[Python-ideas] Json object-level serializer (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Fri Jul 30 12:39:13 CEST 2010

On Fri, Jul 30, 2010 at 8:46 AM, Éric Araujo <merwok at netwok.org> wrote:

Thank you for explaining generic functions so clearly.

Is there a good module out there implementing them without “crazily complex overloading schemes”?

I'm not sure. Most of my exposure to generic functions is through PJE and he's a big fan of pushing them to their limits (hence RuleDispatch and PEAK-rules).

There is an extremely bare bones implementation used internally by pkgutil's emulation of the standard import process, but Guido has said we shouldn't document or promote that in any official way without a PEP (cf. the simple proposal in http://bugs.python.org/issue5135 and the PJE's previous more comprehensive proposal in PEP 3124).

As others have explained more clearly than I did, generic functions work better than magic methods when the same basic operation (e.g. pretty printing, JSON serialisation) is common to many object types, but the details may vary between applications, or even within a single application. By giving the application more control over how different types are handled (through the generic functions' separate type registries) it is much easier to have context dependent behaviour, while still fairly easily sharing code in cases where it makes sense.

E.g. to use Alex Gaynor's example of attendee list serialisation and the issue 5135 syntax:

@functools.simplegeneric def json_unauthenticated(obj): return json.dumps(obj) # Default to a basic dumps() call

@functools.simplegeneric def json_authenticated(obj): return json_unauthenticated(obj) # Default to being the same as unauthenticated info

@json_unauthenticated.register(EventAttendees): def attendee_titles(attendees): return json.dumps([attendee.title for attendee in attendees])

@json_authenticated.register(EventAttendees): def attendee_details(attendees): return json.dumps([attendee.full_details() for attendee in attendees])

(Keep in mind that I don't use JSON, so there are likely plenty of details wrong with the above, but it should give a basic idea of what generic functions are designed to support).

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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