[Distutils] New PEP : dependency specification

[Robert Collins]

On 8 November 2015 at 17:10, Nathaniel Smith <njs at pobox.com> wrote:
> On Sat, Nov 7, 2015 at 12:32 PM, Robert Collins
> <robertc at robertcollins.net> wrote:
>> On unknown names, current pkg_resources raises SyntaxError. So I think
>> we need to update the spec from that perspective to be clear. All PEP
>> 426 defined variables are handled without error by current
>> pkg_resources/markerlib implementation. Perhaps the new variables
>> should raise rather than evaluating to '' / 0 ? Some discussion /
>> thought would be good. Certainly when used and evaluated by an
>> existing pkg_resources they will error - so perhaps we should not add
>> any new variables at this point?
>
> I just thought of one possible strategy for allowing future extensions
> without opening the door wide to every typo: add a variable which
> contains the version number of the environment marker evaluation
> system, define the and/or operators as being short-circuiting, and
> keep the rule that trying to access an unknown variable name raises an
> error. Then you could use write things like:
>
> # 'foo is only really needed if some_new_attr > "2", but doesn't hurt otherwise.
> # So when using an old installation tool that doesn't know about some_new_attr,
> # install it unconditionally.
> Requires-Dist: foo; marker_handling >= "2" and some_new_attr > 2
> Requires-Dist: foo; marker_handling < "2"

It's a decent strategy, but unfortunately a breaking change to
introduce it. I think right now we should just not introduce new
symbols, since we have such a long runway before they can be used, and
instead look for an opportunity where another break is required, and
we can bundle them together.

-Rob

-- 
Robert Collins <rbtcollins at hp.com>
Distinguished Technologist
HP Converged Cloud