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Yes, clarifying buffering for text mode in open() would be nice.
@direprobs: just in case you didn't know, you can achieve what you want with something like the following in pre-3.7:
with open("/dev/null", "wb", buffering=10) as outb, \
 io.TextIOWrapper(outb, write_through=True) as out:
 out.write("x" * 20)
Sadly, write_through can't be passed to open(), but it can be changed on existing TextIOWrapper since 3.7 (via new reconfigure() method).