Improve error for unsupported collections = null

vdirsyncer/cli/utils.py

@@ -137,6 +137,11 @@ def handle_cli_error(status_name=None):
             'the issue tracker at {}'
             .format(e, BUGTRACKER_HOME)
         )
+    except exceptions.CollectionRequired as e:
+        cli_logger.error(
+            'One or more storages don\'t support `collections = null`. '
+            'You probably want to set `collections = ["from a", "from b"]`.'
+        )
     except Exception as e:
         tb = sys.exc_info()[2]
         import traceback

vdirsyncer/exceptions.py

@@ -75,3 +75,7 @@ class InvalidResponse(Error, ValueError):
 
 class UnsupportedMetadataError(Error, NotImplementedError):
     '''The storage doesn't support this type of metadata.'''
+
+
+class CollectionRequired(Error):
+    '''`collection = null` is not allowed.'''

vdirsyncer/storage/google.py

@@ -124,6 +124,9 @@ class GoogleCalendarStorage(dav.CaldavStorage):
 
     def __init__(self, token_file, client_id, client_secret, start_date=None,
                  end_date=None, item_types=(), **kwargs):
+        if not kwargs.get('collection'):
+            raise exceptions.CollectionRequired()
+
         super(GoogleCalendarStorage, self).__init__(
             token_file=token_file, client_id=client_id,
             client_secret=client_secret, start_date=start_date,
@@ -155,6 +158,9 @@ class GoogleContactsStorage(dav.CarddavStorage):
     storage_name = 'google_contacts'
 
     def __init__(self, token_file, client_id, client_secret, **kwargs):
+        if not kwargs.get('collection'):
+            raise exceptions.CollectionRequired()
+
         super(GoogleContactsStorage, self).__init__(
             token_file=token_file, client_id=client_id,
             client_secret=client_secret,

vdirsyncer/storage/remotestorage.py

@@ -263,6 +263,6 @@ class RemoteStorageCalendars(RemoteStorage):
 
     def __init__(self, **kwargs):
         if not kwargs.get('collection'):
-            raise ValueError('The collections parameter is required.')
+            raise exceptions.CollectionRequired()
 
         super(RemoteStorageCalendars, self).__init__(**kwargs)