Executive summary: 

I have just merged a change to firedrake-upgrade which might, in a minority of cases, cause an upgrade fail. If this happens to you then first try running the upgrade again. If that fails please report a bug.

Full version:

Python packages can be installed in editable or non-editable mode. In editable mode, the python files which are run when you import a module are the ones in the source directory. This means that any edits you make apply automatically, without re-installing the package. 

Firedrake used to have developer mode and non-developer mode installs. These corresponded to editable and non-editable Python packages respectively. However the non-developer mode caused confusion among users and some bugs, it also has no advantages. We therefore some time ago removed the --developer install option so that all new installs are in developer mode.

The downside of this is that there are older, non-developer mode installs out there on peoples computers, and this is now an untested and unsupported route. To fix this, latest change to Firedrake will cause firedrake-install to upgrade all installs into developer mode. Once this occurs, it has only advantages. However there are a minority of cases where uninstalling the old packages will fail, so this email is there to provide some explanation about what to do if your update fails.

Regards,

David