python runtime_error ai_generated true

RuntimeError: Set changed size during iteration

ID: python/runtimeerror-set-changed-size

Also available as: JSON · Markdown
95%Fix Rate
95%Confidence
50Evidence
2023-01-01First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
311 active — — —

Root Cause

Modifying a set while iterating. Same issue as dict changed size during iteration.

generic

Workarounds

  1. 95% success Iterate over a copy: for item in set(my_set): my_set.discard(item) 
    for item in set(my_set): my_set.discard(item)

    Sources: https://docs.python.org/3/library/stdtypes.html#frozenset

  2. 92% success Build a new set with set comprehension instead of modifying in place 
    filtered = {x for x in my_set if condition(x)}

    Sources: https://docs.python.org/3/tutorial/datastructures.html#sets

Dead Ends

Common approaches that don't work:

  1. Use try/except to catch and retry 80% fail

    Unreliable — iteration results are undefined after modification

  2. Convert to a thread-safe set 75% fail

    Thread safety doesn't fix single-threaded iteration mutation

Error Chain

Frequently confused with: