subprocess.Popen.communicate is documented as taking a string as the 
input argument. Instead is accepts only a binary stream (bytes).

Python 3.0a4 (r30a4:62126, Apr  3 2008, 15:34:18) [MSC v.1500 32 bit 
(Intel)] on
 win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from subprocess import Popen, PIPE
>>> p = Popen('command.com', stdin=PIPE)
>>> p.communicate("dir\n")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python30\lib\subprocess.py", line 588, in communicate
    self.stdin.write(input)
  File "C:\Python30\lib\io.py", line 844, in write
    raise TypeError("can't write str to binary stream")
TypeError: can't write str to binary stream

What is wrong? Documentation or method?

I say docs.

On subprocess.py the new method communicate() doesn't encode the string:

"""
_communicate(self, input):
...
  if isinstance(input, str):
    input = input.encode()
...
"""

I've attached a patch that adds the str.encode() call on communicate().

Hmm. I think it's better to make people explicity encode their string.

Fixed docs in r64619.

_communicate still encodes the string under the hood

You're right, I fixed that too in r64621.

The doc string refers to "string" instead of "byte string" as well.