我无法使用h5py读取数据。“无法创建组”

我正试图在快速启动页面上完成这个琐碎的例子

http://www.h5py.org/docs/intro/quick.html

import h5py
f = h5py.File('myfile.hdf5','w')
dset = f.create_dataset("MyDataset", (100, 100), 'i')
dset[...] = 42
f.close()

ff = h5py.File('myfile.hdf5','r')
dset = ff.create_group("MyDataset")

输出为

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/h5py/_hl/group.py", line 28, in create_group
    gid = h5g.create(self.id, name, lcpl=lcpl)
  File "h5g.pyx", line 135, in h5py.h5g.create (h5py/h5g.c:2057)
ValueError: unable to create group (Symbol table: Unable to initialize object)

我试着正确地做这件事了吗?