RuntimeError: dictionary changed size during iteration
This error matches known, documented patterns with reliable solutions.
Quick Fix (Most Common Solution)
- Iterate over a copy with dict.copy()
- Collect keys to modify separately
Seeing "RuntimeError: dictionary changed size during iteration"? This error can be frustrating, but it's usually fixable. It typically affects your development workflow or system. Below you'll find clear, step-by-step solutions to resolve this issue.
What This Error Means
You added or removed keys from a dictionary during iteration.
Frequently documented in developer and vendor support forums.
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Common Causes
- Modifying dict in for loop
- Deleting keys during iteration
- Adding keys during iteration
How to Fix
- Iterate over a copy with dict.copy()
- Collect keys to modify separately
- Use dict comprehension instead
Last reviewed: March 2026 How we review solutions
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Frequently Asked Questions
Can I modify values during iteration?
Yes, only changing keys causes this error.
Why does copy work?
You iterate over snapshot while modifying original.
Related Resources
Also Known As
- Python exception
- Python traceback
- Python runtime error
- Python crash
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