First of all, I'm surprised that pip doesn't warn or error in this case.  I think this is certainly a bug that should be fixed.  The problem can come up in much more subtle cases too that are very hard for the user to understand.

The good news is that this is a known problem that happens when doing dependency resolution and has a solution.  The solution, which is referred to as backtracking dependency resolution, basically boils down to saving the state of the dependency resolver whenever you have multiple choices to resolve a dependency.  Then if you reach a later point where there is a conflict, you can backtrack to the point where you made a choice and see if another option would resolve the conflict.  

I have some of the gory details, in Chapter 3.8.5 of my dissertation ( http://isis.poly.edu/~jcappos/papers/cappos_stork_dissertation_08.pdf ).  There is also working Python code out there that shows how this should behave.  (I implemented this as part of Stork, a package manager that was used for years in a large academic testbed. )

Thanks,
Justin





On Wed, Apr 15, 2015 at 7:09 AM, Robin Becker <robin@reportlab.com> wrote:
After again finding that pip doesn't have a correct dependency resolution solution a colleage and I discussed the nature of the problem. We examined the script capture of our install and it seems as though when presented with


level 0 A
  A level 1  1.4<= C


level 0 B
  B level 1  1.6<= C <1.7

pip manages to download version 1.8 of C(Django) using A's requirement, but never even warns us that the B requirement of C was violated. Surely even in the absence of a resolution pip could raise a warning at the end.

Anyhow after some discussion I realize I don't even know the name of the problem that pip should try to solve, is there some tree / graph problem that corresponds? Searching on dependency seems to lead to topological sorts of one kind or another, but here we seem to have nodes with discrete values attached so in the above example we might have (assuming only singleton A & B)

R --> A
R --> B

A --> C-1.4
A --> C-1.6
A --> C-1.6.11
A --> C-1.7
A --> C-1.8

B --> C-1.6
B --> C-1.6.11

so looking at C equivalent nodes seems to allow a solution set. Are there any real problem descriptions / solutions to this kind of problem?
--
Robin Becker
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