Handle (but warn about) missing directories

The previous version of WhiteNoise used `os.walk` which doesn't throw an
error for missing directories, but we now use `os.scandir` which does.
This patch restores the graceful handling of missing directories, but
issues a warning about them.

Closes #166

tests/test_whitenoise.py

@@ -6,6 +6,7 @@
 except ImportError:
     from urlparse import urljoin
 import shutil
+import warnings
 from wsgiref.simple_server import demo_app
 
 from .utils import TestServer, Files
@@ -201,6 +202,11 @@ def test_out_of_range_error(self):
                 response.headers['Content-Range'],
                 'bytes */%s' % len(self.files.js_content))
 
+    def test_warn_about_missing_directories(self):
+        with warnings.catch_warnings(record=True) as warning_list:
+            self.application.add_files(u'/dev/null/nosuchdir\u2713')
+        self.assertEqual(len(warning_list), 1)
+
     def assert_is_default_response(self, response):
         self.assertIn('Hello world!', response.text)
 

whitenoise/base.py

@@ -1,6 +1,7 @@
 import os
 from posixpath import normpath
 import re
+import warnings
 from wsgiref.headers import Headers
 from wsgiref.util import FileWrapper
 
@@ -91,12 +92,15 @@ def add_files(self, root, prefix=None):
         root = root.rstrip(os.path.sep) + os.path.sep
         prefix = decode_if_byte_string(prefix)
         prefix = ensure_leading_trailing_slash(prefix)
+        is_directory = os.path.isdir(root)
+        if not is_directory:
+            warnings.warn(u'No directory at: {}'.format(root))
         if self.autorefresh:
             # Later calls to `add_files` overwrite earlier ones, hence we need
             # to store the list of directories in reverse order so later ones
             # match first when they're checked in "autorefresh" mode
             self.directories.insert(0, (root, prefix))
-        else:
+        elif is_directory:
             self.update_files_dictionary(root, prefix)
 
     def update_files_dictionary(self, root, prefix):