[reportlab-users] PageTemplate unexpectedly copies Frames from earlier PageTemplates

[Nate Silva]

(Hey, another Portlander!) 

Wow, that is subtle. I hope the ReportLab maintainers will fix it. In the meantime, knowing what I know now... I think I can work around it.

Regards,
Nate

On Nov 30, 2009, at 4:59 PM, Tim Roberts wrote:


> Nate Silva wrote:

>> I've been pulling my hair out and finally boiled it down to the following snippet. Is this a bug?

>> 

> 

> My goodness!  It's the famous default parameter closure bug, but it's

> uncommon to find this in code actually released in the wild.

> 

> class PageTemplate:

>    """

>    essentially a list of Frames and an onPage routine to call at the start

>    of a page when this is selected. onPageEnd gets called at the end.

>    derived classes can also implement beforeDrawPage and afterDrawPage

> if they

>    """

>    def __init__(self,id=None,frames=[],onPage=_doNothing,

> onPageEnd=_doNothing,

>                 pagesize=None):

>        if type(frames) not in (ListType,TupleType): frames = [frames]

>        assert filter(lambda x: not isinstance(x,Frame), frames)==[],

> "frames argument error")

>        self.id = id

>        self.frames = frames

>        self.onPage = onPage

> 

> The problem here is that all of the instances of PageTemplate share the

> same instance of [] in the default "frames" parameter.  So, all of the

> self.frames values are bound to the same object.  It starts out being a

> null list, but later, when any instance modifies the list in place, it

> modifies ALL instances.

> 

> Probably this should be something like:

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

>        if frames is None: frames = [:]

> 

> Or, equally good:

> 

>        self.frames = frames[:]

> 

> -- 

> Tim Roberts, timr at probo.com

> Providenza & Boekelheide, Inc.

> 

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