title: check if variable is dataframe
body: <p>when my function f is called with a variable I want to check if var is a pandas dataframe:</p>
  
  <pre><code>def f(var):
      if var == pd.DataFrame():
          print "do stuff"
  </code></pre>
  
  <p>I guess the solution might be quite simple but even with </p>
  
  <pre><code>def f(var):
      if var.values != None:
          print "do stuff"
  </code></pre>
  
  <p>I can't get it to work like expected. </p>