title: Why is there a KEY ERROR in one place but no KEY ERROR in another similar statement?
body: <p>I made a PANDAS Series in the first line of the code below. But I can't understand why
  there is an error in the 3rd line but no error in the 2nd line. Please run the code in Jupyter in 3 separate blocks and help me if you understood the cause of error in 3rd line.</p>
  <pre><code>data = pd.Series([1,2,3,4,5], index=['a', 'b', 'c', 7, 8])
  data['g']=5 # NO ERROR.
  data[9] = 7 # WHY ERROR ??
  </code></pre>