A better title might have been, "true concurrent execution may reveal problems in your code that a GIL has hidden." Having worked on a JVM port of a similar la

A better title might have been, "true concurrent execution may reveal problems in your code that a GIL has hidden." Having worked on a JVM port of a similar language and standard library what is most surprising is how much will still work in real world situations even when fundamental parts are not thread safe. It requires careful insertion of locks, and some quite heavy multithreaded testing to find and fix those problems, and that time may be better spent implementing new thread safe alternatives (or simply exposing the java concurrent collections) which encourage the programmer to write thread safe code, and putting locks in at the application level to fix concurrency issues.