Make it more clear that legacy dependency resolver is still default under Python 2

[wasilukm]

It seems that pip 20.3 under Python 2.7 still uses legacy resolver as default one. I'm executing following:

C:\Users\me>py -2 -m pip install trollius>2 pyshark==0.3.8
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
ERROR: pip's legacy dependency resolver does not consider dependency conflicts when selecting packages. This behavior is the source of the following dependency conflicts.
pyshark 0.3.8 requires trollius==1.0.4, but you'll have trollius 2.2.post1 which is incompatible.

C:\Users\me>py -2 -m pip -V
pip 20.3 from C:\Python27\lib\site-packages\pip (python 2.7)

As contrary for Python 3 I get:

C:\Users\me>py -3 -m pip install trollius>2 pyshark==0.3.8
ERROR: Cannot install pyshark==0.3.8 and trollius because these package versions have conflicting dependencies.
ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

C:\Users\me>py -3 -m pip -V
pip 20.3 from C:\Users\me\AppData\Local\Programs\Python\Python38-32\lib\site-packages\pip (python 3.8)

I would expect similar output on Python 2 and Python 3

[pradyunsg]

This is an intentional choice.

pip's new dependency resolver is stricter across the board and makes significant changes to how we treat package metadata and dependency information. We know that this can be potentially disruptive for various projects. The major benefits from the stricter dependency resolver will slowly become more visible in the future, and we expect that projects would be updated to with better metadata and dependency declarations. Many Python 2 users won't be able to upgrade to newer releases of various projects (since many upstream projects on PyPI have dropped Python 2 support). Given that Python 2 will likely stay alive in various capacities in a lot of places, and that this is our last Python 2 compatible release (ignoring bugfixes), we did not want to be unnecessarily disruptive to those users.

Our anecdotal experience showed that most major applications with Python 2 support would not have benefited with the new resolver, and that improved our error reporting in resolution errors is non-disruptive overall.

It is still possible to opt-in to the new resolver on Python 2 (with --use-feature=2020-resolver), for users who wish to use the new resolver as available in the 20.3 release, on Python 2.

[wasilukm]

Thank you for the explanation. I understand the choice.
In the other hand I've just found information about strategy for Python 2 in documentation at the bottom of sub-chapter https://pip.pypa.io/en/stable/user_guide/#watch-out-for whereas main chapter https://pip.pypa.io/en/stable/user_guide/#using-pip-from-your-program suggest that default resolver doesn't depend on Python version.

Similar is in release notes https://pip.pypa.io/en/stable/news/#id21 (different strategy for Python 2 not mentioned):

Switch to the new dependency resolver by default

Therefore please consider updating documentation regarding Python 2 default resolver to be more visible for the end users.

[brainwane]

Thank you @pradyunsg for your clear explanation! I'll follow up by pointing at #8937 (comment) and #8664 (comment) for more background discussion.

@wasilukm Thanks for the suggestion. I definitely agree that we did not communicate clearly enough about the Python 2 behavior, and I take at least partial responsibility for that. I have some followup stuff to do to mitigate this and have listed them at #8936 (comment) .

[brainwane]

I've made pull request #9269 to improve the documentation and highlight in more places that Python 2 environments will still default to the old resolver. Once it is merged, there will be a new CHANGELOG entry saying this as well.

In that pull request I've also added a link to the Python 2 sunsetting FAQ to the pip Python 2 support policy.

[pradyunsg]

Closing since this was be a part of 20.3's bugfix releases. :)