title: Pandas - how to build and expanding window dataframe from a series
body: <p>I'm not allowed to use any df.expanding().apply() solutions but need to go through an approach as the following.
  Therefore, given a pd.Series such as</p>
  <pre><code>2008-12-31    1.4174
  2009-01-01    1.4184
  2009-01-02    1.4098
  2009-01-05    1.4000
  2009-01-06    1.3882
  2009-01-07    1.4079
  2009-01-08    1.4045
  2009-01-09    1.4148
  2009-01-12    1.4716
  2009-01-13    1.4979
  </code></pre>
  <p>I would like to build a dataframe such as</p>
  <pre><code>2008-12-31  1.4174  1.4174  1.4174  1.4174  1.4174
  2009-01-01  1.4184  1.4184  1.4184  1.4184  1.4184
  2009-01-02  1.4098  1.4098  1.4098  1.4098  1.4098
  2009-01-05  1.4000  1.4000  1.4000  1.4000  1.4000
  2009-01-06  1.3882  1.3882  1.3882  1.3882  1.3882
  2009-01-07  NaN     1.4079  1.4079  1.4079  1.4079
  2009-01-08  NaN     NaN     1.4045  1.4045  1.4045
  2009-01-09  NaN     NaN     NaN     1.4148  1.4148
  2009-01-12  NaN     NaN     NaN     NaN     1.4716
  2009-01-13  NaN     NaN     NaN     NaN     NaN
  </code></pre>
  <p>How can I proceed? Thanks</p>