title: Dictionary lookup causing objects are mutable, thus they cannot be hashed error
body: <p>I have a python dict/hashmap: </p>
  
  <pre><code>yearMapping = {"2016":"6", "2017":"7", "2018":"8", "2019":"9"}
  </code></pre>
  
  <p>I am trying to replace values within a DataFrame object with the corresponding map value. I wrote out an example below with my expected output.</p>
  
  <p>I have 3 columns with data in them. If generated code is null, I am going to populate that column with the Contract Time + the map value.</p>
  
  <pre><code>Generated Code      |      Contract Date      |     Contract Time
      null            |         201607          |          1:31:01
  </code></pre>
  
  <p>Expected output:</p>
  
  <pre><code>Generated Code      |      Contract Date      |     Contract Time
      1:31:016        |         201607          |          1:31:01
  </code></pre>
  
  <p>What I am doing so far:</p>
  
  <pre><code>yearCode = df['Contract Date'].astype(str).apply(lambda x: x[:4])
  
  df.loc[df["Generated Code"].isnull(),'Generated Code'] = df['Contract Time'] + yearMapping.get(yearCode)
  </code></pre>
  
  <p>I keep getting the error: <strong>TypeError: 'Series' objects are mutable, thus they cannot be hashed</strong> </p>
  
  <p>Is this even do-able?</p>