Make filtering loggers pickleable

CHANGELOG.rst

@@ -28,6 +28,8 @@ Changes:
   `#290 <https://github.com/hynek/structlog/pull/290>`_
 - Fix isolation in ``structlog.contextvars``.
   `#302 <https://github.com/hynek/structlog/pull/302>`_
+- The default configuration and loggers are pickleable again.
+  `#301 <https://github.com/hynek/structlog/pull/301>`_
 
 
 ----

src/structlog/_log_levels.py

@@ -94,6 +94,18 @@ def make_filtering_bound_logger(min_level: int) -> Type[FilteringBoundLogger]:
         values.
 
     .. versionadded:: 20.2.0
+    .. versionchanged:: 21.1.0 The returned loggers are now pickleable.
+    """
+
+    return _LEVEL_TO_FILTERING_LOGGER[min_level]
+
+
+def _make_filtering_bound_logger(min_level: int) -> Type[FilteringBoundLogger]:
+    """
+    Create a new `FilteringBoundLogger` that only logs *min_level* or higher.
+
+    The logger is optimized such that log levels below *min_level* only consist
+    of a ``return None``.
     """
 
     def make_method(level: int) -> Callable[..., Any]:
@@ -124,3 +136,21 @@ def meth(self: Any, event: str, **kw: Any) -> Any:
         (BoundLoggerBase,),
         meths,
     )
+
+
+# Pre-create all possible filters to make them pickleable.
+BoundLoggerFilteringAtNotset = _make_filtering_bound_logger(NOTSET)
+BoundLoggerFilteringAtDebug = _make_filtering_bound_logger(DEBUG)
+BoundLoggerFilteringAtInfo = _make_filtering_bound_logger(INFO)
+BoundLoggerFilteringAtWarning = _make_filtering_bound_logger(WARNING)
+BoundLoggerFilteringAtError = _make_filtering_bound_logger(ERROR)
+BoundLoggerFilteringAtCritical = _make_filtering_bound_logger(CRITICAL)
+
+_LEVEL_TO_FILTERING_LOGGER = {
+    CRITICAL: BoundLoggerFilteringAtCritical,
+    ERROR: BoundLoggerFilteringAtError,
+    WARNING: BoundLoggerFilteringAtWarning,
+    INFO: BoundLoggerFilteringAtInfo,
+    DEBUG: BoundLoggerFilteringAtDebug,
+    NOTSET: BoundLoggerFilteringAtNotset,
+}

tests/test_log_levels.py

@@ -3,10 +3,12 @@
 # repository for complete details.
 
 import logging
+import pickle
 
 import pytest
 
 from structlog import make_filtering_bound_logger
+from structlog._log_levels import _LEVEL_TO_NAME
 from structlog.testing import CapturingLogger
 
 
@@ -53,3 +55,12 @@ def test_exception_passed(self, bl, cl):
         bl.exception("boom", exc_info=42)
 
         assert [("error", (), {"event": "boom", "exc_info": 42})]
+
+    @pytest.mark.parametrize("level", tuple(_LEVEL_TO_NAME.keys()))
+    def test_pickle(self, level):
+        """
+        FilteringBoundLogger are pickleable.
+        """
+        bl = make_filtering_bound_logger(level)
+
+        assert bl == pickle.loads(pickle.dumps(bl))