RuntimeError: dictionary changed size during iteration
A dict (or set) was modified while being looped over.
Exact message you may see
dictionary changed size during iterationSet changed size during iteration
Seen on:
Python
Meaning
Adding or deleting keys during iteration invalidates the iterator. Iterate over a copy of the keys, or build a new dict.
Common causes
- del d[k] inside for k in d
- Adding keys while iterating
- Another thread mutating the dict
⚡ Quick fix
- Iterate over list(d) or list(d.items())
- Build a new dict with a comprehension
- Collect keys to delete, then delete after the loop
Detailed fix by platform
Python
active = {k: v for k, v in users.items() if v.active}
How to diagnose
- Loop — Which loop mutates the dict?
- Threads — Is another thread writing to it?
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Last updated 13 Oct 2026