empty transition packages don't seem to work on upgrades

[terceiro]

• Pip version: 9.0.1
• Python version: 3.5.2
• Operating system: Debian unstable

Description:

msgpack-python was recently renamed to just msgpack. To ease upgrades, the msgpack maintainers uploaded a new version of msgpack-python which is empty and depends on msgpack. However, when you do the upgrade, it seems that the upgrade of msgpack-python removes the files that should now be provided by msgpack.

I am not sure whether this is a feature or a bug, but it makes it impossible to provide transition packages to ease upgrades when renaming a package.

What I've run:

$ virtualenv -p python3 /tmp/v
Already using interpreter /usr/bin/python3
Using base prefix '/usr'
New python executable in /tmp/v/bin/python3
Also creating executable in /tmp/v/bin/python
Installing setuptools, pkg_resources, pip, wheel...source /tdone.
$ source /tmp/v/bin/activate
$ pip install msgpack-python==0.4.8
Collecting msgpack-python==0.4.8
Installing collected packages: msgpack-python
Successfully installed msgpack-python-0.4.8
$ pip freeze
msgpack-python==0.4.8
pkg-resources==0.0.0
$ python3 -c 'import msgpack'
$ pip install --upgrade msgpack-python
Collecting msgpack-python
  Using cached msgpack_python-0.5.0-py3-none-any.whl
Collecting msgpack>=0.5 (from msgpack-python)
  Using cached msgpack-0.5.0-cp36-cp36m-manylinux1_x86_64.whl
Installing collected packages: msgpack, msgpack-python
  Found existing installation: msgpack-python 0.4.8
    Uninstalling msgpack-python-0.4.8:
      Successfully uninstalled msgpack-python-0.4.8
Successfully installed msgpack-0.5.0 msgpack-python-0.5.0
$ python3 -c 'import msgpack'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'msgpack'
$ pip freeze
msgpack==0.5.0
msgpack-python==0.5.0
pkg-resources==0.0.0
$ pip --version
pip 9.0.1 from /tmp/v/lib/python3.6/site-packages (python 3.6)

[RonnyPfannschmidt]

this seems to be a order issue, the new package with the shared files is installed before the other one is removed - that way

[chrahunt]

This may be a use case considered for #988, assuming that encompasses the eventual installation order of resolved packages.