Cache operations get and set are used inside a reentrant lock, but cache clear is called from parse_string outside that reentrant lock.

FifoCache set operation does this:

        def set_(_, key, value):
            cache[key] = value
            while len(cache) > size:
                # pop oldest element in cache by getting the first key
                cache_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 RuntimeError dictionary changed size during iteration
To reproduce it easier put another thread to call ParserElement.reset_cache() in loop