Stop reconfiguring the root logger when importing DQX

src/databricks/labs/dqx/__init__.py

@@ -3,7 +3,6 @@
 import warnings
 
 import databricks.sdk.useragent as ua
-from databricks.labs.blueprint.logger import install_logger
 from databricks.labs.dqx.__about__ import __version__
 
 # Suppress Databricks notebook LSP warning
@@ -13,7 +12,9 @@
     category=UserWarning,
 )
 
-install_logger()
+# Do not reconfigure the root logger on import (issue #1136); a library should leave logging
+# configuration to the application, so we attach a NullHandler to our own logger instead.
+logging.getLogger("databricks.labs.dqx").addHandler(logging.NullHandler())
 
 # Route Python warnings through logging for consistent formatting
 # (Some modules like check_funcs still use warnings.warn for backward compatibility)

tests/unit/test_logging_setup.py

@@ -0,0 +1,26 @@
+import importlib
+import logging
+
+import databricks.labs.dqx
+
+
+def test_importing_dqx_does_not_reconfigure_the_root_logger():
+    # Regression test for issue #1136: re-running the import must leave the root logger untouched.
+    root = logging.getLogger()
+    app_handler = logging.StreamHandler()
+    original_level = root.level
+    root.addHandler(app_handler)
+    root.setLevel(logging.WARNING)
+    try:
+        importlib.reload(databricks.labs.dqx)
+        assert app_handler in root.handlers
+        assert root.level == logging.WARNING
+    finally:
+        root.removeHandler(app_handler)
+        root.setLevel(original_level)
+
+
+def test_importing_dqx_attaches_a_null_handler_to_its_own_logger():
+    importlib.reload(databricks.labs.dqx)
+    dqx_logger = logging.getLogger("databricks.labs.dqx")
+    assert any(isinstance(handler, logging.NullHandler) for handler in dqx_logger.handlers)