies` and triggering `Duplicate callback outputs` errors in the renderer. `Dash()` now pre-registers the running main module in `sys.modules` under its canonical import name so the second import reuses it instead of re-executing the file. Fixes [#3818](https://github.com/plotly/dash/issues/3818). |
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When the app module runs as a script, a server that loads it by import string (e.g. uvicorn.run("app:server", reload=True)) executes the same file twice in the worker process: multiprocessing spawn re-runs it as mp_main, then the import string imports it again under its real name. Both passes run the
@callbackdecorators, duplicating every spec in _dash-dependencies and triggering "Duplicate callback outputs" errors in the renderer.Dash() now pre-registers the running main module in sys.modules under its canonical import name (only when that name resolves to the same file and isn't already imported), so the second import reuses the already-executed module instead of re-executing the file.
Fixes #3818