How to Safely Delete Items from a Dictionary While Iterating in Python?

Iterative Dictionary Modification

It is common to need to delete items from a dictionary while simultaneously iterating over it. However, this operation is not natively supported in Python.

Modifying a dictionary while iterating over it can lead to errors. For instance, the code snippet you provided may fail in Python 3 with the error:

RuntimeError: dictionary changed size during iteration.

Python 3 Solution

In Python 3, the solution is to create a list of keys from the dictionary and iterate over that list instead. Here's an example:

<code class="python"># Python 3 or higher

for k in list(mydict.keys()):
    if mydict[k] == 3:
        del mydict[k]</code>

This approach works because a list is immutable and won't be affected by changes to the dictionary.

Python 2 Solution

In Python 2, the keys() method returns an iterator, which cannot be modified during iteration. To modify the dictionary, you can use the following approach:

<code class="python"># Python 2

for k, v in mydict.items():
    if v == 3:
        del mydict[k]</code>

In Python 2, you can also convert the iterator to a list:

<code class="python">for k in mydict.keys():
    if mydict[k] == 3:
        del mydict[k]</code>

Alternative Approach

Alternatively, you can use the pop() method to delete items from the dictionary while iterating:

<code class="python">for k in list(mydict.keys()):
    if k == 3:
        mydict.pop(k)</code>

Note that this approach is more efficient because it doesn't create an additional list.

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