Automatically add key to Python dict
<p>I want to automatically add keys to a Python dictionary if they don't exist already.
For example,</p>

<pre><code>a = "a"
b = "b"
c = "c"

dict = {}
dict[a][b] = c # doesn't work because dict[a] doesn't exist
</code></pre>

<p>How do I automatically create keys if they don't exist?</p>
