Re: Python: Conditional dependencies (environment markers)

On 4 March 2017 at 14:04, Tim Diels <[email protected]> wrote:
> Dear all,
>
> Could support be added for conditional dependencies as available in Python
> packages? See https://www.python.org/dev/peps/pep-0508/#environment-markers
> for how it works in Python. I need this to convert Python packages with
> conditional dependencies to Zero Install feeds. It's not necessary to follow
> the spec exactly, just so long as I can convert to it.
>
> If you are open to adding this feature to ZI, consider the initial design
> below. I would love to hear about better approaches/naming for supporting
> conditional dependencies. The design should be extensible as more special
> cases will likely arise when converting other package types (e.g. conda) in
> the future.
>
> First, I would make the following interfaces:
>
> - cpython.xml: CPython
> - jython.xml: Jython
> - ...
> - python.xml: any python. This includes cpython.xml, jython.xml, ...
>
> Then for python_version, python_full_version,
> platform_python_implementation. I would convert to:
>
>   <if selected='jython.xml' version='{constraint}'>
>     <requires ...>
>
> platform_machine could perhaps be expressed as:
>
>   <if arch={converted_platform_machine}>
>      <requires ...>
>
> where arch refers to the arch of the actual system, not the implementation.
> E.g. its value is never '*-*', always something concrete.
>
> sys_platform (e.g. java1.8.0_51) could be handled like:
>
>   <if selected='java.xml' version='1.8.0_51'>
>     <requires ...>
>
> Something more special would be required for platform_version as its value
> depends on the runtime environment (number of cores, which kernel, ...):
>
>   <if python:platform-version=...>
>
> Or instead of trying to generalise, we could add each environment marker
> variable as `<if python:{environment_variable_name} >`; but likely there is
> interest for conditional dependencies in other languages as well.
>
> I look forward to hearing your suggestions.

Could you just mark the dependencies as importance="recommended"? The
add conflicts for the cases that aren't supported. 0install will
select the dependency whenever possible.


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