[AstroPy] question on matplotlib's loadtxt

[Grigoris Maravelias]

Hello to all!  I would like to ask about an error I face with 
matplotlib's loadtxt...

Suppose that we have a file (called "test.test") with 6 columns like:

1496548.  862.235400937  14.008  0  20110523.201416  tth*g
1690289.  919.424007603  13.875  0  20110523.221527  hgf4
1667241.  996.262754039  13.890  0  20110524.001639  nb.mj
739181.6  881.1753527  14.774  0  20110524.010203  vbfhg

When I use
x, y = loadtxt('test.test', unpack=True, usecols=(0,1), dtype=(float,float))

it prints normally the first and second column.
If I add another column like:

x, y, z = loadtxt('test.test', unpack=True,usecols=(0,1,3), 
dtype=(float,float,float))

the output is this:
   File "/usr/lib/python2.7/site-packages/numpy/lib/io.py", line 584, in 
loadtxt
     dtype = np.dtype(dtype)
TypeError: data type not understood

Even if I use a dictionary, like:
dt = dtype({'names':['n1','n2','n3'],'formats':[float,float,float]})
x, y, z = loadtxt('test.test', unpack=True, usecols=(0,1,2) , dtype=dt)

the output is the same as previous.

Additionaly, when I am trying to load the last column as strings:
x, y = loadtxt('test.test', delimiter='  ', unpack=True,usecols=(0,5), 
dtype=(float,'S5'))

I get this error:
   File "/usr/lib/python2.7/site-packages/numpy/lib/io.py", line 584, in 
loadtxt
     dtype = np.dtype(dtype)
ValueError: mismatch in size of old and new data-descriptor


What am I doing wrong ? I am sorry if it is something really obvious ... 
but I am unable to find it...

Thanks for your time!

Grigoris