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The problem is not only about concurrent prints.
It is about invalid pointer passed to a C function.
Here is an example that reliably crashes the interpreter on my windows
machine:
import bz2, threading
bz2c = bz2.BZ2Compressor()
b = bytearray(b"a" * 1000000)
def f():
 for x in range(10):
 b[:] = b""
 b[:] = bytearray(b"a" * 1000000)
threading.Thread(target=f).start()
for x in range(10):
 bz2c.compress(b)
bz2c.compress is a slow function, that happens to accept bytearray and
to release the GIL. If the other thread reallocates the bytearray,
bz2c.compress will read invalid data.