Cache operationsget andset are used inside a reentrant lock, but cache clear is called fromparse_string outside that reentrant lock.

FifoCache set operation does this:

defset_(_,key,value):cache[key]=valuewhilelen(cache)>size:# pop oldest element in cache by getting the first keycache_pop(next(iter(cache)))

That cache_pop(next(iter(cache)) is not thread safe.

Creating a bunch of threads parsing expressions in loop eventually raise a RuntimeErrordictionary changed size during iteration
To reproduce it easier put another thread to callParserElement.reset_cache() in loop