title: how to create new columns from a list of columns
body: <p>I have an existing dataframe. And I have a list.</p>
  <pre><code>my_list = ['col_10', 'col_11', 'col_12', 'col_13']
  </code></pre>
  <p>I want to add new columns in my dataframe that are have these column names, the same value <code>placeholder</code></p>
  <p>I thought this would do the trick:</p>
  <pre><code>for column_names in my_list:
     my_dataframe[column_names] = 'placeholder'
  </code></pre>
  <p>but I get this error:</p>
  <pre><code>TypeError: list indices must be integers or slices, not str
  </code></pre>
  <p>EDIT: This code worked once I realized that my_dataframe was not a dataframe</p>