Python 'RuntimeError: dictionary changed size during iteration' (or 'Set changed size during iteration'). How do I modify a dict while looping over it?

Cause: iterating over a dict (or its .keys(), .values(), .items() views) while adding or removing keys is not allowed; the views reflect the dict live, and Python raises RuntimeError when the size changes mid-loop to avoid skipping or repeating items. The same applies to sets. It often shows up when deleting entries that match a condition, adding derived keys inside the loop, or in multi-threaded code where another thread changes the dict. Fix (pick one): 1. Loop over a snapshot: for key in list(d): ... (or list(d.items())); then deleting or adding keys inside the loop is safe. 2. Build a new dict instead of mutating: d = {k: v for k, v in d.items() if keep(k, v)}; this is usually clearer and avoids the problem entirely. 3. Collect changes, apply after the loop: to_delete = [k for k, v in d.items() if cond(v)], then for k in to_delete: del d[k]. 4. Changing values for existing keys is fine (d[k] = new_value) since the size doesn't change; only adding or removing keys triggers the error. 5. In threaded code, protect the dict with a lock around both the iteration and the modification, or iterate over a copy taken under the lock. Verify: the loop runs to completion, and a quick test with entries that should be removed or added shows the expected final dict. Sources: https://docs.python.org/3/library/stdtypes.html https://docs.python.org/3/tutorial/datastructures.html