Libraries that implement a client to an HTTP backend often provide multiple implementations such as for tornado, aiohttp or requests. Each implementation comes with its own dependencies. User usually needs only one implementation.

With extras_require developer of the library can specify aiohttp and torndao as optional and install requests for everyone via install_requires.

However, users of aiohttp and tornado do not need requests, hence the request to be able to specify a "default" extra that should be picked when no other extras are specified.

So that users calling pip install mylib or pip install mylib[requests] will get requests installed, but users calling pip install mylib[aiohttp] won't.