How to Avoid the \"RuntimeError: dictionary changed size during iteration\" in Python?

Avoiding the "RuntimeError: dictionary changed size during iteration" Error

Attempting to modify a dictionary while iterating over it, as seen in the code snippet below, can trigger the "RuntimeError: dictionary changed size during iteration" error:

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

for i in d:
    if not d[i]:
        d.pop(i)</code>

To overcome this limitation, various approaches can be employed:

Python 2.x and 3.x:

Force a copy of the keys using 'list':

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

Python 3.x (and later):

Use 'collections.OrderedDict':

<code class="python">from collections import OrderedDict

for i in OrderedDict(d):</code>

Alternative Solutions:

1. Create a new dictionary with the desired modifications:

<code class="python">new_d = {}
for key, value in d.items():
    if value:
        new_d[key] = value</code>

2. Use Python 3.3' s 'popitem' method and iterate over a copy:

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

By leveraging these techniques, you can circumvent the "RuntimeError: dictionary changed size during iteration" error when handling dictionaries in Python.

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