pip/tests/yaml/ERRORS.md

New resolver error messages

Incompatible requirements

Most resolver error messages are due to incompatible requirements. That is, the dependency tree contains conflicting versions of the same package. Take the example:

base:
  available:
    - A 1.0.0; depends B == 1.0.0, C == 2.0.0
    - B 1.0.0; depends C == 1.0.0
    - C 1.0.0
    - C 2.0.0

Here, A cannot be installed because it depends on B (which depends on a different version of C than A itself. In real world examples, the conflicting version are not so easy to spot. I'm suggesting an error message which looks something like this:

A 1.0.0 -> B 1.0.0 -> C 1.0.0
A 1.0.0 -> C 2.0.0

That is, for the conflicting package, we show the user where exactly the requirement came from.

Double requirement

I've noticed that in many cases the old resolver messages are more informative. For example, in the simple example:

base:
  available:
    - B 1.0.0
    - B 2.0.0

Now if we want to install both version of B at the same time, i.e. the requirement B==1.0.0 B==2.0.0, we get:

ERROR: Could not find a version that satisfies the requirement B==1.0.0
ERROR: Could not find a version that satisfies the requirement B==2.0.0
No matching distribution found for b, b

Even though both version are actually available and satisfy each requirement, just not at once. When trying to install a version of B which does not exist, say requirement B==1.5.0, you get the same type of error message:

Could not find a version that satisfies the requirement B==1.5.0
No matching distribution found for b

For this case, the old error message was:

Could not find a version that satisfies the requirement B==1.5.0 (from versions: 1.0.0, 2.0.0)
No matching distribution found for B==1.5.0

And the old error message for the requirement B==1.0.0 B==2.0.0:

Double requirement given: B==2.0.0 (already in B==1.0.0, name='B')