Emit warning when Response.background discards injected BackgroundTasks#15112
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mango766 wants to merge 2 commits intofastapi:masterfrom
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Emit warning when Response.background discards injected BackgroundTasks#15112mango766 wants to merge 2 commits intofastapi:masterfrom
mango766 wants to merge 2 commits intofastapi:masterfrom
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…BackgroundTasks When an endpoint injects BackgroundTasks via dependency injection AND returns a Response that already has its own `background` attribute set, the injected tasks are silently dropped. This is a confusing footgun that causes tasks to disappear without any indication. This change adds a UserWarning in that scenario so the silent data loss becomes visible. The warning message explains how to resolve the conflict (either use the injected BackgroundTasks exclusively, or stop injecting BackgroundTasks when using Response.background directly). Ref: fastapi#11215 Co-Authored-By: Claude (claude-opus-4-6) <noreply@anthropic.com>
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Summary
UserWarningwhen an endpoint returns aResponsethat already has abackgroundattribute while dependency-injectedBackgroundTasksare also present, since the injected tasks would otherwise be silently discardedCloses #15111
Related: #11215
Details
The fix is intentionally conservative (warning only, no behavior change) as discussed by @YuriiMotov:
The warning message explains to the developer how to resolve the conflict:
BackgroundTasksinstance instead of settingbackgroundon the response, ORBackgroundTaskswhen usingResponse.backgrounddirectly.Test plan
test_warn_when_response_background_overwrites_injected_tasks- verifies UserWarning is emitted when both injected BackgroundTasks and Response.background are presenttest_no_warn_when_response_has_no_background- verifies no warning when response has no background (injected tasks get attached normally)test_no_warn_when_no_injected_background_tasks- verifies no warning when no BackgroundTasks were injectedAll existing tests pass unchanged.