import tables as tb


class MyClass:
    foo = "bar"


# An object of my custom class.
my_object = MyClass()

with tb.open_file("test.h5", "w") as h5f:
    h5f.root._v_attrs.obj = my_object  # store the object
    print(h5f.root._v_attrs.obj.foo)  # retrieve it

# Delete class of stored object and reopen the file.
del MyClass, my_object

with tb.open_file("test.h5", "r") as h5f:
    print(h5f.root._v_attrs.obj.foo)
    # Let us inspect the object to see what is happening.
    print(repr(h5f.root._v_attrs.obj))
    # Maybe unpickling the string will yield more information:
    import pickle

    pickle.loads(h5f.root._v_attrs.obj)
    # So the problem was not in the stored object,
    # but in the *environment* where it was restored.