[Distutils] (How) do setuptools/distribute/pip handle circular dependencies?

[Daniel Holth]

On Nov 12, 2012, at 6:53 PM, Donald Stufft <donald.stufft at gmail.com> wrote:

> On Monday, November 12, 2012 at 6:10 PM, Vinay Sajip wrote:
>> PJ Eby <pje <at> telecommunity.com> writes:
>> 
>> Thanks for the explanation. It seems to me that the new metadata formats make
>> dependency resolution more difficult because they allow for e.g.
>> 'Provides-Dist' as a multi-value field. Perhaps I'm misunderstanding, but this,
>> it seems to me, opens the door for a project named A on PyPI to provide e.g.
>> "A (1.0)", "B (1.5)" and "C (2.0)", and likewise, projects B and C on PyPI
>> could provide slightly different versions of "A", "B" and "C". You can soon get
>> a rat's nest of dependencies in the resolver - and if you get something like
>> the case that Carl linked to, where some element of backtracking might be in
>> order, it doesn't seem computationally straightforward to resolve dependencies,
>> perhaps even with a SAT solver in the mix. Is this a case of practicality
>> losing out to purity? Assuming it's easy to pull any version from an index, I
>> can't see a compelling case for any distribution archive for A to ever provide
>> anything other than e.g. "A (x.y)". Can someone point me to the real need for
>> multi-valued "Provides" fields? Or have I completely misunderstood this aspect
>> of the metadata?
> I think Provides is a misfeature as well. Even RPM, debs, etc which do have a
> provides feature do not use it as it is used here. They allow you to use it for
> a virtual package (e.g. email) which any number of packages could provide
> but it's not there to allow one package to masquerade as another. 

I think it's necessary, but you would never hunt for "providing" packages as part of dependency resolution. You would have to install those manually first.

>> 
>> Regards,
>> 
>> Vinay Sajip
>> 
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