title: Pandas isna() and isnull(), what is the difference?
body: <p>Pandas has both <code>isna()</code> and <code>isnull()</code>. I usually use <code>isnull()</code> to detect missing values and have never met the case so that I had to use other than that.
  So, when to use <code>isna()</code>?</p>