How to Safely Remove Items from a Dictionary During Iteration?

Avoiding "RuntimeError: dictionary changed size during iteration" Error with Dictionary Modifications

When iterating over a dictionary, adding or removing entries can lead to the "RuntimeError: dictionary changed size during iteration" error. This article explores a scenario where you want to remove key-value pairs with empty list values from a dictionary.

Problem Statement:

Given a dictionary d containing key-value pairs where values are lists, you want to remove key-value pairs where the values are empty lists. However, attempting to do so using a for loop with conditional checks results in the aforementioned error.

Solution:

To avoid this error, you can make a copy of the dictionary's keys using the list() function. This creates a separate list of keys that can be iterated over independently of the dictionary's modifications:

<code class="python">for i in list(d):
    if not d[i]:
        d.pop(i)</code>

Alternative Approach for Python 2.x:

In Python 2.x, calling the .keys() method on a dictionary returned a copy of the keys. Therefore, you could use the following approach:

<code class="python">for i in d.keys():</code>

Note for Python 3.x:

In Python 3.x, the .keys() method returns a view object instead of a copy. Consequently, the second approach will not work in Python 3.x.

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