If you try to change a dictionary while you’re looping through it, Python will stop you with this error. This usually happens when you’re adding or deleting keys during the loop. Let’s look at why this happens and how to fix it.<\/p>\n\n\n\n
You’re iterating over a dictionary and modifying it at the same time, and Python throws this error:<\/p>\n\n\n\n
my_dict = {\"a\": 1, \"b\": 2, \"c\": 3}\n\nfor key in my_dict:\n if key == \"b\":\n del my_dict[key]<\/code><\/pre>\n\n\n\nOutput:<\/strong><\/p>\n\n\n\nRuntimeError: dictionary changed size during iteration<\/code><\/pre>\n\n\n\nEven if your logic seems correct, this exception appears and halts your script.<\/p>\n\n\n\n
Why This Happens<\/h2>\n\n\n\n
When Python loops through a dictionary, it keeps track of the size. If the size changes while it’s looping, like if you delete a key, Python gets confused and shows an error to prevent mistakes.<\/p>\n\n\n\n
This applies to:<\/strong><\/p>\n\n\n\n\n- for key in dict<\/li>\n\n\n\n
- for key, value in dict.items()<\/li>\n\n\n\n
- Even dict.keys() and dict.values() if modified during iteration<\/li>\n<\/ul>\n\n\n\n
Steps to Resolve the Issue<\/h2>\n\n\n\nStep 1: Use a Copy of the Keys for Safe Iteration<\/h3>\n\n\n\n
Instead of modifying the dictionary directly while looping, iterate over a copy of its keys:<\/p>\n\n\n\n
for key in list(my_dict.keys()):\n if key == \"b\":\n del my_dict[key]<\/code><\/pre>\n\n\n\nThis avoids the runtime error because you’re not looping over the original dictionary structure.<\/p>\n\n\n\n
Step 2: Build a New Dictionary Instead<\/h3>\n\n\n\n
Another safer and often cleaner approach is to build a new dictionary with filtered items:<\/p>\n\n\n\n
filtered_dict = {k: v for k, v in my_dict.items() if k != \"b\"}<\/code><\/pre>\n\n\n\nThis approach avoids mutation altogether and leads to more predictable results.<\/p>\n\n\n\n
Step 3: Mark Keys for Deletion, Then Remove Later<\/h3>\n\n\n\n
If deletion logic is more complex, first collect the keys to remove, then delete them in a second pass:<\/p>\n\n\n\n
to_remove = []\n\nfor key in my_dict:\n if should_remove(key):\n to_remove.append(key)\n\nfor key in to_remove:\n del my_dict[key]<\/code><\/pre>\n\n\n\nThis avoids modifying the dictionary during iteration and gives you full control over the deletion logic.<\/p>\n\n\n\n
Step 4: Be Cautious with .items(), .keys() and .values()<\/h3>\n\n\n\n
All these views reflect real-time changes to the dictionary. So modifying the dictionary while looping through them will also throw errors. Convert them to list() first if you intend to modify.<\/p>\n\n\n\n
Conclusion<\/h2>\n\n\n\n
This error shows up when you change a dictionary while looping through it. Python blocks it to avoid problems.<\/p>\n\n\n\n
To fix it<\/strong>:<\/strong><\/p>\n\n\n\n\n- Use list(dict.keys()) or list(dict.items()) to safely loop and modify<\/li>\n\n\n\n
- Prefer building a new filtered dictionary if you’re removing elements<\/li>\n\n\n\n
- Separate iteration and modification steps for cleaner, safer code<\/li>\n<\/ul>\n\n\n\n