Re: [h5md-user] Unit attribute versus non-dimensionless quantities

[Felix Höfling]

Am 31.07.2013, 22:40 Uhr, schrieb Peter Colberg  
<address@hidden>:

> Hi all,
>
> I have discovered the perfect implementation of units in H5MD :-).
>
> Please marvel at the output of the attached example:
>
>   HDF5 "h5md_units.h5" {
>   GROUP "/" {
>      ATTRIBUTE "data" {
>         DATATYPE  "/units/acceleration"
>         DATASPACE  SCALAR
>         DATA {
>         (0): 9.81
>         }
>      }
>      GROUP "units" {
>         DATATYPE "acceleration" H5T_IEEE_F64LE;
>            ATTRIBUTE "unit" {
>               DATATYPE  H5T_STRING {
>                  STRSIZE H5T_VARIABLE;
>                  STRPAD H5T_STR_NULLTERM;
>                  CSET H5T_CSET_UTF8;
>                  CTYPE H5T_C_S1;
>               }
>               DATASPACE  SCALAR
>               DATA {
>               (0): "meter per square second"
>               }
>            }
>      }
>   }
>   }
>
> I will prepare a complete H5MD units proposal for your consideration.
>
> Peter

Hi Peter,

Thanks for working out the example. It is an interesting solution, and I  
consider it superior over the compound type way.

I played a bit with h5py, which seems to ignore user-defined types  
unfortunately. The type is immediately converted to a NumPy type, and it  
is not possible (for me) to retrieve the actual HDF5 type of the attribute  
(and the 'unit' attribute of this type):

import h5py
f = h5py.File('h5md_units.h5')
t = f['units/acceleration']
print f.attrs['data'].dtype
f.create_dataset('dataset', dtype=t, data=(1,2))
print type(f['dataset'])
print f['dataset'].dtype
f.close()

The output is:

float64
<class 'h5py._hl.dataset.Dataset'>
float64

And the new dataset is shown by h5dump as:

   DATASET "dataset" {
      DATATYPE  H5T_IEEE_F64LE
      DATASPACE  SIMPLE { ( 2 ) / ( 2 ) }
      DATA {
      (0): 1, 2
      }
   }

Maybe there is still a trick with h5py. But a solution that works only  
with low-level APIs would not be practical either :-(

Cheers,

Felix