[Distutils] Adding Provides-Extra as an edit to PEP 345

[Daniel Holth]

You're right, the example could be better.

It works like so:

beaglevote's METADATA:

Provides-Extra: quux
Provides-Extra: pdf
Requires-Dist: doghouse
Requires-Dist: sphinx; extra == 'quux'
Requires-Dist: reportlab; extra == 'pdf'

"install beaglevote" -> evaluate Requires-Dist: with extra == None: ->
requires doghouse

"install beaglevote[pdf]" -> set(Requires-Dist: with extra == None) +
set(Requires-Dist: with extra == 'pdf') -> requires doghouse,
reportlab

"install beaglevote[quux]" -> set(Requires-Dist: with extra == None) +
set(Requires-Dist: with extra == 'quux') -> requires doghouse, sphinx

"install beaglevote[pearl]" -> error, 'pearl' is not mentioned in
Provides-Extra:


Extras are only supposed to add requirements. To make things as
predictable as possible, extra is a string, not a list, requirements
are always evaluated with extra == None first, and the requirements
are re-evaluated for each requested extra individually. This is to
avoid mischief like

Requires-Dist: something; 'foo' in extra and not 'bar' in extra

Requires-Dist: another; extra != 'quux'


Parenthesis would be desirable so you can say:

Requires-Dist: splort; (python_version < '2.6' or sys.platform ==
'win32') and extra == 'blurble'