title: Saving lists of variable length to HDF5 in pandas
body: <p>I'm pretty new to pandas, so please apologize if my questions are trivial.</p>
  
  <p>I want to save a pandas data frame to hdf5 that contains a list of lists of variable length as a column. My goal is to write and read in this data frame in chuncks later on. This seems to be only possible if I store the data frame in table format.
  However if I save my data frame in table format I get the following error:</p>
  
  <pre><code>TypeError: Cannot serialize the column [pdf] because its data contents are [mixed] object dtype
  </code></pre>
  
  <p>In the following I show a minimal example to reproduce the issue:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  df = pd.DataFrame()
  df['ID'] = [el for el in np.arange(100)]
  df['pdf'] = [np.arange(i) for i in df['ID']]
  df.to_hdf('test_format_table.h5','df', format='table')
  </code></pre>
  
  <p>If I save the data frame with format = 'fixed' everything will work, however I want to save in table format. </p>
  
  <p>Is it possible to easily resolve this issue?</p>
  
  <p>Thank you very much for your assistance.</p>
  
  <p>Markus</p>