Have _ defined in notebook.transutils

Import it in conf.py (docs)

Fixes https://github.com/jupyter/notebook/issues/2798

docs/source/conf.py

@@ -345,3 +345,6 @@ intersphinx_mapping = {
 
 spelling_lang='en_US'
 spelling_word_list_filename='spelling_wordlist.txt'
+
+# import before any doc is built, so _ is guaranteed to be injected
+import notebook.transutils

notebook/notebookapp.py

@@ -35,11 +35,7 @@ except ImportError: #PY2
 
 from jinja2 import Environment, FileSystemLoader
 
-# Set up message catalog access
-base_dir = os.path.realpath(os.path.join(__file__, '..', '..'))
-trans = gettext.translation('notebook', localedir=os.path.join(base_dir, 'notebook/i18n'), fallback=True)
-trans.install()
-_ = trans.gettext
+from notebook.transutils import trans, _
 
 # Install the pyzmq ioloop. This has to be done before anything else from
 # tornado is imported.

notebook/transutils.py

@@ -0,0 +1,14 @@
+"""Translation related utilities. When imported, injects _ to builtins"""
+
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+import os
+import gettext
+
+
+# Set up message catalog access
+base_dir = os.path.realpath(os.path.join(__file__, '..', '..'))
+trans = gettext.translation('notebook', localedir=os.path.join(base_dir, 'notebook/i18n'), fallback=True)
+trans.install()
+_ = trans.gettext