AFAIK there is nothing you can do between after calling fork(2) to "reinitialise" the ObjC runtime. And I don't think that's the issue anyway: I suspect that the actual problem is that Apple's system frameworks use multithreading (in particular libdispatch) and don't have code to ensure a sane state after calling fork. 

In Python 3 there is another workaround to avoid problems using multiprocessing: use multiprocessing.set_start_method() to switch away from the "fork" startup handler to "spawn" or "forkserver" (the latter only when calling set_start_method before calling any code that might call into Apple system frameworks.