dev-python/dask-1.2.0 is the current version in the tree. Packages (such as) dev-python/xarray start depending on more recent releases of dask, i.e., >=2.9.0. Therefore, please add a more recent version of dask. (At the time of writing, dask 2.21.0 is just out.) Reproducible: Always
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f2f7ad28e8870825582f6d0fbae49c33b6a641cb