pip always warns that wheel is not installed

[zeha]

Environment

• pip version: 20.1
• Python version: 3.7.6
• OS: Debian

Description

When trying to install a package that is already installed, there is a spurious warning about wheel:

# pip3 install lxml
Looking in indexes: XXX
Requirement already satisfied: lxml in /venv/lib/python3.7/site-packages (4.5.0)
Could not build wheels for lxml, since package 'wheel' is not installed.

Expected behavior

Should not say "Could not build wheels for lxml, since package 'wheel' is not installed." when there was no wheel to build.

How to Reproduce

1. Install pip 20.1 in a venv with no wheel package installed
2. Run pip3 install lxml that uses a prebuilt .whl
3. Run pip3 install lxml again
4. See the useless complaint

Output

Paste the output of the steps above, including the commands themselves and
pip's output/traceback etc.

# pip3 install lxml
Looking in indexes: XXX
Requirement already satisfied: lxml in /venv/lib/python3.7/site-packages (4.5.0)
Could not build wheels for lxml, since package 'wheel' is not installed.

# pip list
Package            Version
------------------ ----------
attrs              19.3.0
certifi            2020.4.5.1
coverage           5.1
importlib-metadata 1.6.0
lxml               4.5.0
more-itertools     8.2.0
packaging          20.3
parameterized      0.7.4
pip                20.1
pluggy             0.13.1
py                 1.8.1
pyparsing          2.4.7
pytest             5.3.5
setuptools         41.2.0
six                1.14.0
wcwidth            0.1.9
zipp               3.1.0

Related: #7768

[sbidoul]

This error message has been improved in #8180, which should be part of pip 20.1.1.
In a nutshell pip is progressively evolving to always build a wheel then installing from it.

I'll therefore close this one as duplicate of #8178. See also #8102 for an ongoing discussion about this topic.

Thanks for reporting!