How to Avoid \'RuntimeError: dictionary changed size during iteration\'?

Avoiding "RuntimeError: dictionary changed size during iteration"

When attempting to modify a dictionary while iterating through it, you may encounter the "RuntimeError: dictionary changed size during iteration" error. This arises because modifying the dictionary's size (adding or removing entries) while looping over it invalidates the iterator.

To address this issue, the recommended approach is to create a copy of the dictionary keys using list() or iterate over the keys via d.keys(). For Python 3.x, d.keys() returns a view object, requiring you to create an explicit copy using list().

For example, consider the following dictionary:

<code class="python">d = {'a': [1], 'b': [1, 2], 'c': [], 'd':[]}</code>

To remove key-value pairs with empty lists, you can use the following code:

<code class="python">for i in list(d):  # Python 3.x
# or
for i in d.keys():  # Python 2.x
    if not d[i]:
        d.pop(i)</code>

By creating a copy of the keys using list(), we avoid the issue of modifying the dictionary size during iteration and ensure that the loop completes successfully.

The above is the detailed content of How to Avoid \'RuntimeError: dictionary changed size during iteration\'?. For more information, please follow other related articles on the PHP Chinese website!