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: April 2026 How we review solutions
OS-Specific Behavior
Dictionary Iteration Error on Windows: Path, Shell, and Script Differences
Dictionary Iteration Error on Windows because of path separator differences, case sensitivity, shell availability, and how Windows handles scripts.
The most impactful differences: Windows uses backslash (\) as the path separator — use pathlib.Path or os.path.join() rather than string concatenation. Windows is case-insensitive for paths, so import MyModule and import mymodule both work on Windows but only one works on Linux. os.system() or subprocess commands that call bash, grep, sed, or awk fail on Windows unless run inside WSL2 or Git Bash. Python scripts without a shebang line need python script.py explicitly — not ./script.py. For PowerShell users, the activation script for venv is .venv\Scripts\Activate.ps1, but PowerShell's execution policy may block it — run Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser once to allow signed local scripts.Optional follow-up
Some users ask whether saving fixes for recurring errors would be useful when the same issue appears again.
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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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