title: how to keep fractions in a pandas dataframe
body: <p>I got a numpy array which contains some fractions.
  When I generate a dataframe from this array, the fractions turned into floats.
  How do I still keep all the fractions in the dataframe?
  My code is something like this:</p>
  <pre><code>row=[&quot;A&quot;,&quot;B&quot;]
  col=[&quot;X1&quot;,&quot;X2&quot;]
  my_array=np.array([[1/3,2/5],[4/7,1/6]])
  df = pd.DataFrame(data = my_array, 
                  index = row, 
                  columns = col)
  print(my_array)
  print(df)
  </code></pre>
  <p>Output:</p>
  <pre><code>[[1/3 2/5]
  [4/7 1/6]]
           X1        X2
  A  0.333333  0.400000
  B  0.571429  0.166667
  </code></pre>