How to deal with array of variable size while creating a HDF5 Dataset in Python?

How to create a HDF5 dataset when size of one dimension of a multidimensional array is not fixed. I tried the following toy code, but it seems that I am missing some point here.

import numpy as np

import h5py


Polyline=h5py.special_dtype(vlen=np.float32)

f=h5py.File('dataset.hdf5', mode='w')

var_features=f.create_dataset('var_features', (10,), dtype=Polyline )

features = np.empty(shape=(10,), dtype=Polyline)

for i in range(10):

    a=10+i*2
    features[i]=np.arange(a).reshape(a/2,2) 

var_features[...]=features

print features[0].shape 

print var_features[0].shape