Oleg Broytmann writes:
 > .... functions as default parameters to external method functions
 > cause import problems ....
 > ....
 >    "Cannot import default_render from module __main__". Oops! What's that?
 > The default_render isn't in __main__, sure. I tried to create External
 > Method before importing, but this didn't help :(
This is a severe danger of "pickle" and the external method

  *  the implementation does not import the Python source files
     but reads them in and executes them.

     All functions and classes appear to be defined in the
     module "__main__".

  *  The implementation places the function default arguments
     into the ZODB (as a pickle).

  *  "unpickle" cannot load the corresponding object,
     as it imports "__main__" and looks there for the
     function -- in vain.

As a rule:

  Avoid the use of class of function definitons from
  an external method source file for anything that
  may be written to the ZODB (or more generally
  be pickled; this applies e.g. to session context, too).
  This includes created object instances that may
  end up as attributes of Zope objects or
  function default parameters to external methods.

If you defined such classes or functions in a true
Python module, you should have no problem.

What can you do, if your zexp is valuable?

  *  put a dummy definition of "default_render" in the
     "__main__" module.

     You should then be able to import your zexp
     and change the problematic external method.

     I do not know precisely, what Zope's "__main__"
     module is. I expect the "z2.py".
     If this is not the case, then a small dummy product
     that does "import __main__; __main__.default_render= 0"
     should do the trick, whatever "__main__" may be.


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