title: thresh in dropna for DataFrame in pandas in python
body: <pre><code>df1 = pd.DataFrame(np.arange(15).reshape(5,3))
  df1.iloc[:4,1] = np.nan
  df1.iloc[:2,2] = np.nan
  df1.dropna(thresh=1 ,axis=1)
  </code></pre>
  
  <p>It seems that no nan value has been deleted.</p>
  
  <pre><code>    0     1     2
  0   0   NaN   NaN
  1   3   NaN   NaN
  2   6   NaN   8.0
  3   9   NaN  11.0
  4  12  13.0  14.0
  </code></pre>
  
  <p>if i run </p>
  
  <pre><code>df1.dropna(thresh=2,axis=1)
  </code></pre>
  
  <p>why it gives the following?</p>
  
  <pre><code>    0     2
  0   0   NaN
  1   3   NaN
  2   6   8.0
  3   9  11.0
  4  12  14.0
  </code></pre>
  
  <p>i just dont understand what thresh is doing here. If a column has more than one nan value, should the column be deleted?</p>