Numpy array to gzip file

[Sean Davis]

I have a set of numpy arrays which I would like to save to a gzip
file.  Here is an example without gzip:

b=numpy.ones(1000000,dtype=numpy.uint8)
a=numpy.zeros(1000000,dtype=numpy.uint8)
fd = file('test.dat','wb')
a.tofile(fd)
b.tofile(fd)
fd.close()

This works fine.  However, this does not:

fd = gzip.open('test.dat','wb')
a.tofile(fd)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: first argument must be a string or open file

In the bigger picture, I want to be able to write multiple numpy
arrays with some metadata to a binary file for very fast reading, and
these arrays are pretty compressible (strings of small integers), so I
can probably benefit in speed and file size by gzipping.

Thanks,
Sean