[Bf-committers] Python API breakage - squeaking wheels

[Domino Marama]

On 05/28/2013 11:26 AM, Campbell Barton wrote:
> I still am a bit concerned that we get many complaints but very little
> real evidence of the cause of addon breakage - just that it happens, I
> realize not every project is opensource but enough are that links to
> scripts that break would be very helpful for API devs. 

For my scripts the changes to UV handling caused the most problems..
Supporting multiple Blender versions from same script version results in
code like this:

        u_points = []
        v_points = []
        mesh = obj.data

        if svn <= 44243:
            for f in mesh.uv_textures[uv_name].data:
                for u, v in f.uv:
                    u_points.append(u)
                    v_points.append(v)
        else:
            if svn > 45996:
                uv_loops = mesh.uv_layers[uv_name].data
            else:
                loopindex = mesh.uv_textures.keys().index(uv_name)
                uv_loops = mesh.uv_layers[loopindex].data
            u_points = [v.uv.x for v in uv_loops]
            v_points = [v.uv.y for v in uv_loops]

And other changes mean I have to do this to get the svn version :)

    try:
        svn = bpy.app.build_revision.decode().split(':')[-1].rstrip('M')
    except:
        svn = bpy.app.build_revision.split(':')[-1].rstrip('M')
    svn = int(svn)

I think the major problem is the changes such as these where the new way
replaces the old way. I test against SVN builds so usually catch changes
quickly and before my users do. But if I developed against release
versions, then a new RC is the first time I'd be aware of issues. If I
was on holiday, then it could even get to a release version and more
users hitting problems before I got chance to do anything. Without a
release or two with depreciated APIs it's something I need to constantly
watch out and allow time for.

Things have been better the past few versions though. I've not needed to
add any more svn checks since 2.65 - currently in this particular addon,
there's over 20 checks to adjust for API changes but it does work on
2.59 to 2.67. For each version from 2.59 to 2.65 there is at least one
check for the specific svn version to tweak things.

While the majority of the changes are due to API improvements, sometimes
they seem to be changes that maybe should have been reconsidered. One
bit of my code walks over the edges of faces, so the only change needed was:

    if svn <= 44243:
        faces = mesh.faces
    else:
        faces = mesh.polygons

I am overdue for a code cleanup though. My official stance is that I
support 3 older versions of Blender, so the majority of the svn checks
are due for removal. For my first example, that results in the much nicer:

        mesh = obj.data
        uv_loops = mesh.uv_layers[uv_name].data
        u_points = [v.uv.x for v in uv_loops]
        v_points = [v.uv.y for v in uv_loops]

So although the API breakage is a hassle, I do see the long term benefit
to the changes.

It's my choice to support multiple Blender versions rather than
constantly recommending Blender updates as I get fewer requests for
support that way :)

Hope that helps clarify the issues API changes create and how I manage them.