title: Specifying dtype float32 with pandas.read_csv on pandas 0.10.1
body: <p>I'm attempting to read a simple space-separated file with pandas <code>read_csv</code> method.  However, pandas doesn't seem to be obeying my <code>dtype</code> argument.  Maybe I'm incorrectly specifying it?</p>
  
  <p>I've distilled down my somewhat complicated call to <code>read_csv</code> to this simple test case.  I'm actually using the <code>converters</code> argument in my 'real' scenario but I removed this for simplicity.</p>
  
  <p>Below is my ipython session:</p>
  
  <pre><code>&gt;&gt;&gt; cat test.out
  a b
  0.76398 0.81394
  0.32136 0.91063
  &gt;&gt;&gt; import pandas
  &gt;&gt;&gt; import numpy
  &gt;&gt;&gt; x = pandas.read_csv('test.out', dtype={'a': numpy.float32}, delim_whitespace=True)
  &gt;&gt;&gt; x
           a        b
  0  0.76398  0.81394
  1  0.32136  0.91063
  &gt;&gt;&gt; x.a.dtype
  dtype('float64')
  </code></pre>
  
  <p>I've also tried this using this with a <code>dtype</code> of <code>numpy.int32</code> or <code>numpy.int64</code>.  These choices result in an exception:</p>
  
  <pre><code>AttributeError: 'NoneType' object has no attribute 'dtype'
  </code></pre>
  
  <p>I'm assuming the <code>AttributeError</code> is because pandas will not automatically try to convert/truncate the float values into an integer?</p>
  
  <p>I'm running on a 32-bit machine with a 32-bit version of Python.</p>
  
  <pre><code>&gt;&gt;&gt; !uname -a
  Linux ubuntu 3.0.0-13-generic #22-Ubuntu SMP Wed Nov 2 13:25:36 UTC 2011 i686 i686 i386 GNU/Linux
  &gt;&gt;&gt; import platform
  &gt;&gt;&gt; platform.architecture()
  ('32bit', 'ELF')
  &gt;&gt;&gt; pandas.__version__
  '0.10.1'
  </code></pre>