diff --git a/src/_pytest/main.py b/src/_pytest/main.py
index d67468887..de0740744 100644
--- a/src/_pytest/main.py
+++ b/src/_pytest/main.py
@@ -489,7 +489,7 @@ class Session(nodes.FSCollector):
 
         names = self._parsearg(arg)
         argpath = names.pop(0).realpath()
-        paths = []
+        paths = set()
 
         root = self
         # Start with a Session root, and delve to argpath item (dir or file)
@@ -537,14 +537,12 @@ class Session(nodes.FSCollector):
                 if dirpath not in seen_dirs:
                     seen_dirs.add(dirpath)
                     pkginit = dirpath.join("__init__.py")
-                    if pkginit.exists() and not any(
-                        x in parts(pkginit.strpath) for x in paths
-                    ):
+                    if pkginit.exists() and parts(pkginit.strpath).isdisjoint(paths):
                         for x in root._collectfile(pkginit):
                             yield x
-                            paths.append(x.fspath.dirpath())
+                            paths.add(x.fspath.dirpath())
 
-                if not any(x in parts(path.strpath) for x in paths):
+                if parts(path.strpath).isdisjoint(paths):
                     for x in root._collectfile(path):
                         key = (type(x), x.fspath)
                         if key in self._node_cache:
diff --git a/src/_pytest/pathlib.py b/src/_pytest/pathlib.py
index c907b495c..430e1ec1d 100644
--- a/src/_pytest/pathlib.py
+++ b/src/_pytest/pathlib.py
@@ -307,4 +307,4 @@ def fnmatch_ex(pattern, path):
 
 def parts(s):
     parts = s.split(sep)
-    return [sep.join(parts[:i+1]) or sep for i in range(len(parts))]
+    return {sep.join(parts[: i + 1]) or sep for i in range(len(parts))}