RuntimeError: Set Changed Size During Iteration

Why This Happens

Like dictionaries, sets cannot be modified during iteration. Adding or removing elements changes the internal hash table, invalidating the iterator.

The Problem

numbers = {1, 2, 3, 4, 5}
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)

The Fix

numbers = {1, 2, 3, 4, 5}
for n in numbers.copy():
    if n % 2 == 0:
        numbers.remove(n)

# Or comprehension:
numbers = {n for n in numbers if n % 2 != 0}

Step-by-Step Fix

1. 1

   Copy before iterating

   Use set.copy() or list(set) to iterate over a copy.

2. 2

   Use set comprehension

   Create a new filtered set instead of modifying in place.

3. 3

   Collect removals

   Build a list of items to remove, apply after the loop.