[systemd-devel] Python journal reader

[Steven Hiscocks]

On 15/04/13 04:05, David Strauss wrote:
> Maybe I'm missing something in the Gist patch, but won't
> Reader.get('__REALTIME_TIMESTAMP') fail with "field name is not
> valid"?
>
You're right, but `get` is changed to `_get`, so is "private". Use 
`get_next` and `get_previous` does on the traversal and getting of both 
special and standard fields to simplify use of the Reader.
> On Sun, Apr 14, 2013 at 10:49 AM, Zbigniew Jędrzejewski-Szmek
> <zbyszek at in.waw.pl> wrote:
>> On Sun, Apr 14, 2013 at 03:28:31PM +0100, Steven Hiscocks wrote:
>>> On 14/04/13 03:36, David Strauss wrote:
>>>> I keep writing lengthy emails about how we can use this as an
>>>> opportunity to reduce redundancy and improve consistency, but I should
>>>> probably ping you on #systemd IRC to hash it out. I can't think of
>>>> anything elegant that doesn't involve altering the existing journal.py
>>>> or _reader.c code.
>> Hi Steven, hi David,
>>
>> I read your discussion on IRC... I agree that backwards compatibility
>> is not (yet) something that we need to keep very. I think that the
>> number of people using those Python interfaces is quite small so far,
>> and having a nice interface is more important than some small breakage.
>>
>>>> As a starter, I want to enumerate the places where
>>>> __REALTIME_TIMESTAMP has special handling:
>>>>   (1) Existing code: A C-based member function in _reader.c.
>>>>   (2) Existing code: A native Python conversion function in journal.py.
>>>>   (3) Existing code: An entry in DEFAULT_CONVERTERS.
>>>>   (4) Added in your patch: A key setting in get_next().
>>>>   (5) Added in your patch: A Python-based member function in journal.py
>>>> that overrides (1).
>>>>   (6) Added in your patch: A condition in get() that invokes (1) when
>>>> the specific field is requested.
>>>>
>>>> While I want to fix this bug, I also don't want six scattered special
>>>> cases for __REALTIME_TIMESTAMP and similar fields.
>>>>
>>>
>>> Thanks for the chat earlier on IRC David, especially given your
>>> localtime :-).
>>>
>>> I thought about the points discussed: keeping things similar to
>>> journalctl -o json output (i.e. include special fields); get,
>>> get_next, and get_realtime, etc methods create duplication and
>>> multiple ways to get the same/some values (could cause confusion);
>>> issue with `next` clashing with python2 __iter__ next; trying to
>>> stabilise the interface (note: not declared stable yet); performance
>>> issues with unnecessary dictionary field conversions.
>>>
>>> I've therefore created another patch for consideration:
>>> https://gist.github.com/kwirk/5382783
>> This looks pretty nice, and I think it can be committed. Could
>> you create a patch with a commit message, including some of the
>> rationale, so that people understand what's happenning? ;)
>>
>>> I've removed the get_next and get_previous methods from _Reader, as
>>> these didn't align with the C API. This is replaced with:`_get_all`
>>> private method (effectively a wrapper around C API macro
>>> sd_journal_{enumerate,restart}_data/SD_JOURNAL_FOREACH_DATA);
>>> renamed `next` method to `_next`, such to avoid name clash (and I
>>> believe should be private); `_previous` method for consistency.
>>>
>>> The `get_next` and `get_previous` methods are now in Reader only.
>>> These call `_next` method, followed by `_get_all` and then fetch the
>>> special fields, before passing whole dictionary to converters. This
>>> makes all the traversal, and getting standard and special fields
>>> from the journal transparent to the user.
>>>
>>> With the changes of removing `get_next` in _Reader, the iter methods
>>> don't function as you'd expect. To that end, I've moved them to
>>> Reader with get_next as iter next call as before.
>>>
>>> I understand the points about performance, but don't think the
>>> performance hit is that bad, or critical in most use cases. These
>>> changes should allow a custom class to inherit _Reader, and the
>>> create a more optimised version if that is required. (One option if
>>> this is a major issue, is a custom dictionary class could be
>>> returned by `get_next`, which uses the __get__ method to lazy
>>> convert the fields on access. This is starting to get complicated
>>> thought...)
>> I think we can always do that later on. Maybe it makes sense
>> to document that get_next() returns something dictionary-like
>> (collections.abc.Mapping) that right now is just a dict, but
>> could become something different later on.
>>
>> Zbyszek
>
>
>


-- 
Steven Hiscocks