Add on_missing_context parameter to get_run_logger()#21028

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zionts wants to merge 2 commits intoPrefectHQ:mainfrom

zionts:fix/get-run-logger-thread-fallback

zionts commented Mar 9, 2026

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


          Add on_missing_context parameter to get_run_logger()

b5bab27

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.

zionts requested review from chrisguidry, cicdw, desertaxle and zzstoatzz as code owners

March 9, 2026 01:11

codspeed-hq bot commented Mar 9, 2026 •

edited

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Merging this PR will not alter performance

✅ 2 untouched benchmarks

_{Comparing zionts:fix/get-run-logger-thread-fallback (a86170f) with main (4fc6103)}


          Fix ruff formatting: split long typing import

a86170f

desertaxle requested changes

View reviewed changes

src/prefect/logging/loggers.py

    
                  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``.

Member

desertaxle Mar 9, 2026

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`.

src/prefect/logging/loggers.py

                   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

Member

desertaxle Mar 9, 2026

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

src/prefect/logging/loggers.py

Comment on lines +117 to +122

+                      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.

Member

desertaxle Mar 9, 2026

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.

src/prefect/logging/loggers.py

Comment on lines +127 to +128

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

Member

desertaxle Mar 9, 2026

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"`

src/prefect/logging/loggers.py

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

Member

desertaxle Mar 9, 2026

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

src/prefect/logging/loggers.py

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

Member

desertaxle Mar 9, 2026

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

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Reviewers

desertaxle desertaxle requested changes

cicdw Awaiting requested review from cicdw cicdw is a code owner

zzstoatzz Awaiting requested review from zzstoatzz zzstoatzz is a code owner

chrisguidry Awaiting requested review from chrisguidry chrisguidry is a code owner

Requested changes must be addressed to merge this pull request.

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