Tal had written this on the original issue21765:
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Finally, since the defaultdict is kept around as long as IDLE is running, I decided to avoid having it grow continually and consume memory unnecessarily. So I wrote a simple Mapping class, which wraps a normal dict and uses a custom default value instead of None, ord('x') in this case. Works like a charm :)
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So maybe I misunderstood and this shouldn't be a defaultdict?