bpo-29762: More use "raise from None". (#569)

This hides unwanted implementation details from tracebacks.

Lib/multiprocessing/context.py

@@ -189,7 +189,7 @@ class BaseContext(object):
         try:
             ctx = _concrete_contexts[method]
         except KeyError:
-            raise ValueError('cannot find context for %r' % method)
+            raise ValueError('cannot find context for %r' % method) from None
         ctx._check_available()
         return ctx
 

Lib/multiprocessing/pool.py

@@ -720,14 +720,14 @@ class IMapIterator(object):
                 item = self._items.popleft()
             except IndexError:
                 if self._index == self._length:
-                    raise StopIteration
+                    raise StopIteration from None
                 self._cond.wait(timeout)
                 try:
                     item = self._items.popleft()
                 except IndexError:
                     if self._index == self._length:
-                        raise StopIteration
-                    raise TimeoutError
+                        raise StopIteration from None
+                    raise TimeoutError from None
 
         success, value = item
         if success:

Lib/multiprocessing/process.py

@@ -199,7 +199,7 @@ class BaseProcess(object):
         try:
             return self._sentinel
         except AttributeError:
-            raise ValueError("process not started")
+            raise ValueError("process not started") from None
 
     def __repr__(self):
         if self is _current_process: