title: maintaining ordering when using unique fuction in python
body: <p>I have some code, where the following, say, are the columns of my df. </p>
  
  <pre><code>df.columns = ['A1', 'A2', 'B1', 'B2', 'C1', 'C2', 'D1', 'D2', 'E1', 'E2']
  
  list = df.columns.str[:1]
  list = np.unique(list)
  </code></pre>
  
  <p>I am trying to get the unique values of the letters, and numbers, but in the correct order. </p>
  
  <p>My code doesn't maintain the ordering and I cant figure out how to do so. </p>
  
  <p>Thank you</p>
  
  <p>expected output:</p>
  
  <pre><code>letters = [A, B, C, D, E]
  numbers = [1, 2]
  </code></pre>