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).