title: pandas: Notnull() or Notna() does not drop Nan value
body: <p>I have a dataframe which contains two columns (as shown in the figure).
  <a href="https://i.stack.imgur.com/whoY4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/whoY4.png" alt="enter image description here"></a></p>
  
  <p>I am trying to drop Nan from one of the columns (i.e. FACILITYID). I tried to use the following commands to drop the NaN</p>
  
  <pre><code>temp = temp[temp['FACILITYID'].notnull()]
  temp = temp[temp['FACILITYID'].notna()]
  temp = temp[~temp['FACILITYID'].isnull()]
  temp = temp[~temp['FACILITYID'].isna()]
  temp = temp[temp['FACILITYID']!='']
  </code></pre>
  
  <p>However, none of them remove NaN. I followed the instruction of the existing thread (<a href="https://stackoverflow.com/questions/58420868/nan-does-not-drop-out-in-python/58421136#58421136">Nan does not drop out in Python</a>) but no luck. Could anyone point out where am I making the mistake?</p>