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As discovered in #3139, io.BufferedWriter mutates the size of its
internal bytearray object. Consequently, invocations of write() from
multiple threads can produce exceptions when one thread gets a buffer to
the bytearray while the other thread tries to resize it. This especially
affects calling print() from multiple threads.
A solution is to use a fixed-size (preallocated) bytearray object.
Another solution is to get rid of the bytearray approach and replace it
with a growing list of immutable bytes objects.
Here is the test script provided by Amaury:
import sys, io, threading
stdout2 = io.open(sys.stdout.fileno(), mode="w")
def f(i):
 for i in range(10):
 stdout2.write(unicode((x, i)) + '\n')
for x in range(10):
 t = threading.Thread(target=f, args=(x,))
 t.start()
(with py3k, replace "stdout2.write" with a simple "print")