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@ronaldoussoren: The order of the imports made no difference. Even with the call at the top, I got endless errors with both 'spawn' and 'forkserver', with or without importing a graphics backend. Only 'fork' works, and only if a graphics package is not imported.
** HOWEVER **
Setting method='spawn' or 'forkserver' and force=True at top, or calling multiprocessing.set_start_method in __main__ does work (when run as scripts from command line). It's a shame to have to give up the convenience of 'fork', but this is a start.
```
import multiprocessing
multiprocessing.set_start_method("spawn", force=True)
import numpy as np
# import _tkinter
def parallel_matmul(x):
 R = np.random.randn(3, 3)
 return np.matmul(R, x)
if __name__ == '__main__':
 # multiprocessing.set_start_method("spawn", force=False)
 pool = multiprocessing.Pool(4)
 results = pool.map(parallel_matmul, [np.random.randn(3, 5000) for i in range(2)])
```