Yes, the IDD can be the same for all projects and EPPy just needs a path to the version corresponding to the IDF(s) you're working with, which can be the default EP directory.

The error you're seeing is normal and a result of the way the current code is written (ref: Issue 49). To fix it, try starting a new session. Often I'm able to "debug" strange errors in IPython by simply starting fresh.

as of version 5.0 the following is true:

• eppy can have only one IDD file at a time.
• if you try to set a second IDD file, it will throw an exception.

For the near future you eppy will work with only one IDD file. What this means is that:

• you cannot open idf files of different versions simultaneously.
• Once you have set an IDD file, you cannot change it

In theory this is should not be a problem, but in practice it is a real pain. The next version of eppy will have updates that will make it easier to use. Some of these potential updates are described in issue 41

eppy will work with any IDD version. It should even work with future IDD versions.

The error message you are having "IDDAlreadySetError: IDD file is set to: ../eppy/resources/iddfiles/Energy+V7_2_0.idd" means that the iddfile has already been set to Energy+V7_2_0.idd. If you restart python again, then import eppy and try set iddfile to new directory C:\EnergyPlusV8-1-0\Energy+.idd. I personally did not try using v8.1 of .idd file. I am not sure whether it works or not. Coz in resource file, it only contains up to date version of v8.0 if I remember correct.