Send files to OS trash mechanism on delete

This adds a config option, defaulting to use trash, and adds a
dependency on the pure-Python 'send2trash' package. Linux, Mac and
Windows should all be supported.

Alternatively, we could make it default to hard delete (the current
behaviour), and let users opt in to trash behaviour. Then Send2Trash
could be a soft dependency.

This doesn't touch the UI yet, so you still get a confirmation dialog
which inaccurately says it will 'permanently delete' the file. If we
want to do this, we'll need some way for the contents manager to pass
the UI a hint about whether deleting is permanent or not.

Closes gh-165

notebook/services/contents/filemanager.py

@@ -13,6 +13,7 @@ import warnings
 import mimetypes
 import nbformat
 
+from send2trash import send2trash
 from tornado import web
 
 from .filecheckpoints import FileCheckpoints
@@ -144,6 +145,11 @@ class FileContentsManager(FileManagerMixin, ContentsManager):
     def _checkpoints_class_default(self):
         return FileCheckpoints
 
+    delete_to_trash = Bool(True, config=True,
+        help="""If True (default), deleting files will send them to the
+        platform's trash/recycle bin, where they can be recovered. If False,
+        deleting files really deletes them.""")
+
     def is_hidden(self, path):
         """Does the API style path correspond to a hidden directory or file?
 
@@ -464,6 +470,14 @@ class FileContentsManager(FileManagerMixin, ContentsManager):
         elif not os.path.isfile(os_path):
             raise web.HTTPError(404, u'File does not exist: %s' % os_path)
 
+        if self.delete_to_trash:
+            self.log.debug("Sending %s to trash", os_path)
+            # Looking at the code in send2trash, I don't think the errors it
+            # raises let us distinguish permission errors from other errors in
+            # code. So for now, just let them all get logged as server errors.
+            send2trash(os_path)
+            return
+
         if os.path.isdir(os_path):
             self.log.debug("Removing directory %s", os_path)
             with self.perm_to_403():

setup.py

@@ -152,6 +152,7 @@ install_requires = [
     'nbformat',
     'nbconvert',
     'ipykernel', # bless IPython kernel for now
+    'Send2Trash',
 ]
 extras_require = {
     ':sys_platform != "win32"': ['terminado>=0.3.3'],