[reportlab-users] Reusing flowables can cause problems

[Christoph Zwerschke]

I just noticed a very nasty pitfall in ReportLab's Platypus, namely the 
_postponed attribute of Flowables that may be set during the build 
process. The problem arises if you do something like this:

spacer = Spacer(...)

doc = SimpleDocTemplate(...)
# build a document
doc.build([spacer, ..., spacer, ...])
...
doc = SimpleDocTemplate(...)
# build another document
doc.build([spacer, ..., spacer, ...])

During the first build process, the _postponed attribute may have been 
sticked to the spacer, so that it will not behave correctly in the 
second build process.

I noticed that the undocumented multiBuild method solves this problem by 
  doing the following after each build process:

     #clean up so multi-build does not go wrong - the frame
     #packer might have tacked an attribute onto some flowables
     for elem in story:
         if hasattr(elem, '_postponed'):
             del elem._postponed

Maybe this loop should be run automatically at the beginning of the 
build method instead?

Otherwise, it should be better documented that Flowables should not be 
reused after they have gone through a build process.

-- Christoph