title: pandas._libs.tslib.NaTType is different from pandas.tslib.NaTType
body: <p>In <code>pandas</code>, why <code>pandas._libs.tslib.NaTType</code> is different from <code>pandas.tslib.NaTType</code>, I thought <code>_libs.tslib.NaTType</code> exposes to <code>tslib.NaTType</code>? <code>val_type</code> is <code>pandas._libs.tslib.NaTType</code>,</p>
  
  <pre><code>issubclass(val_type, pd.tslib.NaTType)
  
  False
  
  issubclass(val_type, pd._libs.tslib.NaTType)
  
  True
  </code></pre>
  
  <p>Also when I tried to display an error message,</p>
  
  <pre><code>raise TypeError(
          'type error for value {} of type {}'.format(val, val_type))
  </code></pre>
  
  <p>it only displays as </p>
  
  <pre><code>type error for value {} of type {}
  </code></pre>
  
  <p>I am wondering how to fix it. </p>
  
  <p>UPDATE</p>
  
  <p>Running <code>Python 3.5.2</code> on <code>Linux Mint 18.1 X64</code></p>
  
  <p>updated <code>pandas</code> using</p>
  
  <pre><code>sudo pip3 install pandas --upgrade
  
  Successfully installed numpy-1.13.3 pandas-0.21.0 pytz-2017.3 six-1.11.0
  </code></pre>
  
  <p>show <code>pandas</code> version using</p>
  
  <pre><code>pip3 show pandas
  Version: 0.21.0
  
  pip3 show numpy
  Version: 1.10.4
  </code></pre>
  
  <p>updated <code>numpy</code> to <code>1.13.3</code>, using</p>
  
  <pre><code>sudo pip3 install numpy --upgrade
  </code></pre>
  
  <p>show <code>numpy</code> version using</p>
  
  <pre><code>sudo pip3 show numpy
  Version: 1.13.3
  </code></pre>
  
  <p>running the same code, it still shows <code>False</code>. when <code>issubclass(val_type, pd.tslib.NaTType)</code> or <code>val_type is pd.tslib.NaTType</code>. </p>