=== modified file 'duplicity/backends/pydrivebackend.py'
--- duplicity/backends/pydrivebackend.py	2015-05-31 19:14:43 +0000
+++ duplicity/backends/pydrivebackend.py	2015-06-10 12:26:45 +0000
@@ -21,6 +21,7 @@
 
 import duplicity.backend
 from duplicity.errors import BackendException
+from duplicity import log
 
 
 class PyDriveBackend(duplicity.backend.Backend):
@@ -74,17 +75,36 @@
             parent_folder_id = folder['id']
         self.folder = parent_folder_id
 
-    def FilesList(self):
-        return self.drive.ListFile({'q': "'" + self.folder + "' in parents"}).GetList()
+    def FilesList(self, filename=None):
+        q = "('%s' in parents)" % self.folder
+        if filename is not None:
+            q += " and (title='%s')" % filename
+        fields = 'items(title,id,fileSize),nextPageToken'
+        return self.drive.ListFile({'q': q, 'fields': fields}).GetList()
+
+    def file_by_name(self,filename):
+        flist = self.FilesList(filename)
+        flist_length = len(flist)
+
+        if flist_length == 0:
+            return {'id': '', 'fileSize': -1, 'title': filename }
+        else:
+            if flist_length > 1:
+                log.Warn(_("PyDrive backend: filename %s is not unique in backup folder! Proceeding anyway...") % (filename))
+            return flist[0]
 
     def id_by_name(self, filename):
-        try:
-            return next(item for item in self.FilesList() if item['title'] == filename)['id']
-        except:
-            return ''
+        return self.file_by_name(filename)['id']
 
     def _put(self, source_path, remote_filename):
-        drive_file = self.drive.CreateFile({'title': remote_filename, 'parents': [{"kind": "drive#fileLink", "id": self.folder}]})
+        id = self.id_by_name(remote_filename)
+        if id == '':
+            # no existing file, make a new one
+            target = {'title': remote_filename, 'parents': [{"kind": "drive#fileLink", "id": self.folder}]}
+        else:
+            # upload over existing file
+            target = {'id': id }
+        drive_file = self.drive.CreateFile(target)
         drive_file.SetContentFile(source_path.name)
         drive_file.Upload()
 
@@ -97,14 +117,12 @@
 
     def _delete(self, filename):
         file_id = self.id_by_name(filename)
-        drive_file = self.drive.CreateFile({'id': file_id})
-        drive_file.auth.service.files().delete(fileId=drive_file['id']).execute()
+        if id != '':
+            drive_file = self.drive.CreateFile({'id': file_id})
+            drive_file.auth.service.files().delete(fileId=drive_file['id']).execute()
 
     def _query(self, filename):
-        try:
-            size = int((item for item in self.FilesList() if item['title'] == filename).next()['fileSize'])
-        except:
-            size = -1
+        size = int(self.file_by_name(filename)['fileSize'])
         return {'size': size}
 
 duplicity.backend.register_backend('pydrive', PyDriveBackend)