NOTE: Only wheels downloaded over HTTPS are cached. If you are using a custom repo over plain old HTTP, the cache is disabled.

For new Pip versions:

Newer Pip versions by default now cache downloads. See this documentation:

https://pip.pypa.io/en/stable/topics/caching/

For old Pip versions:

Create a configuration file named ~/.pip/pip.conf, and add the following contents:

[global]
download_cache = ~/.cache/pip

In one command:

printf '[global]\ndownload_cache = ~/.cache/pip\n' >> ~/.pip/pip.conf