Solution 1. The class provides the transformation into a dict of hdf-compliant objects

Principle

The object which can be reduced to and reconstructed from a dictionary whose keys are strings, and whose values are of one of the following types:

a scalar (int, float, complex ….)

a numpy array of scalars

an hdf-compliant object

a list, a tuple or a dict of any of the types above.

A class cls has to implement :

__reduce_to_dict__()

Returns a dictionary of objects implementing hdf-compliant or basic objects as number, strings, and arrays of numbers (any type and shape).

Return type:: a dictionary

__factory_from_dict__(cls,name,D) :

A classmethod which reconstructs a new object of type cls from the dictionary returned by __reduce_to_dict__().

Parameters:

cls – the class
name – the attribute cls.name of the class
D – the dictionary returned by __reduce_to_dict__.

Return type:

a new object of type cls

Example

A little example:

class Example:

   def __init__(self,d,t):
       self.d,self.t = d,t # some data

   def __reduce_to_dict__(self):
       return {'d': self.d, 't': self.t}

   @classmethod
   def __factory_from_dict__(cls,name,D) :
       return cls(D['d'],D['t'])

# registering my class
from h5.formats import register_class
register_class (myclass)

# Testing it:
HDFArchive("myfile2.h5")['e'] = Example( [1,2,3], 56)

What happens in detail?

Let us consider an object Ob of class Cls, interacting with and HDFArchive/HDFArchiveGroup H.

Writing
```
H[Name] = Ob
```
- a subgroup S of path Root_of_H/Name is created.
- Ob.__reduce_to_dict__() is called and returns a dictionary D of scalar/numpy arrays/hdf-compliant objects.
- The objects in D are then stored S in a regular way (scalar/arrays) or using the same recursive procedure (other objects).
- S is tagged with the attribute HDF5_data_scheme of Cls.
Reading
```
res = H[Name]
```
- The subgroup S of path Root_of_H/Name is explored. All objects are taken to build a dictionary D: name -> values. This procedure is recursive: the hdf-compliant objects in the subgroup are rebuilt.
- An attribute HDF5_data_scheme is searched for S.
  - If it is found and it corresponds to a registered class Cls :
    
    Cls.__factory_from_dict__(name,D) is called and returns a new object Obj of type Cls, which is returned as res.
  - Otherwise, a new HDFArchiveGroup is constructed with S as root, and returned as res.