==== install a package ====
<code>
pip install --ignore-installed PyYAML
</code>

Ref:- https://stackoverflow.com/questions/53807511/pip-cannot-uninstall-package-it-is-a-distutils-installed-project

==== install a specific version of a package ====
<code>
pip install <package>==<version>
</code>
For example
<code>
pip install pandas==1.5.3
</code>

==== reinstall an existing package ====
<code>
pip install --upgrade --force-reinstall <package>
</code>

  * https://stackoverflow.com/questions/51913361/difference-between-pip-install-options-ignore-installed-and-force-reinstall - explains the difference between --force-reinstall and --ignore-installed
==== pip cache ====
install a package without using the cache, just for this run.
  pip install --no-cache-dir <package>

configure pip to not use the cache "globally" (in all commands)
  pip config set global.cache-dir false

remove all wheel files related to matplotlib from pip's cache.
  pip cache remove matplotlib

clear all files from pip's cache.
  pip cache purge

pip cache directory
  pip cache dir

Availability: {'pip cache': 'pip >= 20.1', 'pip config': 'pip >= 10.0', '--no-cache-dir': 'pip >= 6.0'}

Ref:-
  * https://stackoverflow.com/a/61762308/6305733 - response from a pip maintainer.

==== upgrade pip ====
<code>
python -m pip install --upgrade pip
</code>

==== links I came across ====
  * https://caremad.io/posts/2013/07/setup-vs-requirement/ - setup.py vs requirements.txt - an article by Donald Stufft
    * Easy to understand, well written.
    * found it in https://pip.pypa.io/en/stable/user_guide/#requirements-files
  * Changes to the pip dependency resolver in 20.3 - https://pip.pypa.io/en/stable/user_guide/#changes-to-the-pip-dependency-resolver-in-20-3-2020
  * https://pip.pypa.io/en/stable/news/ - pip changelog