Pythonic representation of a list of values with optional flags
<p>What first pops into my head is a dictionary with keys acting as list values and dictionary values defaulting to <code>None</code>, but this feels sub-optimal to me. Can anyone propose a single-structure (i.e. no separate 'options' dict) alternative where <code>dict[key]</code> on an item with no flags set returns <code>None</code> and otherwise returns a list of flags? Apologies if I am overlooking some obvious convention.</p>

<p>A simple use for this structure could be an <code>order</code> instance where each item can optionally contain modifications (i.e. no nuts).</p>
