Fix callbacks double-registered when app file is loaded by import string#3883
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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>
camdecoster
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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 |
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Should a warning be logged here?
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I added a comment explaining why it's needed.
Co-authored-by: Cameron DeCoster <cameron.decoster@gmail.com>
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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