Shifting log levels¶

A library may log at a level that does not suit your application, for example an ERROR for a failure it recovers from. Raising the level of its logger hides those records altogether. ShiftingFilter keeps them but changes their level.

Shift the records of a library¶

ShiftingFilter(shift_by, logger) moves the level of every record of logger and its children by shift_by steps. -1 turns ERROR into WARNING, 1 turns INFO into WARNING. Attach it to a handler, so it sees the records of every logger:

>>> import logging
>>> from gouge.colourcli import Simple
>>> from gouge.filters import ShiftingFilter
>>> handlers = Simple.basicConfig()
>>> for handler in handlers:
...     handler.addFilter(
...         ShiftingFilter(-1, logger="noisy")
...     )
>>> logging.getLogger("noisy.client").error(
...     "Connection reset, retrying"
... )
2026-10-27 13:34:48,899 WARNING    [noisy.client] Connection reset, retrying
>>> log = logging.getLogger("app")
>>> log.error("Payment failed")
2026-10-27 13:34:48,899 ERROR      [app] Payment failed

Levels are checked before filters run: a record must pass the level of its logger and of the handler before the filter changes it. Shifting up therefore cannot bring back records that these levels drop.

min and max limit the result, and offset shifts by a number of level values instead of steps (a step is 10):

>>> limited = ShiftingFilter(
...     2, max=logging.WARNING
... )

Attach to loggers¶

A filter on a logger only sees records logged on exactly that logger, not those of its children. inject() attaches the filter to a logger and every existing descendant; cleanup() removes it again:

>>> lib = logging.getLogger("lib")
>>> lib_db = logging.getLogger("lib.db")
>>> quiet = ShiftingFilter(-2)
>>> quiet.inject("lib")
>>> lib_db.error("Deadlock, retrying")
2026-10-27 13:34:48,899 INFO       [lib.db] Deadlock, retrying
>>> quiet.cleanup()

Loggers created after inject() do not get the filter.