Add ability to specify "default" extras_require

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.

[jonas-eschle]

No, not a single package will break. Just to introduce a new syntax: you can define None in the extras_require as a key which defines requirements that are only additional to the default. Nothing will break since None is currently not allowed as a key, so no one has it. So the default is equal to an “extra” None.

if you install any extra, it takes the install_requires and the extra. If no extra is specified, it takes the install_requires and the None extra (trivially: which is empty in case it is not specified).

This makes the default less of a special case, in fact it makes everything more consistent.

[dholth]

I like the idea of allowing install_requires to be replaced by extra: { None: [list, of, requirements] }. This is how pkg_resources used to work and it reduces the total number of concepts in the system, with install_requires being a special case of extra. The idea of removing a special extra if any others have been chosen would be a new feature.