title: python numpy.savetext a matrix with mixed format
body: <p>I am trying to save as text a matrix which has in each row 288 float and 1 string at the end, i've used the savetxt like this:</p>
  
  <pre><code>np.savetxt('name', matrix, delimiter='  ', header='string', comments='', fmt= '%3f'*288 + '%s')
  </code></pre>
  
  <p>but when I try to run the code it raises exceptions like this:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\lib\site-packages\numpy\lib\npyio.py", line 1371, in savetxt
      v = format % tuple(row) + newline
  TypeError: must be real number, not numpy.str_
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\lib\site-packages\numpy\lib\npyio.py", line 1375, in savetxt
      % (str(X.dtype), format))
  TypeError: Mismatch between array dtype ('&lt;U32') and format specifier ('%3f(repeated 288 times without spaces)%s')
  </code></pre>
  
  <p>I really can't understand where I am wrong.</p>