Export is a new feature in HDMF 2.0. You can use export to take a container that was read from a file and write it to a different file, with or without modifications to the container in memory. The in-memory container being exported will be written to the exported file as if it was never read from a file.

To export a container, first read the container from a file, then create a new HDF5IO object for exporting the data, then call export on the HDF5IO object, passing in the IO object used to read the container and optionally, the container itself, which may be modified in memory between reading and exporting.

For example:

with HDF5IO(self.read_path, manager=manager, mode='r') as read_io:
    with HDF5IO(self.export_path, mode='w') as export_io:


Can I read a container from disk, modify it, and then export the modified container?

Yes, you can export the in-memory container after modifying it in memory. The modifications will appear in the exported file and not the read file.

  • If the modifications are removals or additions of containers, then no special action must be taken, as long as the container hierarchy is updated correspondingly.
  • If the modifications are changes to attributes, then Container.set_modified() must be called on the container before exporting.


Modifications to h5py.Dataset objects act directly on the read file on disk. Changes are applied immediately and do not require exporting or writing the file. If you want to modify a dataset only in the new file, than you should replace the whole object with a new array holding the modified data. To prevent unintentional changes to the source file, the source file should be opened with mode='r'.

Can I export a newly instantiated container?

No, you can only export containers that have been read from a file. The src_io argument is required in HDMFIO.export.

Can I read a container from disk and export only part of the container?

It depends. You can only export the root container from a file. To export the root container without certain other sub-containers in the hierarchy, you can remove those other containers before exporting. However, you cannot export only a sub-container of the container hierarchy.

Can I write a newly instantiated container to two different files?

HDMF does not allow you to write a container that was not read from a file to two different files. For example, if you instantiate container A and write it file 1 and then try to write it to file 2, an error will be raised. However, you can read container A from file 1 and then export it to file 2, with or without modifications to container A in memory.

What happens to references when I export?

References will be preserved in the exported file. NOTE: Exporting a file involves loading into memory all datasets that contain references and attributes that are references. The HDF5 reference IDs within an exported file may differ from the reference IDs in the original file.

What happens to object IDs when I export?

After exporting a container, the object IDs of the container and its child containers will be identical to the object IDs of the read container and its child containers. The object ID of a container uniquely identifies the container within a file, but should not be used to distinguish between two different files.

If you would like all object IDs to change on export, then first call the method generate_new_id on the root container to generate a new set of IDs for the root container and all of its children, recursively. Then export the container with its new IDs. Note: calling the generate_new_id method changes the object IDs of the containers in memory. These changes are not reflected in the original file from which the containers were read unless the HDF5IO.write method is subsequently called.