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    <p>A simple test in the console shows you cannot modify a dictionary while iterating over it:</p> <pre><code>&gt;&gt;&gt; mydict = {'one': 1, 'two': 2, 'three': 3, 'four': 4} &gt;&gt;&gt; for k, v in mydict.iteritems(): ... if k == 'two': ... del mydict[k] ... ------------------------------------------------------------ Traceback (most recent call last): File "&lt;ipython console&gt;", line 1, in &lt;module&gt; RuntimeError: dictionary changed size during iteration </code></pre> <p>As stated in delnan's answer, deleting entries causes problems when the iterator tries to move onto the next entry. Instead, use the <code>keys()</code> method to get a list of the keys and work with that:</p> <pre><code>&gt;&gt;&gt; for k in mydict.keys(): ... if k == 'two': ... del mydict[k] ... &gt;&gt;&gt; mydict {'four': 4, 'three': 3, 'one': 1} </code></pre> <p>If you need to delete based on the items value, use the <code>items()</code> method instead:</p> <pre><code>&gt;&gt;&gt; for k, v in mydict.items(): ... if v == 3: ... del mydict[k] ... &gt;&gt;&gt; mydict {'four': 4, 'one': 1} </code></pre> <p><strong>EDIT:</strong></p> <p>Above will not work for Python3 and will give a <code>RuntimeError</code>.</p> <blockquote> <p>RuntimeError: dictionary changed size during iteration.</p> </blockquote> <p>This happens because <code>mydict.keys()</code> returns an iterator not a list. As pointed out in comments simply convert <code>mydict.keys()</code> to a list by <code>list(mydict.keys())</code> and it should work.</p>
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    1. POHow to delete items from a dictionary while iterating over it?
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    1. POHow to delete items from a dictionary while iterating over it?
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    1. CONote that in Python 3, dict.items() returns an iterator (and dict.iteritems() is gone).
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      1. USTim Lesher
    2. COTo elaborate on @TimLesher comment... This will NOT work in Python 3.
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    3. COTo elaborate on @max's elaboration, it will work if you convert the above code with 2to3. One of the default fixers will make the loop look like `for k, v in list(mydict.items()):` which works fine in Python 3. Same for `keys()` becoming `list(keys())`.
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      1. USWalter Mundt
 

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