title: how to use lists as values in pandas dataframe?
body: <p>I have a dataframe that requires a subset of the columns to have entries with multiple values. below is a dataframe with a "runtimes" column that has the runtimes of a program in various conditions:</p>
  
  <pre><code>df = [{"condition": "a", "runtimes": [1,1.5,2]}, {"condition": "b", "runtimes": [0.5,0.75,1]}]
  df = pandas.DataFrame(df)
  </code></pre>
  
  <p>this makes a dataframe:</p>
  
  <pre><code>  condition        runtimes
  0         a     [1, 1.5, 2]
  1         b  [0.5, 0.75, 1]
  </code></pre>
  
  <p>how can I work with this dataframe and get pandas to treat its values as a numeric list? for example calculate the mean for "runtimes" column across the rows?</p>
  
  <pre><code>df["runtimes"].mean()
  </code></pre>
  
  <p>gives the error: <code>"Could not convert [1, 1.5, 2, 0.5, 0.75, 1] to numeric"</code></p>
  
  <p>it'd be useful to work with this dataframes and also to serialize them as csv files where a list like:  <code>[1, 1.5, 2]</code> gets converted into <code>"1,1.5,2"</code> so that it's still a single entry in the csv file.</p>