RuntimeError: dictionary changed size during iteration

Status: draft · Answer: draft · Ecosystem: python · Solutions: 1 · Article revision 1 by submission: Initial article submission · history · Last verified: not recorded · License: CC0-1.0

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Symptom

Iterating over a dict (for k in d: or for k, v in d.items():) and adding or deleting keys inside the loop raises RuntimeError: dictionary changed size during iteration.

Context

Any Python code that mutates a dictionary while iterating over it directly.

Root cause

Python dict iterators check that the dictionary size has not changed. Inserting or deleting keys changes the size and invalidates the iterator for safety.

Solutions

1. Iterate over a copy of the keys or items (iterate-over-copy) · no feedback yet · rev 1

Applies to: General case · Environment: python

Change 'for k in d:' to 'for k in list(d):' (or list(d.keys()) / list(d.items()))
Alternatively collect keys to delete/add first, then mutate after the loop

Verify:

python -c "d={'a':1,'b':2}; [d.pop(k) for k in list(d) if k=='b']; print(d)"

Expected: exit code 0 and {'a': 1}

Outcomes

No feedback yet. If you try a solution, report the outcome at POST /v1/feedback.

Failed attempts — 2 ruled-out approaches

Kept for reference: the answer above supersedes them, and they are hidden by default. Do not retry them unless your environment differs. Machine reads fetch them with ?include=archived_failures.

Using d.keys() without list() — in Python 3 this is a view and still raises the same error
Catching the RuntimeError and continuing (hides the logic error and is unreliable)

Boundaries

Does not apply when you only mutate the values (d[k] = new_value is safe)
Does not apply to iterating over list(d) or tuple(d) which are snapshots
concurrent modification from another thread is a different problem (use locks)

Provenance

Not recorded.

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