Merge pull request #419 from itamarst/418.support-old-python-3.5

Make error message on Python 3.5.2 more useful

Author: itamarst

docs/source/news.rst

@@ -4,6 +4,10 @@ What's New
 1.9.0
 ^^^^^
 
+Deprecation:
+
+* Python versions older than 3.5.3, e.g. the 3.5.2 on Ubuntu Xenial, don't work with Eliot, so added a more informative error message explaining that. Fixes #418.
+
 Features:
 
 * If you call ``to_file()/FileDestination()`` with a non-writable file, an

eliot/__init__.py

@@ -6,6 +6,14 @@
 
 # Enable asyncio contextvars support in Python 3.5/3.6:
 if version_info < (3, 7):
+    # On Python 3.5.2 and earlier, some of the necessary attributes aren't exposed:
+    if version_info < (3, 5, 3):
+        raise RuntimeError(
+            "This version of Eliot doesn't work on Python 3.5.2 or earlier. "
+            "Either upgrade to Python 3.5.3 or later (on Ubuntu 16.04 "
+            "you can use https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa "
+            "to get Python 3.6), or pin Eliot to version 1.7."
+        )
     import aiocontextvars
 
     dir(aiocontextvars)  # pacify pyflakes