Fix tests, document, and make install_hook() install only once.

news/imphook.rst

@@ -0,0 +1,21 @@
+**Added:** None
+
+**Changed:**
+
+* ``xonsh.imphooks`` does not install the import hooks automatically, you now
+  need to explicitly call the  `install_hook()` method defined in this module.
+  For example: ``from xonsh.imphooks import install_hook; install_hook()``. The
+  ``install_hook`` method can safely be called several times. If you need
+  compatibility with previous versions of Xonsh you can use the following::
+
+    from xonsh import imphooks
+    getattr(imphooks, 'install_hook', lambda:None)()
+
+
+**Deprecated:** None
+
+**Removed:** None
+
+**Fixed:** None
+
+**Security:** None

tests/test_imphooks.py

@@ -2,13 +2,13 @@
 """Testing xonsh import hooks"""
 import pytest
 
-from xonsh import imphooks  # noqa
-from xonsh import built_ins
+from xonsh import imphooks
 from xonsh.environ import Env
-from xonsh.execer import Execer
 from xonsh.built_ins import load_builtins, unload_builtins
 import builtins
 
+imphooks.install_hook()
+
 
 @pytest.yield_fixture(autouse=True)
 def imp_env(xonsh_execer):

xonsh/imphooks.py

@@ -85,4 +85,13 @@ class XonshImportHook(MetaPathFinder, SourceLoader):
 
 
 def install_hook():
-    sys.meta_path.append(XonshImportHook())
+    """
+    Install Xonsh import hook in `sys.metapath` in order for `.xsh` files to be
+    importable.
+
+    Can safely be called many times, will be no-op if a xonsh import hook is
+    already present.
+    """
+
+    if XonshImportHook not in {type(hook) for hook in sys.meta_path}:
+        sys.meta_path.append(XonshImportHook())