Pre-release versions not included when specifying version interval

[andy-maier]

Environment

• pip version: 20.1.1
• Python version: 3.8.2
• OS: macOS 10.15.5

Description

Since we are about to release our first pre-release version of a package, I did some experiments using pip in that area, and I found that pip does not properly consider the versioning order for pre-releases in some cases. I did read some of the issues in that area (specifically #4969, #7579, https://discuss.python.org/t/version-specifier-sets-and-prereleases/3000) but I think this issue reported here is not covered in those).

I used the "apispec" package for these experiments, which has some pre-release versions before its 1.0.0 version:
..., 0.39.0, 1.0.0b1, 1.0.0b2, 1.0.0b3, 1.0.0b4, 1.0.0b5, 1.0.0b6, 1.0.0rc1, 1.0.0, 1.1.0, ...

Here are the experiments, and my judgement of the result (I uninstalled the package in between the experiments)::

• pip install 'apispec>0.39.0,<1.0.0' - does not find a version - OK
• pip install 'apispec>0.39.0,<1.0.0' --pre - does not find a version - STRANGE
• pip install 'apispec>0.39.0,<1.0.0rc1' - does not find a version - STRANGE
• pip install 'apispec>0.39.0,<=1.0.0rc1' - installs apispec-1.0.0rc1 - OK
• pip install 'apispec>=1.0.0b1,<1.0.0' - does not find a version - STRANGE
• pip install 'apispec>=1.0.0b1,<1.0.0' --pre - does not find a version - STRANGE
• pip install 'apispec==1.0.0b1' - installs apispec-1.0.0b1 - OK

In all cases marked as STRANGE, there is a trigger to include pre-release versions, either by means of specifying a pre-release version in the version specification, or by means of specifying the --pre option, or both, and there are one or more pre-release versions that match the version specification. Nevertheless, no pre-release version is found to be installed. According to the version sequence shown further up, each of these cases should have found one or more pre-release versions that satisfy the version specification.

Expected behavior

1. If the inclusion of pre-releases is enabled by any trigger, the order of versions according to PEP 440 including any pre-release versions should be used to evaluate the version constraints.

2. I wish there would be a more detailed description of the behavior for pre-releases in the pip documentation.

How to Reproduce

See the experiments listed above. Make sure that the package gets uninstalled before each experiment.

[uranusjr]

Without looking into the actual implementation, this should likely be reported to pypa/packaging, which implements the actual version comparison logic.

[andy-maier]

I have opened pypa/packaging#315, pointing back to here for details.

[pradyunsg]

As noted in the packaging issue, this is expected behavior. Thanks for filing this @andy-maier, I'm going to go ahead and eagerly close this.

If we decide over on packaging that this behavior should be changed, we can reopen/revisit this issue. :)