Sorry. I misunderstood one of the error messages I was getting:
IOError: could not spawn `h5ls simulation_a=2_b=7.h5`: no such file or directory (ENOENT)
The “file or directory” it’s talking about is not the file I was talking about; it’s the h5ls command. I happened to test these things only with binaries that are in my own PATH, but not whatever run is using.
So yes, this works just fine. Thanks for your help.