title: Iterating over a large dataframe and grouping by date
body: <p>I have a very large dataframe  that can't all fit into memory at once, the file is ordered by date but each date has a different number of rows. I know I can read the file into chunks via:</p>
  <pre><code>with pd.read_csv(filename, chunksize=chunksize) as reader:
      for df_chunk in reader:
          for g in df_chunk.groupby('date'):
              process(g)
  </code></pre>
  <p>However since the number of rows on a given date are different I can't guarantee that I can get all the rows on a given <code>groupby('date')</code>. How can I iterate over the large dataframe and group by date to get all rows for each date in the group by?</p>