bpo-5001, bpo-31169: Fix two uninformative asserts in multiprocessing/managers.py (#3078)

* Make error message more informative

Replace assertions in error-reporting code with more-informative version that doesn't cause confusion over where and what the error is.

* Additional clarification + get travis to check

* Change from SystemError to TypeError

As suggested in PR comment by @pitrou, changing from SystemError; TypeError appears appropriate.

* NEWS file installation; ACKS addition (will do my best to justify it by additional work)

1 files changed, 10 insertions, 7 deletions

diff --git a/Lib/multiprocessing/managers.py b/Lib/multiprocessing/managers.py
index cae1c10..c672277 100644
--- a/Lib/multiprocessing/managers.py
+++ b/Lib/multiprocessing/managers.py
@@ -84,14 +84,17 @@ def dispatch(c, id, methodname, args=(), kwds={}):
 def convert_to_error(kind, result):
     if kind == '#ERROR':
         return result
-    elif kind == '#TRACEBACK':
-        assert type(result) is str
-        return  RemoteError(result)
-    elif kind == '#UNSERIALIZABLE':
-        assert type(result) is str
-        return RemoteError('Unserializable message: %s\n' % result)
+    elif kind in ('#TRACEBACK', '#UNSERIALIZABLE'):
+        if not isinstance(result, str):
+            raise TypeError(
+                "Result {0!r} (kind '{1}') type is {2}, not str".format(
+                    result, kind, type(result)))
+        if kind == '#UNSERIALIZABLE':
+            return RemoteError('Unserializable message: %s\n' % result)
+        else:
+            return RemoteError(result)
     else:
-        return ValueError('Unrecognized message type')
+        return ValueError('Unrecognized message type {!r}'.format(kind))
 
 class RemoteError(Exception):
     def __str__(self):