Is checking a dictionary for a key an atomic operation?
<p>Is checking if a key is in a dictionary  - <code>if key in mydict</code> - an atomic operation?</p>

<p>If not, would there be any negative effects if a thread was checking for a key while another thread was modifying the dictionary? The checking thread doesn't modify the dictionary, it just behaves differently depending on the presence of a key.</p>
