[New-bugs-announce] [issue23698] Fix documentation for multiprocessing.Manager

Anand B Pillai report at bugs.python.org
Wed Mar 18 07:33:05 CET 2015

New submission from Anand B Pillai:

multiprocessing.Manager seems to have an inconsistency in behaviour/documentation or both.

The behaviour inconsistency is documented in the attached test script which should run for both Python2 and Python3. 

Briefly, multiprocessing.managers.BaseManager class starts a Python subprocess only after a call to its "start" method. Whereas its subclass, multiprocessing.managers.SyncManager does at the time of object creation. This is undocumented and against the base classe's documented behaviour.

Also, the SyncManager is more commonly advertised via the facade multiprocessing.Manager() which is again at odds with the BaseManager's interface in that,

1. It takes no arguments (Python2) and different arguments (Python 3).
2. You can't call "start" on it but you can call "start" if you initialize it via multiprocessing.managers.SyncManager directly (!)
3. Even if you can't call a start on it, apparently you can shut it down via a call to "shutdown"
4. When you try to start such a manager you get a strange AssertionError complaining the state is not INITIAL. A better error is required here.

Please find the attached file for all the sequence of tests done.

Suggested changes are,

1. Fix the inconsistency in documented behaviour between SyncManager and its base class. Either don't start the child process upon object creation or document it.
2. Fix the inconsistency in SyncManager object creation interface and behaviour via "mulitprocessing.Manager()" and directly via "multiprocessing.managers.SyncManager(...)" . One should be able to start both objects cleanly. And the former should also take the address argument. If not document it properly.
3. The AssertionError when trying to start the SyncManager object obtained via a call to "multiprocessing.Manager()" looks like a bug. It should be fixed - otherwise a bette error should be raised and documentation updated.

assignee: docs at python
components: Documentation, Library (Lib)
files: mp_test.py
messages: 238379
nosy: docs at python, pythonhacker
priority: normal
severity: normal
status: open
title: Fix documentation for multiprocessing.Manager
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6
Added file: http://bugs.python.org/file38537/mp_test.py

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