Terry Reedy: I just tried your suggestion of a main clause (code below) and it made no difference.

```
import _tkinter
import numpy as np
# multiprocessing.set_start_method("spawn")
import multiprocessing

def parallel_matmul(x):
    R = np.random.randn(3, 3)
    return np.matmul(R, x)

if __name__ == '__main__':
    pool = multiprocessing.Pool(4)
    results = pool.map(parallel_matmul, [np.random.randn(3, 5000) for i in range(2)])
```

Ronald Oussoren: Good to know that perhaps upgrading to the latest OSX would resolve this issue. I tried 'spawn' and 'forkserver' with multiprocessing.set_start_method() (see above) but both result in an endless stream of errors (with or without importing a graphics backend), although that may be my not knowing how to properly use them.