title: Need to overwrite cells in a column without overwriting the other cells in the same column
body: <p>I need to overwrite the SERIALNUMBER to 'NONE' only if the DESCRIPTION contains 'fuel'.</p>
  
  <pre><code>cols = ['DESCRIPTION', 'SERIALNUMBER']
  data = [['vehicle; truck', '123456'],
          ['fuel; truck', 'nan'],
          ['vehicle; car', '789456'],
          ['fuel; car', 'nan'],
          ['vehicle; semi', '653123'],
          ['fuel; semi', 'nan']
         ]
  df = pd.DataFrame(data=data, columns=cols)
  
       DESCRIPTION     SERIALNUMBER
  0    vehicle; truck   123456
  1    fuel; truck       nan
  2    vehicle; car     789456
  3    fuel; car         nan
  4    vehicle; semi    653123
  5    fuel; semi        nan
  </code></pre>
  
  <p>I tried using the <code>df.replace</code>, but it replaces the serial numbers that are currently there with 0.</p>
  
  <pre><code>c1 = df['DESCRIPTION'].str.contains('fuel;')
  df['SERIALNUMBER'].replace(c1, value='NONE', inplace=True)
  </code></pre>
  
  <p>What I end up with is </p>
  
  <pre><code>DESCRIPTION     SERIALNUMBER
  0    vehicle; truck   0
  1    fuel; truck       NONE
  2    vehicle; car     0
  3    fuel; car         NONE
  4    vehicle; semi    0
  5    fuel; semi        NONE
  </code></pre>