Add support for __all__ in xontribs.

docs/tutorial_xontrib.rst

@@ -69,8 +69,8 @@ default, they cannot be unloaded (easily).
 .. note::
 
     When a xontrib is loaded from a config file or via the xontrib command,
-    its public variables are placed in the current execution context, just
-    like variables set in run control files.
+    its public variables are placed in the current execution context unless
+    `__all__` is defined, just like in regular Python modules.
 
 Loading xontribs in the config file is as simple as adding a list of string
 xontrib names to the top-level ``"xontribs"`` key. For example, the following

news/xontrib-all.rst

@@ -0,0 +1,13 @@
+**Added:** None
+
+**Changed:**
+
+* Xontribs may now define ``__all__`` as a module top-level to limit what gets exported to the shell context
+
+**Deprecated:** None
+
+**Removed:** None
+
+**Fixed:** None
+
+**Security:** None

xonsh/xontribs.py

@@ -36,7 +36,10 @@ def xontrib_context(name):
                           ImportWarning)
         return {}
     m = importlib.import_module(spec.name)
-    ctx = {k: getattr(m, k) for k in dir(m) if not k.startswith('_')}
+    if hasattr(m, '__all__'):
+        ctx = {k: getattr(m, k) for k in m.__all__}
+    else:
+        ctx = {k: getattr(m, k) for k in dir(m) if not k.startswith('_')}
     return ctx