# What’s new in h5py 3.4

## New features

- The pre-built wheels now bundle HDF5 1.12.1 (PR 1945).

- `len()` now works on `dset.astype()`, `.asstr()` and `.fields()` wrappers (PR 1913).

## Bug fixes

- Fix bug introduced in version 3.3 that did not allow the creation of files using the flag “a” for certain drivers (e.g. mpiio, core and stdio) (PR 1922).

- Dataset indexing will now use the optimized fast path, which was accidentally disabled in a previous version (PR 1944).

- Fix an error building with Cython 3.0 alpha 8 (`cpdef` inside functions) (PR 1923).