Chris Withers wrote at 2009-3-27 16:02 -0500:
> ...
>> Got zope.principalregistry 3.7.0.
>> While:
>>    Installing zopetest.
>> Error: There is a version conflict.
>> We already have: zope.component 3.5.1
>> but 3.7.0 requires 'zope.component>=3.6.0'.
>Okay, so I thought I'd be smart and try the following buildout.cfg:
> ...
>What the hell is 3.7.0 doing being dragged down?
>Is this a buildout snafu? Jim?

I fear we will have to learn to live with version conflicts
until we are ready to pin each and every version used, either
in the buildout itself or via a KGS (Known Good Set index).

If we let "buildout/setuptools" decide which version to use,
there is a good chance that it gets it wrong.

The problem would be far smaller if "setuptools" (yes, this is
the major component to blame in this case, not "buildout") 
would first construct the complete dependency graph and then
try to search a global solution. Unfortunately, "setuptools"
resolves dependancies incrementally (not globally): when
it has to handle a dependency, it tries to find a distribution
satisfying the requirements taking into account the distributions
it already has -- but not the dependency requirements it will
also have to satisfy in a short time. This way, it may take
a distribution that after a short while will conflict with
another dependancy requirement.

Your observation above might (other chains are possible) result from:

   Some dependency specifies: "zope.component==3.5.1".

   Later some dependancy specifies: ""
   "setuptools" looks what distribution it can choose.
   Lacking a more specific prescription, it uses the newest
   one "".

   Later, it tries to resolve the dependencies of ""
   and finds "zope.component==3.6.0". Conflict!

To avoid the conflict, "setuptools" would need to backtrack and
revise its distribution decision for ""
or alternatively first build the complete dependency graph and
search a global solution.

However, finding such a global solution requires a complex algorithm
at least when efficiency is important (which probably is
the case as the dependency graphs for larger applications will be huge).

Thus: I appreaciate much your wish to solve this problem but expect
that it will be quite complex.

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