Thank you for feedback. I will try to reproduce the issue with 3.6. By the way, haven't you used gdb with python pretty-printers enabled to examine the state of the program? I've got the same error message, then I breaked the execution in debugger and tried to examine the callstack of threads, that stucked in UnlockGILandSleep. The reason for it is clear: then the debugger tries to build a callstack, some of pretty printers try to execute some python code to give a better representation of interpreter objects. The code is executed at the top of the stack of the examined thread. Since this thread explicitly released the GIL before going to sleep, these functions hit the assert about calling the memory allocator without holdng the GIL. Disabling pretty-printers makes these error messages to disappear.