Creating Hdf5 Compound Attributes Using H5py

I'm trying to create some simple HDF5 datasets that contain attributes with a compound datatype using h5py. The goal is an attribute that has two integers. Here are two example o

Solution 1:

To make an array with dt_type, you have to properly nest lists and tuples:

In [162]: arr = np.array([(['23','3'],)], dt_type)                                             
In [163]: arr                                                                                  
Out[163]: array([([23,  3],)], dtype=[('val1', '<i4', (2,))])

This is (1,) array with a compound dtype. The dtype has 1 field, but 2 values within that field.

With the alternative dtype:

In [165]: dt2 = np.dtype({"names": ["val1", "val2"],"formats": ['<i4', '<i4']})                
In [166]: arr2 = np.array([('23','3',)], dt2)                                                  
In [167]: arr2                                                                                 
Out[167]: array([(23, 3)], dtype=[('val1', '<i4'), ('val2', '<i4')])

or the simplest array:

In [168]: arr3 = np.array([23,2])                                                              
In [169]: arr3                                                                                 
Out[169]: array([23,  2])

Writing to a dataset:

In [170]: ds.attrs.create('arr', arr)                                                          
In [172]: ds.attrs.create('arr2', arr2)                                                        
In [173]: ds.attrs.create('arr3', arr3) 

check the fetch:

In[175]: ds.attrs['arr']Out[175]: array([([23,  3],)], dtype=[('val1', '<i4', (2,))])
In[176]: ds.attrs['arr2']Out[176]: array([(23, 3)], dtype=[('val1', '<i4'), ('val2', '<i4')])
In[177]: ds.attrs['arr3']Out[177]: array([23,  2])

dump:

1203:~/mypy$ h5dump compound.h5
HDF5 "compound.h5" {
GROUP "/" {
   DATASET "test" {
      DATATYPE  H5T_STD_I64LE
      DATASPACE  SIMPLE { ( 10 ) / ( 10 ) }
      DATA {
      (0): 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
      }
      ATTRIBUTE "arr" {
         DATATYPE  H5T_COMPOUND {
            H5T_ARRAY { [2] H5T_STD_I32LE } "val1";
         }
         DATASPACE  SIMPLE { ( 1 ) / ( 1 ) }
         DATA {
         (0): {
               [ 23, 3 ]
            }
         }
      }
      ATTRIBUTE "arr2" {
         DATATYPE  H5T_COMPOUND {
            H5T_STD_I32LE "val1";
            H5T_STD_I32LE "val2";
         }
         DATASPACE  SIMPLE { ( 1 ) / ( 1 ) }
         DATA {
         (0): {
               23,
               3
            }
         }
      }
      ATTRIBUTE "arr3" {
         DATATYPE  H5T_STD_I64LE
         DATASPACE  SIMPLE { ( 2 ) / ( 2 ) }
         DATA {
         (0): 23, 2
         }
      }
   }
}
}