pip: Why pip install don't cache packages?

Hi,

With the following environment:

$ pip --version pip 20.0.2 from /home/werner/.pyenv/versions/3.8.1/envs/test1/lib/python3.8/site-packages/pip (python 3.8)

Tested with the following steps:

(test1) werner@ubuntu-01:~$ pip install six
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
Collecting six
  Downloading https://pypi.tuna.tsinghua.edu.cn/packages/65/eb/1f97cb97bfc2390a276969c6fae16075da282f5058082d4cb10c6c5c1dba/six-1.14.0-py2.py3-none-any.whl (10 kB)
Installing collected packages: six
Successfully installed six-1.14.0

(test1) werner@ubuntu-01:~$ pip uninstall six
Found existing installation: six 1.14.0
Uninstalling six-1.14.0:
  Would remove:
    /home/werner/.repo/github.com/pyenv/versions/linux-x86_64/3.8.1/envs/test1/lib/python3.8/site-packages/six-1.14.0.dist-info/*
    /home/werner/.repo/github.com/pyenv/versions/linux-x86_64/3.8.1/envs/test1/lib/python3.8/site-packages/six.py
Proceed (y/n)? y
  Successfully uninstalled six-1.14.0

(test1) werner@ubuntu-01:~$ pip install six
Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
Collecting six
  Downloading https://pypi.tuna.tsinghua.edu.cn/packages/65/eb/1f97cb97bfc2390a276969c6fae16075da282f5058082d4cb10c6c5c1dba/six-1.14.0-py2.py3-none-any.whl (10 kB)
Installing collected packages: six
Successfully installed six-1.14.0

As you can see, when I run ``pip install’’ for the second time, it will re-downloading the package instead of using the local cache.

And I also cannot find the downloaded package at the following location:

(test1) werner@ubuntu-01:~$ find ~/.cache/pip -type f -name 'six-1.14.0-py2.py3-none-any.whl'
(test1) werner@ubuntu-01:~$ 

Any hints?

Regards