title: Filter pandas data frame for col == None
body: <p>I have a data frame <code>data_df</code> with multiple columns, one of which is <code>c</code> which holds country names. How do I filter out the rows where <code>c == None</code>.</p>
  <p>My first attempt was to do this:</p>
  <pre><code>countries_df = data_df[data_df.c != None]
  </code></pre>
  <p>However, that yielded 0 rows. This, however, worked:</p>
  <pre><code>countries_df = data_df[~data_df.c.isin([None])]
  </code></pre>
  <p>Can someone explain why? It seems that from the Pandas doc, the first should be able to filter correctly.</p>
  <p>Some sample rows:</p>
  <pre><code>  _heartbeat_                           a                    al     c      cy     g
  0   NaN Mozilla/5.0 (Linux; U; Android 4.1.2; en-us; H...   en-US   US  Anaheim 15r91
  1   NaN Mozilla/4.0 (compatible; MSIE 7.0; Windows NT ...   en-us   None    NaN ifIpBW
  2   NaN Mozilla/5.0 (Windows NT 6.1; rv:21.0) Gecko/20...   en-US,en;q=0.5  US  Fort Huachuca   10DaxOu
  3   NaN Mozilla/5.0 (Linux; U; Android 4.1.2; en-us; S...   en-US   US  Houston TysVFU
  4   NaN Opera/9.80 (Android; Opera Mini/7.5.33286/29.3...   en  None    NaN 10IGW7m
  5   NaN Mozilla/5.0 (compatible; MSIE 10.0; Windows NT...   en-US   US  Mishawaka   13GrCeP
  6   NaN Mozilla/5.0 (Windows NT 6.1; WOW64; rv:20.0) G...   en-US,en;q=0.5  US  Hammond YmtpnZ
  7   NaN Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_5 li...   en-us   None    NaN 13oM0hV
  8   NaN Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_3 like ...   en-us   AU  Sydney  15r91
  9   NaN Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKi...   en-US,en;q=0.8  None    NaN 109LtDc
  10  NaN Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_3 like ...   en-us   US  Middletown  109ar5F
  11  NaN Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_3 like ...   en-us   US  Germantown  107xZnW
  </code></pre>