The signal module is really special. In Python, only the main thread of the main interpreter is supposed to be allowed to run Python signal handlers (but the C signal handler can be executed by any thread).
The exec function of the _signal module runs actions each time a new instance of the _signal module is created, whereas some actions must only be done exactly once:
* modify Handlers[i].tripped
* modify Handlers[i].func
* Set SIGINT signal handler
* Initialize sigint_event event (Windows only): see bpo-41686
For example, calling signal.signal() in a subinterpreter raises ValueError("signal only works in main thread of the main interpreter").