[Zope] ZPL and GPL: What should one consider when choosing a license?

David Pratt fairwinds at eastlink.ca
Fri Dec 21 17:41:16 EST 2007

Hi Ross. The ZPL is a brief and concise license. It is clear on 
providing attribution of authors and copyright. The other requirements 
it imposes are fairly minimal. Other than this, it permits the code to 
be used in virtually any manner. The components in the Zope repository 
upon which Zope and Plone depend upon are licensed this way. It is 
roughly equivalent to BSD or MIT license. These are liberal licenses and 
in wide use. TurboGears, Twisted, Zope, and Django all use similar 

The rationale for these liberal licenses is more or less that the use of 
the licensed code does not impact code using it (other than for the user 
of the code to include the license in the source and perhaps identify 
last date of change in mixed sources). This means that not all code or 
derivatives using the licensed code need be disclosed but when it is 
offered it must contain the licensing in the source. Mixing code that 
GPL with nonGPL code on the other hand generally in a derivative that is 
GPL almost without exception. The requirements and disclosures required 
by derivative code is detailed in the GPL.

In any case, licensing is a thorny issue. No one really wishes to debate 
it since it often digresses into heated and divisive exchanges on moral, 
social or commercial benefit or consequence. That said, it remains a 
necessary consideration for developers and businesses in relation to 
their intellectual assets (and also the responsibility one assumes when 
working with open source).

In relation to Chris's post, my understanding of the Zope repository is 
that a committer is required to sign an agreement with Zope Corp or ZF. 
This requires the code to be licensed as ZPL with 50% of intellectual 
rights to Zope Corp or ZF. Hope this helps.


Ross Patterson wrote:
> Recently, someone wrote to me regarding one of my z3c packages which is
> licensed under the GPL.  They asked me to license them under the ZPL
> saying that the ZPL would allow the widest range of use for those
> packages.  They also implied that using the GPL impacted the licensing
> of other code that uses mine.
> Mostly this is just a debate I've never taken up before.  I've always
> just chosen the GPL because it's the most aligned with my values and
> ideals on a "visceral" level.  IOW, it's a gut choice not an educated
> choice.
> I realize this is an old debate.  I found some lengthy threads on
> zope-dev from 2001, but I think there have been some license changes
> since then.  I also read the following blog posts:
> http://radio.weblogs.com/0116506/2005/11/21.html#a370
> http://awkly.org/2005/11/21/gpl-considered-harmful/
> Unfortunately, the comment by Chris McDonough mentioned in the latter
> doesn't seem to be accessible any more.  I'd love to read it.
>>From those blog posts, I don't really see any corroboration of the
> claims of the person who made the request that I switch to the ZPL.
> What exactly about the GPL narrows the range of developers who can use
> my code?  What about the GPL impacts the license of other code that
> depends on my code?
> Here's what I want.  I want anyone who modifies or forks my code to make
> their modifications just as accessible to myself and the community as my
> code is in the first place.  I also want anyone who forks my code to
> credit me and other contributors as authors of the code they forked.  I
> have a slight preference to allow proprietary code to depend on my code
> without having to open source their code.  I definitely want commercial
> entities to be allowed to sell products and services that include my
> code as long as credit is given to the authors.
> That's just what I want from my admittedly naive perspective.  I'd
> like to hear comments on why others think I should or shouldn't want
> any of those things from a license.  I'd also like to hear how others
> think the license available relate to what I want from a license.
> Thoughts?  Is there documentation somewhere about choosing a license?
> Maybe a howto I missed in my googling?
> Ross
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