Add on_missing_context parameter to get_run_logger() by zionts · Pull Request #21028 · PrefectHQ/prefect

zionts · 2026-03-09T01:11:33Z

Summary

Closes #21027 — get_run_logger() raises MissingContextError in threaded contexts with no fallback option.

When using ThreadPoolExecutor inside Prefect tasks for parallel processing, worker threads don't inherit the Prefect run context. Any code that calls get_run_logger() in these threads crashes with MissingContextError. The only workaround today is to monkey-patch prefect.get_run_logger globally.

Changes

New on_missing_context parameter on get_run_logger():

"raise" (default): current behavior, fully backward compatible
"warn": return a standard prefect logger and emit a debug message
"ignore": return a standard prefect logger silently

# Before: crashes in threads
logger = get_run_logger()  # MissingContextError in ThreadPoolExecutor workers

# After: works everywhere
logger = get_run_logger(on_missing_context="warn")
logger.info("Processing file")  # Prefect logger in context, fallback in threads

The fallback logger is a standard prefect.run_fallback logger created via the existing get_logger() function, so it inherits Prefect's logging configuration.

Testing

7 new tests in tests/logging/test_on_missing_context.py
Verifies default "raise" behavior is unchanged (backward compat)
Verifies "warn" returns logger and emits debug message
Verifies "ignore" returns logger silently
Verifies fallback logger is functional

Checklist

This pull request references any related issue by including "closes <link to issue>"
- Closes get_run_logger() raises MissingContextError in threaded contexts with no fallback option #21027
If this pull request adds new functionality, it includes unit tests that cover the changes
If this pull request adds functions or classes, it includes helpful docstrings

get_run_logger() raises MissingContextError when called outside a flow or task run context. This makes it unusable from threads (e.g. ThreadPoolExecutor workers) where no Prefect context exists, forcing users to monkey-patch the function or wrap every call in try/except. Add an on_missing_context parameter that controls this behavior: - "raise" (default): current behavior, fully backward compatible - "warn": return a standard prefect logger + emit debug message - "ignore": return a standard prefect logger silently This lets users write code that works both inside and outside Prefect run contexts without monkey-patching.

codspeed-hq · 2026-03-09T01:16:27Z

Merging this PR will not alter performance

✅ 2 untouched benchmarks

_{Comparing zionts:fix/get-run-logger-thread-fallback (00126f2) with main (316330b)}

desertaxle · 2026-03-09T15:27:58Z

    Get a Prefect logger for the current task run or flow run.

-    The logger will be named either `prefect.task_runs` or `prefect.flow_runs`.
+    The logger will be named either ``prefect.task_runs`` or ``prefect.flow_runs``.


Suggested change

The logger will be named either ``prefect.task_runs`` or ``prefect.flow_runs``.

The logger will be named either `prefect.task_runs` or `prefect.flow_runs`.

desertaxle · 2026-03-09T15:28:06Z

    Contextual data about the run will be attached to the log records.

-    These loggers are connected to the `APILogHandler` by default to send log records to
+    These loggers are connected to the ``APILogHandler`` by default to send log records to


Suggested change

These loggers are connected to the ``APILogHandler`` by default to send log records to

These loggers are connected to the `APILogHandler` by default to send log records to

desertaxle · 2026-03-09T15:28:46Z

+        on_missing_context: Controls behavior when no active flow or task run context
+            exists. ``"raise"`` (default) raises ``MissingContextError`` for backward
+            compatibility. ``"warn"`` returns a standard ``prefect`` logger and emits a
+            warning. ``"ignore"`` returns a standard ``prefect`` logger silently. Use
+            ``"warn"`` or ``"ignore"`` when calling from threads or other contexts where
+            a Prefect run context may not be available.


Suggested change

on_missing_context: Controls behavior when no active flow or task run context

exists. ``"raise"`` (default) raises ``MissingContextError`` for backward

compatibility. ``"warn"`` returns a standard ``prefect`` logger and emits a

warning. ``"ignore"`` returns a standard ``prefect`` logger silently. Use

``"warn"`` or ``"ignore"`` when calling from threads or other contexts where

a Prefect run context may not be available.

on_missing_context: Controls behavior when no active flow or task run context

exists. `"raise"` (default) raises `MissingContextError` for backward

compatibility. `"warn"` returns a standard `prefect` logger and emits a

warning. `"ignore"` returns a standard `prefect` logger silently. Use

`"warn"` or `"ignore"` when calling from threads or other contexts where

a Prefect run context may not be available.

desertaxle · 2026-03-09T15:29:09Z

+        MissingContextError: If no context can be found and ``on_missing_context`` is
+            ``"raise"``


Suggested change

MissingContextError: If no context can be found and ``on_missing_context`` is

``"raise"``

MissingContextError: If no context can be found and `on_missing_context` is

`"raise"`

desertaxle · 2026-03-09T15:30:05Z

        logger = logging.getLogger("null")
        logger.disabled = True
+    elif on_missing_context == "warn":
+        logger = get_logger("prefect.run_fallback")


I'd feel better about adding this if users had the option to customize this logger name.

desertaxle · 2026-03-09T15:30:40Z

+    elif on_missing_context == "ignore":
+        logger = get_logger("prefect.run_fallback")
    else:
        raise MissingContextError("There is no active flow or task run context.")


It'd be good to mention the new on_missing_context kwarg in this exception message.

Apply docstring suggestions, add fallback_logger_name parameter for customizing the logger returned in warn/ignore modes, and mention on_missing_context in the MissingContextError message.

zionts · 2026-03-26T21:28:29Z

Friendly bump @desertaxle — all review feedback has been addressed and CI is green. Let me know if there's anything else needed!

desertaxle · 2026-03-27T17:51:09Z

@zionts it looks like there are some test failures on this PR. Those need to be addressed before we can merge.

github-actions · 2026-04-10T18:12:41Z

This pull request is stale because it has been open 14 days with no activity. To keep this pull request open remove stale label or comment.

zionts requested review from chrisguidry, cicdw, desertaxle and zzstoatzz as code owners March 9, 2026 01:11

Fix ruff formatting: split long typing import

a86170f

desertaxle requested changes Mar 9, 2026

View reviewed changes

zionts and others added 2 commits March 18, 2026 00:13

Address review feedback on get_run_logger on_missing_context

7968321

Apply docstring suggestions, add fallback_logger_name parameter for customizing the logger returned in warn/ignore modes, and mention on_missing_context in the MissingContextError message.

Merge branch 'main' into fix/get-run-logger-thread-fallback

5529887

desertaxle mentioned this pull request Mar 27, 2026

feat: add on_missing_context parameter to get_run_logger #21313

Closed

Merge branch 'main' into fix/get-run-logger-thread-fallback

8d14dd4

desertaxle reviewed Mar 27, 2026

View reviewed changes

Comment thread src/prefect/logging/loggers.py Outdated

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Add on_missing_context parameter to get_run_logger()#21028

Add on_missing_context parameter to get_run_logger()#21028
zionts wants to merge 6 commits intoPrefectHQ:mainfrom
zionts:fix/get-run-logger-thread-fallback

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	The logger will be named either ``prefect.task_runs`` or ``prefect.flow_runs``.
	The logger will be named either `prefect.task_runs` or `prefect.flow_runs`.

	These loggers are connected to the ``APILogHandler`` by default to send log records to
	These loggers are connected to the `APILogHandler` by default to send log records to

		MissingContextError: If no context can be found and ``on_missing_context`` is
		``"raise"``

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zionts commented Mar 9, 2026

Summary

Changes

Testing

Checklist

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Merging this PR will not alter performance

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