[nprescott]

PEP-508[0] explains the grammar for "extras":

> Optional components of a distribution may be specified using the extras field:

  identifier_end = letterOrDigit | (('-' | '_' | '.' )* letterOrDigit)
  identifier    = letterOrDigit identifier_end*
  name          = identifier
  extras_list   = identifier (wsp* ',' wsp* identifier)*
  extras        = '[' wsp* extras_list? wsp* ']'

as well as explaining their behavior, albeit briefly:

> Extras union in the dependencies they define with the dependencies of the distribution they are attached to.

The resolution on . is explained by the pip documentation[1]:

> pip looks for packages in a number of places: on PyPI (if not disabled via --no-index), in the local filesystem, and in any additional repositories specified via --find-links or --index-url. There is no ordering in the locations that are searched. Rather they are all checked, and the “best” match for the requirements (in terms of version number - see PEP 440 for details) is selected.

[0]: https://peps.python.org/pep-0508/#grammar

[1]: https://pip.pypa.io/en/stable/cli/pip_install/#finding-packa...