title: Concatenate current value to previous value based on other column condition
body: <p>I'm new to Pandas as well as Python.  I'm trying to mimic the task with Python similar to what I created in excel file to find concatenate the current value to previous value based on condition</p>
  
  <p>if A = false then B, else current value in B column concatenate to previous value in B</p>
  
  <pre><code>A         B          C
  False     "bird"     "bird"
  True      "fish"     "bird,fish"
  True      "Tiger"    "bird,fish,Tiger"
  False     "Elephant" "Elephant"
  </code></pre>