don't immediately crash when importing Pyro4 when the selectors/selec…

…tors34 module is not available, but raise a meaningful exception instead. (added to facilitate Debian, that doesn't package this module at this time)

docs/source/changelog.rst

@@ -12,6 +12,11 @@ Change Log
   to be the user's problem if they attempt to use this combination.
 - Fixed a few IronPython issues with several unit tests.
 - Applied version detection patch from Debian package to contrib/init.d/pyro4-nsd
+- Don't crash immediately at importing Pyro4 when the 'selectors' or 'selectors34' module is not available.
+  (This is normally a required dependency so the situation should not occur.
+   But it is problematic on Debian (and perhaps other distributions) at this time, because this module is not packaged.
+   So we now raise a proper error message, but only when an attempt is made to actually create a multiplex server.
+   Note that all other parts of Pyro4 are usable just fine in this case. The problem is absent when using Python 3.4 or newer.)
 
 
 **Pyro 4.53**

src/Pyro4/socketserver/multiplexserver.py

@@ -15,7 +15,10 @@
 try:
     import selectors
 except ImportError:
-    import selectors34 as selectors
+    try:
+        import selectors34 as selectors
+    except ImportError:
+        selectors = None
 from Pyro4 import socketutil, errors, util
 import Pyro4.constants
 
@@ -26,6 +29,9 @@ class SocketServer_Multiplex(object):
     """Multiplexed transport server for socket connections (uses select, poll, kqueue, ...)"""
     def __init__(self):
         self.sock = self.daemon = self.locationStr = None
+        if selectors is None:
+            raise RuntimeError("This Python installation doesn't have the 'selectors34' module installed, " +
+                               "which is required to use Pyro's multiplex server. Install it, or use the threadpool server instead.")
         self.selector = selectors.DefaultSelector()
         self.shutting_down = False