I struck it out because I had missed the sleep in the OP's code. With the sleep I too would have expected the output to be essentially synchronised. I thought my comment may be misleading and I am not authoritative enough in this area to leave it stand.

Optimising for fewest key strokes only makes sense transmitting to Pluto or beyond

In reply to Re^3: buffering when reading from unnamed pipe with child process by GrandFather
in thread buffering when reading from unnamed pipe with child process by Santasha

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