[systemd-devel] Python journal reader

[David Strauss]

Maybe I'm missing something in the Gist patch, but won't
Reader.get('__REALTIME_TIMESTAMP') fail with "field name is not
valid"?

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



-- 
David Strauss
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