[yocto] What's up with python3

[Alexander Kanavin]

On 06/08/2017 10:28 AM, Gary Thomas wrote:
> I'm building using Poky/Yocto master
> 576821ea0a7558b626ccc87e9ae0e9ee40864956
> and I've noticed that python3 (target) is being built for my image, but it
> doesn't end up in the actual image.  None of my own recipes mention python3
> at all, yet virtually every recipe that's used in the build depend on it.
>
> I've listed all the recipes used in my build (from bitbake -g =>
> recipe-depends.dot)
> and sucked out the ones that claim to depend on python3.  I don't see
> why, for
> example, python(2) depends on python3?
>
> Can someone explain this?  I've added the appropriate files to this email.
>
> I'm mostly curious, but also a bit concerned that my builds are doing more
> than they need (and hence just taking longer than they might need to)

Typically for two reasons:

a) a recipe needs to execute python during build, and host python is not 
suitable for some reason - so native python is built and used;

b) a supplementary package needs python, while the main package does not 
- most often this is the -ptest package, which will not end up in the 
image, unless you ask.

Alex