nixops on the master branch has moved to python3, which means a command like this will fail if the selected plugins have not all moved to python3:

$ ./dev-shell --arg p "(p: [ p.aws (p.callPackage ../nixops-gce/release.nix {})])"

WORKAROUNDS:

A. In nixops and all local plugins you're using, checkout the pre-python3 branch, then build as normal.

B. Port the plugin to python3. All plugins should be ported before the next release, so help is appreciated here! For inspiration/tips/tools, see the sequence of PRs for porting the nixops-aws plugin.