title: problem with nan in python ( pandas) the program can't recognize the nan
body: <p>I stock in strange situation</p>
  <p>i have Data frame , some of the values are nulls</p>
  <p>when I try do change the null , i cant do it , for some reason the console dosent recognize the null</p>
  <p><strong>for example:</strong></p>
  <pre><code>
  df[&quot;volume_fermentor&quot;]=df[&quot;volume_fermentor&quot;].replace(np.nan,999)
  </code></pre>
  <p>when i run it , the console success do it but nothing change in the data frame</p>
  <p><strong>another example:</strong></p>
  <pre><code>df[&quot;volume_fermentor&quot;][4]
  
  Out[79]: nan
  
  df[&quot;volume_fermentor&quot;][4]==df[&quot;volume_fermentor&quot;][4]
  
  Out[80]: False
  
  df[&quot;volume_fermentor&quot;][4]==np.nan
  
  Out[81]: False'
  </code></pre>