h5py notes

[azazellochg]

• By default, objects inside group are iterated in alphanumeric order. However, if group is created with track_order=True, the insertion order for the group is remembered (tracked) in HDF5 file, and group contents are iterated in that order. The latter is consistent with Python 3.7+ dictionaries.
• consider https://docs.h5py.org/en/stable/high/group.html#Group.visit and https://docs.h5py.org/en/stable/high/group.html#Group.visititems
• consider using datasets to store as numpy strings with/without compression (gzip)
• eman2 stores md as:
  int numbers - int32
  float - most as f32 (very few as f64)
  strings as fixed length ascii :

            ATTRIBUTE "EMAN.SPIDER.date" {
               DATATYPE  H5T_STRING {
                  STRSIZE 1;
                  STRPAD H5T_STR_NULLTERM;
                  CSET H5T_CSET_ASCII;
                  CTYPE H5T_C_S1;
               }
               DATASPACE  SCALAR
               DATA {
               (0): ""
               }

Now we store strings as utf8 var length. More on strings here: https://docs.h5py.org/en/stable/strings.html