Strings in HDF5


The rules around reading & writing string data were redesigned for h5py 3.0. Refer to the h5py 2.10 docs for how to store strings in older versions.

Reading strings

String data in HDF5 datasets is read as bytes by default: bytes objects for variable-length strings, or numpy bytes arrays ('S' dtypes) for fixed-length strings. Use Dataset.asstr() to retrieve str objects.

Variable-length strings in attributes are read as str objects. These are decoded as UTF-8 with surrogate escaping for unrecognised bytes.

Storing strings

When creating a new dataset or attribute, Python str or bytes objects will be treated as variable-length strings, marked as UTF-8 and ASCII respectively. Numpy bytes arrays ('S' dtypes) make fixed-length strings. You can use string_dtype() to explictly specify any HDF5 string datatype.

When writing data to an existing dataset or attribute, data passed as bytes is written without checking the encoding. Data passed as Python str objects is encoded as either ASCII or UTF-8, based on the HDF5 datatype. In either case, null bytes ('\x00') in the data will cause an error.


Fixed-length string datasets will silently truncate longer strings which are written to them. Numpy byte string arrays do the same thing.

Fixed-length strings in HDF5 hold a set number of bytes. It may take multiple bytes to store one character.

What about NumPy’s U type?

NumPy also has a Unicode type, a UTF-32 fixed-width format (4-byte characters). HDF5 has no support for wide characters. Rather than trying to hack around this and “pretend” to support it, h5py will raise an error if you try to store data of this type.

How to store raw binary data

If you have a non-text blob in a Python byte string (as opposed to ASCII or UTF-8 encoded text, which is fine), you should wrap it in a void type for storage. This will map to the HDF5 OPAQUE datatype, and will prevent your blob from getting mangled by the string machinery.

Here’s an example of how to store binary data in an attribute, and then recover it:

>>> binary_blob = b"Hello\x00Hello\x00"
>>> dset.attrs["attribute_name"] = np.void(binary_blob)
>>> out = dset.attrs["attribute_name"]
>>> binary_blob = out.tostring()

Object names

Unicode strings are used exclusively for object names in the file:


You can supply either byte or unicode strings (on both Python 2 and Python 3) when creating or retrieving objects. If a byte string is supplied, it will be used as-is; Unicode strings will be encoded down to UTF-8.

In the file, h5py uses the most-compatible representation; H5T_CSET_ASCII for characters in the ASCII range; H5T_CSET_UTF8 otherwise.

>>> grp = f.create_dataset(b"name")
>>> grp2 = f.create_dataset("name2")