[Zope-dev] pickling limitation in external methods?

Tue, 04 May 1999 07:53:56 -0400

Wolf Logan wrote:
> 
> back in january, i asked a question about this, and jim fulton started to
> answer. unfortunately, time pressures were such that i had to abandon that
> line of development for a while. well, i'm back now, and with more
> information!
> 
> can somebody tell my why i might be seeing a problem in using pickles in my
> external methods? i have an external method which uses the pickle module to
> store and retrieve instances of DateTime (from the mxDateTime module). the
> mxDateTime module imports fine, but no longer seems to cooperate with the
> pickler. that is, the external method works fine until it tries to unpickle
> a DateTime object, whereupon it crashes (with one of the least helpful
> tracebacks in recent memory):
> -----
> Traceback (innermost last):
>   File C:\Program Files\WebSite\lib\python\ZPublisher\Publish.py, line 877,
> in publish_module
>   File C:\Program Files\WebSite\lib\python\ZPublisher\Publish.py, line 590,
> in publish
>     (Info: /testMethod)
>   File C:\Program
> Files\WebSite\lib\python\Products\ExternalMethod\ExternalMethod.py, line
> 254, in __call__
>     (Object: CopySource)
>     (Info: ((), {}, None))
> TypeError: read-only buffer, class
> -----

This is probably due to a bug in the ExternalMethod error handling.
It assumes that the error type is a string, when it is really a 
class.  Check the indicated source line.  This will be fixed in the
next release.

> 
> here's the code itself:
> -----
> def test():
>         d = dbConnection.dbConnection()
>         r = d.execAndGet("select serial, startDate from myData where
> state='started'")
>         return '<html><body>%s</body></html>' % (r,)
> -----
> 
> the dbConnection module proxies a database connection. internally, it
> processes results as pickled python objects.
> 
> i've isolated the trouble to the DateTime objects. in the execAndGet call
> above, everything's fine if the result set doesn't include a DateTime object
> (if i remove the "startDate" from the query). if the result set has a
> DateTime object, it chokes. they pickle and unpickle just fine and dandy
> under "normal" python, but not inside an external method.
> 
> we make heavy use of these objects (and the pickling code) in our backend
> processing. i've tried replacing the cPickle module that comes with ZOPE
> with the one i have in my standard python installation, and it makes no
> difference. is there anything anyone can suggest to make this work? would
> writing a product (or factory, i can't remember which) be a possible
> solution?

Are mxDateTime objects picklable?

If you create one and bind it to 'd', does the following
code work:

  cPickle.loads(cPickle.dumps(d))

does it return a copy of d?

Jim

P.S.  There is an issue with pickling 
      instances of classes defined in external
      method files.  These files are not real modules
      and can't work with the pickling machinery.  This
      may change in the future.

--
Jim Fulton           mailto:jim@digicool.com
Technical Director   (540) 371-6909              Python Powered!
Digital Creations    http://www.digicool.com     http://www.python.org

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