diff --git a/dvc/utils/collections.py b/dvc/utils/collections.py
index 2d7b259fa0..e1367d10a8 100644
--- a/dvc/utils/collections.py
+++ b/dvc/utils/collections.py
@@ -8,7 +8,14 @@ def compact(args):
 
 
 def apply_diff(src, dest):
-    """Recursively apply changes from stc to dest"""
+    """Recursively apply changes from src to dest.
+
+    Preserves dest type and hidden info in dest structure,
+    like ruamel.yaml leaves when parses files. This includes comments,
+    ordering and line foldings.
+
+    Used in Stage load/dump cycle to preserve comments and custom formatting.
+    """
     Seq = (list, tuple)
     Container = (Mapping, list, tuple)
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 733a1bb2f3..e58eddd48f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -3,12 +3,13 @@
 from git.exc import GitCommandNotFound
 
 from dvc.repo import Repo as DvcRepo
-from .basic_env import TestDirFixture, logger
+from .basic_env import TestDirFixture
 
 
 @pytest.fixture(autouse=True)
-def debug():
-    logger.setLevel("DEBUG")
+def debug(caplog):
+    with caplog.at_level("DEBUG", logger="dvc"):
+        yield
 
 
 # Wrap class like fixture as pytest-like one to avoid code duplication