Create directory and then file

dask/config.py

@@ -136,7 +136,8 @@ def ensure_config_file(
         source,
         destination=os.path.join(os.path.expanduser('~'), '.config', 'dask'),
         comment=True):
-    """ Copy file to default location if it does not already exist
+    """
+    Copy file to default location if it does not already exist
 
     This tries to move a default configuration file to a default location if
     if does not already exist.  It also comments out that file by default.
@@ -154,13 +155,13 @@ def ensure_config_file(
     comment: bool, True by default
         Whether or not to comment out the config file when copying
     """
-    if not os.path.exists(os.path.dirname(destination)):
-        os.makedirs(os.path.dirname(destination), exists_ok=True)
-
-    if os.path.isdir(destination):
+    if not os.path.splitext(destination)[1].strip('.'):
         _, filename = os.path.split(source)
         destination = os.path.join(destination, filename)
 
+    if not os.path.exists(os.path.dirname(destination)):
+        os.makedirs(os.path.dirname(destination))
+
     if not os.path.exists(destination):
         # Atomically create destination.  Parallel testing discovered
         # a race condition where a process can be busy creating the
@@ -170,7 +171,10 @@ def ensure_config_file(
             lines = list(f)
 
         if comment:
-            lines = ['#' + line if line else line for line in lines]
+            lines = ['# ' + line
+                     if line.strip() and not line.startswith('#')
+                     else line
+                     for line in lines]
 
         with open(tmp, 'w') as f:
             f.write(''.join(lines))

dask/tests/test_config.py

@@ -143,11 +143,13 @@ def test_set_config():
         assert config['abc'] == {'x': 1, 'y': 2, 'z': {'a': 3}}
 
 
-def test_ensure_config_file_directory():
+@pytest.mark.parametrize('mkdir', [True, False])
+def test_ensure_config_file_directory(mkdir):
     a = {'x': 1, 'y': {'a': 1}}
     with tmpfile(extension='yaml') as source:
         with tmpfile() as destination:
-            os.mkdir(destination)
+            if mkdir:
+                os.mkdir(destination)
             with open(source, 'w') as f:
                 yaml.dump(a, f)