I have a program that needs to output binary data to stdout.
I don't want to convert it to a string.

for example something like
sys.stdout.write('0o377')
to write a byte with all the bits turned on.

When I try this, I get an error like:
sys.stdout.write(data)
File "/usr/local/lib/python3.0/io.py", line 1484, in write
s.__class__.__name__)
TypeError: can't write bytes to text stream

I know I can open a file in 'wb' mode and write to it, but what
I want to do is somehow switch the mode of stdout to 'wb' mode.
I read lots of python 3 documentation, as well as searched without
finding a way.

You can write to the underlying raw stream:

sys.stdout.buffer.write(b'abc')

Is that an official way? If yes, it needs to be documented.

Documented in r67365.

I don't consider sys.stdout.buffer.write(b'abc')
to be an acceptable solution. There are many programs that need
to produce binary output with standard output. Consider uudecode
and similar programs.

There needs to be a standard, portable, documented way to put stdout
into binary mode in order to write binary output. For example,
all the flavors of the print command need to be able to send binary
data to standard output.

I don't have a problem changing open statements to support binary
file i/o, but I do have a problem changing every print or write statement
in order to support binary file i/o.