I have code which does a logging.addLevelName(logging.INFO, '') so that the logging level name is not displayed in INFO messages, but is in all other levels. I have been running this code fine since many versions of Python 2 through to 3.5.

Now running with python 3.6.0-1 (on Arch Linux), the name is output in messages as "Level 20". Changing the empty string '' in the call above to a space ' ' is my current work-around.

This change was due to fixing #27937. Since yours is not a common use case, and since the workaround works for you, I propose not to change things back.

Well my work-around is not ideal as I get an extra embedded space in those messages. I'm surprised this is "not a common use case"? It doesn't really worry me but it is a 3.6 incompatibility change which I would think is always far better to avoid.

New changeset 99ad6e871459 by Vinay Sajip in branch 'default':
Closes #29220: Fixed regression in logging.getLevelName().
https://hg.python.org/cpython/rev/99ad6e871459

Could you provide a working example of using your trick Mark? I think that even with empty name the output contains extra separators.

As for the patch itself, I would add an indentation for the second "if":

    result = _levelToName.get(level)
    if result is None:
        result = _nameToLevel.get(level)
        if result is None:
            result = "Level %s" % level
    return result

or even use multiple returns for the sake of microoptimization:

    result = _levelToName.get(level)
    if result is not None:
        return result
    result = _nameToLevel.get(level)
    if result is not None:
        return result
    return "Level %s" % level

But I'm not sure that empty name is valid. It can cause problems when parse a configuration file or logs.

I don't understand the use of _nameToLevel. getLevelName('CRITICAL') returns 50, that even is not a string.

> I think that even with empty name the output contains extra separators

Not sure what you mean - please clarify with an example.

> I would add an indentation for the second "if" ... or even use multiple returns for the sake of microoptimization

I will do a further update soon.

> I don't understand the use of _nameToLevel. getLevelName('CRITICAL') returns 50, that even is not a string.

Historically, getLevelName() gets the name for a level *but also the level for a name* (I perhaps should have done a separate API for that, like getLevelForName(), but it's too late now due to backward compatibility requirements - it's been like this for a long time.)

> Historically, getLevelName() gets the name for a level *but also the level for a name*

Ah, then getLevelName() was broken by issue27937.

>>> logging.getLevelName('NOTSET')
'Level NOTSET'

The fix should be applied to 3.5.

> The fix should be applied to 3.5.

Are you sure?

$ python3.5
Python 3.5.2 (default, Nov 17 2016, 17:05:23) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import logging
>>> logging.getLevelName(logging.NOTSET)
'NOTSET'
>>> logging.getLevelName('NOTSET')
0
>>> 

I think just 3.6 and default are affected - doesn't 3.5 seem OK?

I'm sure. e4b6faf22e8d is in 3.5 branch.

OK, will apply to 3.5 -> default.

New changeset adf8312f377f by Vinay Sajip in branch '3.5':
Issue #29220: Improved fix and test.
https://hg.python.org/cpython/rev/adf8312f377f

New changeset a76eed0baa0f by Vinay Sajip in branch 'default':
Issue #29220: Merged fixes from 3.6.
https://hg.python.org/cpython/rev/a76eed0baa0f