New dependency resolver triggers unnecessary downloads

[bersbersbers]

What did you want to do?
Start with a fresh environment, in particular:

pip uninstall astroid lazy-object-proxy

Then install the two packages again:

pip install --upgrade --upgrade-strategy eager --no-cache-dir --use-feature=2020-resolver astroid lazy-object-proxy

Output
Note how pip downloads two versions of lazy-object-proxy. Obviously, only once can be installed:

Collecting lazy-object-proxy
  Downloading lazy_object_proxy-1.5.1-cp38-cp38-manylinux1_x86_64.whl (54 kB)
     |████████████████████████████████| 54 kB 3.3 MB/s
Collecting astroid
  Downloading astroid-2.4.2-py3-none-any.whl (213 kB)
     |████████████████████████████████| 213 kB 11.2 MB/s
Requirement already satisfied: wrapt~=1.11 in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (from astroid) (1.12.1)
Requirement already satisfied: six~=1.12 in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (from astroid) (1.15.0)
Collecting lazy-object-proxy
  Downloading lazy_object_proxy-1.4.3-cp38-cp38-manylinux1_x86_64.whl (58 kB)
     |████████████████████████████████| 58 kB 22.3 MB/s
Installing collected packages: lazy-object-proxy, astroid
Successfully installed astroid-2.4.2 lazy-object-proxy-1.4.3

Running an upgrade later again downloads the unneeded package version 1.5.1 without installing it (which is correct as astroid==2.4.2 pins lazy-object-proxy==1.4.*):

pip install --upgrade --upgrade-strategy eager --no-cache-dir --use-feature=2020-resolver astroid lazy-object-proxy

Requirement already satisfied: astroid in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (2.4.2)
Requirement already satisfied: lazy-object-proxy in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (1.4.3)
Collecting lazy-object-proxy
  Downloading lazy_object_proxy-1.5.1-cp38-cp38-manylinux1_x86_64.whl (54 kB)
     |████████████████████████████████| 54 kB 3.3 MB/s
Requirement already satisfied: wrapt~=1.11 in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (from astroid) (1.12.1)
Requirement already satisfied: lazy-object-proxy in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (1.4.3)
Requirement already satisfied: six~=1.12 in /home/bers/.pyenv/versions/3.8.6/lib/python3.8/site-packages (from astroid) (1.15.0)

Additional information

• Issue is not fixed in today's master.

[uranusjr]

This is unavoidable. pip does not know about the lazy-object-proxy==1.4.* specification before it downloads astroid, so it has to start with the best version allowed by the unqualified lazy-object-proxy from the command line).

See discussion in #8893 if you want to ask the next natural question “why doesn’t pip try astroid and its dependencies first?”