[reportlab-users] Bug Report: Default parameter closure bug in several Graphics and Platypus classes

[Nate Silva]

SUMMARY

On this list yesterday I complained about, and Tim Roberts identified, a bug that causes Frames to be duplicated between PageTemplates.

See Tim Roberts' e-mail dated 30-Nov-2009 (US) / 01-Dec-2009 (EU).

Here is part of PageTemplate.__init__:

    def __init__(self,id=None,frames=[],...):
        self.frames = frames
        # If the caller did not explicitly pass a value for frames,
        # then self.frames now points to a list that is unique per
        # CLASS, not per instance as you might expect.
        
In Python the default value of [] assigned to "frames" is shared by all instances of PageTemplate. That means that, if you do not explicitly specify a value for "frames," other instances of PageTemplate will share the list of Frames with previous instances:

    template1 = PageTemplate(id='template1')
    template1.frames.append(Frame(...))

    template2 = PageTemplate(id='template2')
    template2.frames.append(Frame(...))
    
    # template2 now unexpectedly contains two frames!

WORKAROUND
    
As a workaround, users should always specify a value for the "frames" argument when creating a PageTemplate.

FIX

While the problem is subtle, the fix is simple. Quoting Tim Roberts:


> Probably this should be something like:

>    def __init__(self,id=None,frames=None,...):

>        if frames is None: frames = [:]

> 

> Or, equally good:

> 

>        self.frames = frames[:]


FINDINGS

The bug occurs anywhere an empty list [] default argument is assigned to an instance variable. These are in the latest Subversion (revision 3604):

   reportlab.graphics.shapes.Polygon (shapes.py:1208)
   reportlab.graphics.shapes.PolyLine (shapes.py:1230)
   reportlab.platypus.doctemplate.PageTemplate (doctemplate.py:246)
   reportlab.platypus.flowables.KeepInFrame (flowables.py:878)
   reportlab.platypus.flowables.FrameSplitter (flowables.py:1145)
   reportlab.platypus.flowables.DocIf (flowables.py:1288)

SAMPLE CODE

A short example, showing how Python handles this:

http://gist.github.com/246507

A short example showing the bug in ReportLab itself:

http://gist.github.com/246511

Regards,
Nate