diff --git a/starlette/staticfiles.py b/starlette/staticfiles.py
index 4199e510..da8a2aa3 100644
--- a/starlette/staticfiles.py
+++ b/starlette/staticfiles.py
@@ -23,7 +23,7 @@ class StaticFiles:
         if scope["method"] not in ("GET", "HEAD"):
             return PlainTextResponse("Method not allowed", status_code=406)
         path = os.path.normpath(os.path.join(*scope["path"].split("/")))
-        if path.startswith('..'):
+        if path.startswith(".."):
             return PlainTextResponse("Not found", status_code=404)
         path = os.path.join(self.directory, path)
         if self.config_checked:
diff --git a/tests/test_staticfiles.py b/tests/test_staticfiles.py
index 7d4a2f24..ac5a6aef 100644
--- a/tests/test_staticfiles.py
+++ b/tests/test_staticfiles.py
@@ -125,7 +125,7 @@ def test_staticfiles_config_check_occurs_only_once(tmpdir):
 
 
 def test_staticfiles_prevents_breaking_out_of_directory(tmpdir):
-    directory = os.path.join(tmpdir, 'foo')
+    directory = os.path.join(tmpdir, "foo")
     os.mkdir(directory)
 
     path = os.path.join(tmpdir, "example.txt")
@@ -134,6 +134,6 @@ def test_staticfiles_prevents_breaking_out_of_directory(tmpdir):
 
     app = StaticFiles(directory=directory)
     # We can't test this with 'requests', so we call the app directly here.
-    response = app({'method': 'GET', 'path': '/../example.txt'})
+    response = app({"method": "GET", "path": "/../example.txt"})
     assert response.status_code == 404
     assert response.body == b"Not found"