Make alabaster and sphinx_rtd_theme dependencies optional

There is no need to pull these dependencies if i.e. the documentation is
using the classic theme.

There will be no change in behavior for projects using alabaster or
sphinx_rtd_theme.

1 files changed, 2 insertions, 3 deletions

diff --git a/sphinx/theming.py b/sphinx/theming.py
index b4d940dfe..4b885a22e 100644
--- a/sphinx/theming.py
+++ b/sphinx/theming.py
@@ -26,9 +26,6 @@ except ImportError:
 from sphinx import package_dir
 from sphinx.errors import ThemeError
 
-import alabaster
-import sphinx_rtd_theme
-
 NODEFAULT = object()
 THEMECONF = 'theme.conf'
 
@@ -73,10 +70,12 @@ class Theme(object):
     def load_extra_theme(cls, name):
         if name in ('alabaster', 'sphinx_rtd_theme'):
             if name == 'alabaster':
+                import alabaster
                 themedir = alabaster.get_path()
                 # alabaster theme also requires 'alabaster' extension, it will be loaded
                 # at sphinx.application module.
             elif name == 'sphinx_rtd_theme':
+                import sphinx_rtd_theme
                 themedir = sphinx_rtd_theme.get_html_theme_path()
             else:
                 raise NotImplementedError('Programming Error')