Downgrade and upgrade: cannot import name 'SourceDistribution'

Environment

• pip version: 20.1
• Python version: 3.6, 3.8
• OS: Mojave

Description

Expected behavior

Upgrades smoothly after downgrade.

How to Reproduce

python3.6 --version
python3.6 -m pip --version
python3.6 -m pip install -U pip
python3.6 -m pip install pip==20
python3.6 -m pip --version

Output

⌂66% [hugo:/private/tmp] 130 % python3.6 --version
Python 3.6.10

⌂68% [hugo:/private/tmp] % python3.6 -m pip --version
pip 20.1 from /Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip (python 3.6)

⌂62% [hugo:/private/tmp] % python3.6 -m pip install -U pip
Requirement already up-to-date: pip in /Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages (20.1)

[hugo:/private/tmp] 2s % python3.6 -m pip install pip==20
Collecting pip==20
  Using cached pip-20.0-py2.py3-none-any.whl (1.5 MB)
Installing collected packages: pip
  Attempting uninstall: pip
    Found existing installation: pip 20.1
    Uninstalling pip-20.1:
      Successfully uninstalled pip-20.1
Successfully installed pip-20.0
[hugo:/private/tmp] 10s % python3.6 -m pip --version
pip 20.0 from /Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip (python 3.6)

[hugo:/private/tmp] % python3.6 -m pip install -U pip
Traceback (most recent call last):
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/__main__.py", line 19, in <module>
    sys.exit(_main())
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/cli/main.py", line 73, in main
    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/commands/__init__.py", line 96, in create_command
    module = importlib.import_module(module_path)
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 24, in <module>
    from pip._internal.cli.req_command import RequirementCommand
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/cli/req_command.py", line 20, in <module>
    from pip._internal.operations.prepare import RequirementPreparer
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/operations/prepare.py", line 16, in <module>
    from pip._internal.distributions import (
  File "/Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip/_internal/distributions/__init__.py", line 1, in <module>
    from pip._internal.distributions.source import SourceDistribution
ImportError: cannot import name 'SourceDistribution'

Can recover with get-pip.py:

[hugo:/private/tmp] 1 % python3.6 get-pip.py
Collecting pip
  Using cached pip-20.1-py2.py3-none-any.whl (1.5 MB)
Installing collected packages: pip
  Attempting uninstall: pip
    Found existing installation: pip 20.0
    Uninstalling pip-20.0:
      Successfully uninstalled pip-20.0
Successfully installed pip-20.1

⌂66% [hugo:/private/tmp] 13s % python3.6 -m pip --version
pip 20.1 from /Users/hugo/.pyenv/versions/3.6.10/lib/python3.6/site-packages/pip (python 3.6)

[pradyunsg]

Now that PyPI has support for yanked releases, I’ve gone ahead and marked pip 20.0 as “yanked”.

This results in installing it via pip install pip==20 printing a warning now:

WARNING: The candidate selected for download or install is a yanked version: 'pip' candidate (version 20.0 at https://files.pythonhosted.org/packages/60/65/16487a7c4e0f95bb3fc89c2e377be331fd496b7a9b08fd3077de7f3ae2cf/pip-20.0-py2.py3-none-any.whl#sha256=eea07b449d969dbc8c062c157852cf8ed2ad1b8b5ac965a6b819e62929e41703 (from https://pypi.org/simple/pip/) (requires-python:>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*))

[pradyunsg]

Can you provide a reason at PyPI?

Nope. 😦

pypa/warehouse#7856