Create LogCaptureHandler if necessary (closes #6240)

AUTHORS

@@ -99,6 +99,7 @@ Erik M. Bray
 Evan Kepner
 Fabien Zarifian
 Fabio Zadrozny
+Felix Nieuwenhuizen
 Feng Ma
 Florian Bruhin
 Floris Bruynooghe

changelog/6240.bugfix.rst

@@ -0,0 +1,2 @@
+Fixes an issue where logging during collection step caused duplication of log
+messages to stderr.

src/_pytest/logging.py

@@ -613,6 +613,12 @@ class LoggingPlugin:
                 with catching_logs(self.log_file_handler, level=self.log_file_level):
                     yield
             else:
+                # Add a dummy handler to ensure logging.root.handlers is not empty.
+                # If it were empty, then a `logging.warning()` call (and similar) during collection
+                # would trigger a `logging.basicConfig()` call, which would add a `StreamHandler`
+                # handler, which would cause all subsequent logs which reach the root to be also
+                # printed to stdout, which we don't want (issue #6240).
+                with catching_logs(logging.NullHandler()):
                     yield
 
     @contextmanager

testing/test_capture.py

@@ -1493,3 +1493,32 @@ def test__get_multicapture() -> None:
     pytest.raises(ValueError, _get_multicapture, "unknown").match(
         r"^unknown capturing method: 'unknown'"
     )
+
+
+def test_logging_while_collecting(testdir):
+    """Issue #6240: Calls to logging.xxx() during collection causes all logging calls to be duplicated to stderr"""
+    p = testdir.makepyfile(
+        """\
+        import logging
+
+        logging.warning("during collection")
+
+        def test_logging():
+            logging.warning("during call")
+            assert False
+        """
+    )
+    result = testdir.runpytest_subprocess(p)
+    assert result.ret == ExitCode.TESTS_FAILED
+    result.stdout.fnmatch_lines(
+        [
+            "*test_*.py F*",
+            "====* FAILURES *====",
+            "____*____",
+            "*--- Captured log call*",
+            "WARNING * during call",
+            "*1 failed*",
+        ]
+    )
+    result.stdout.no_fnmatch_line("*Captured stderr call*")
+    result.stdout.no_fnmatch_line("*during collection*")