Coding⏱️ 2 min read📅 2026-05-31

How to Fix: How to avoid "RuntimeError: dictionary changed size during iteration" error?

Avoid RuntimeError: dictionary changed size during iteration by using a copy of the dictionary and iterating over it instead of the original dictionary.

Quick Answer: Use d = dict((k, v) for k, v in d.items()) to create a new dictionary without modifying the original one.

The RuntimeError: dictionary changed size during iteration error occurs when you try to modify a dictionary while iterating over it. This can happen in Python due to the way dictionaries are implemented.

🛑 Root Causes of the Error

  • Modifying a dictionary while iterating over it can lead to unexpected behavior or errors.

🛠️ Step-by-Step Verified Fixes

Method 1: Iterating Over Dictionary Items Before Modifying

  1. Step 1: Iterate over the dictionary items using a for loop or enumerate.

Method 2: Creating a New Dictionary with Filtered Items

  1. Step 1: Use a dictionary comprehension to create a new dictionary that only includes key-value pairs where the value is not an empty list.

✨ Wrapping Up

To avoid the RuntimeError: dictionary changed size during iteration error, you can use either of these methods. By iterating over the dictionary items before modifying them or creating a new dictionary with filtered items, you can safely remove key-value pairs where the values are empty lists.

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