title: Get a list from Pandas DataFrame column headers
body: <p>I want to get a list of the column headers from a Pandas DataFrame.  The DataFrame will come from user input, so I won't know how many columns there will be or what they will be called.</p>
  <p>For example, if I'm given a DataFrame like this:</p>
  <pre><code>&gt;&gt;&gt; my_dataframe
      y  gdp  cap
  0   1    2    5
  1   2    3    9
  2   8    7    2
  3   3    4    7
  4   6    7    7
  5   4    8    3
  6   8    2    8
  7   9    9   10
  8   6    6    4
  9  10   10    7
  </code></pre>
  <p>I would get a list like this:</p>
  <pre><code>&gt;&gt;&gt; header_list
  ['y', 'gdp', 'cap']
  </code></pre>