Creating a list using a date as index
<p>I have made a list that uses a list of dates as an index, like so:</p>

<pre><code>&gt;&gt;&gt; import datedlist
&gt;&gt;&gt; import datetime
&gt;&gt;&gt; dates = [datetime.date(2012,1,x) for x in range(2,6)]
&gt;&gt;&gt; values = range(4,8)
&gt;&gt;&gt; dates
[datetime.date(2012, 1, 2), datetime.date(2012, 1, 3), datetime.date(2012, 1, 4), datetime.date(2012, 1, 5)]
&gt;&gt;&gt; dl = datedlist.DatedList(values, dates)
&gt;&gt;&gt; dl
[4, 5, 6, 7]
&gt;&gt;&gt; dl[datetime.date(2012,1,3)]
5
</code></pre>

<p>So far all is well, but I also want to be able to use slicing (not extended slicing), like so (the following does <strong>not</strong> work - it is the result I want):</p>

<pre><code>&gt;&gt;&gt; datedlist[datetime.date(2012,1,3):datetime.date(2012,1,4)]
[5, 6]
</code></pre>

<p>Here is my attempt at this (which obviously doesn't work):</p>

<pre><code>class DatedList(list):

    def __init__(self, values, dates):
        self.dates = dates
        list.__init__(self, values)

    def __getitem__(self, date):
        if isinstance(date, slice):
            start = self.dates.index(slice[0])
            end = self.dates.index(slice[1])
            return [list.__getitem__(self, index) for index in range(start, end)]
        elif isinstance( date, datetime.date ) :
            index = self.dates.index(date)
            return list.__getitem__(self, index)
        elif isinstance(date, int):
            if date &lt; 0:
                date += len(self)
            if date &gt;= len(self):
                raise IndexError, "index out of range {}".format(date)
            return list.__getitem__(self, date)
        else:
            raise TypeError, "Invalid argument type."
</code></pre>

<p>The slice[0] and slice[1] are only there to explain my intention. The isinstance(date, int) is only there for debugging - will be removed for production code.</p>

<p><strong>Here is the question</strong>: how can I implement slicing that uses datetime.date objects as indices?</p>

<p>EDIT (after gnibblers 2nd comment):
I did try getslice too (even though the docs say that getslice is obsolete).
The class then looks like this (isinstance-slice bit commented out because of syntax):</p>

<pre><code>class DatedList(list):

    def __init__(self, values, dates):
        self.dates = dates
        list.__init__(self, values)

    def __contains__(self, date):
        return date in self.dates

    def __getslice__(self, fromdate, todate):
        i_from = self.get_index(fromdate)
        i_to = self.get_index(todate)
        print i_from, i_to
        return [list.__getitem__(self, i) for i in range(i_from, i_to)]

    def __getitem__(self, date):
        if isinstance(date, slice):
            pass
#            start = self.dates.index(slice[0])
#            end = self.dates.index(slice[1])
#            return [list.__getitem__(self, i) for i in range(start, end)]
        elif isinstance(date, datetime.date):
            index = self.get_index(date)
            return list.__getitem__(self, index)
        elif isinstance(date, int):
            if date &lt; 0:
                date += len(self)
            if date &gt;= len(self):
                raise IndexError, "index out of range {}".format(date)
            return list.__getitem__(self, date)
        else:
            raise TypeError, "Invalid argument type."

    def get_index(self, date):
        if date in self.dates:
            index = self.dates.index(date)
        elif date &lt; self.dates[0]:
            index = 0
        elif date &gt; self.dates[-1]:
            index = len(self.dates) - 1
        return index
</code></pre>

<p>The result is:</p>

<pre><code>&gt;&gt;&gt; print dl[datetime.date(2012,1,3):datetime.date(2012,1,5)]
&gt;&gt;&gt; None
</code></pre>

<p>Apparently getslice does not get used at all, because the print is not executed. It appears that getitem is executed when a slice is requested, but I don't seem to be able to use a datetime.date in a slice.
/EDIT</p>

<p>Note: apparently it is not a good idea to subclass list, but none of the alternatives I tried so far seemed to work better (or at all):</p>

<ul>
<li><p>Building a class from scratch: I could not get the [] notation to work:</p>

<blockquote>
<pre><code>dl = DatedList(values, dates)
value = dl[some_date]       # I want this to work
value = dl.value(same_date) # I don't want this
</code></pre>
</blockquote></li>
<li><p>I have considered using a dict, but my list needs to be ordered and I need to use slicing as well.</p></li>
<li><p>I also tried to subclass collections.Sequence, but that resulted in:</p>

<blockquote>
  <p>TypeError: descriptor '<strong>init</strong>' requires a 'list' object but received a 'DatedList'</p>
</blockquote></li>
</ul>
