Create a python dictionary with unique keys that have list as their values
<p>I want to create a dictionary as following - </p>

<pre><code>{'a':[1, 2, 3, 4, 5], 'b':[1, 3, 5], 'c':[2, 3, 5]}
</code></pre>

<p>The way I have implemented it is</p>

<pre><code>mydict = dict()
letters = ['a', 'b', 'a', 'c', 'a'] 

#please mark the list has multiple occurence of a, 
#hence I would want to check if a key with 'a' exists. Please do not advise to make the list unique. 
for l in letters:
    if not mydict.get(l):
        mydict[l] =  &lt;values from another place via some filter&gt;
    else:
        mydict[l].append(&lt;values from another dict&gt;)
</code></pre>

<p>Is there a better approach to do this?</p>
