>>> getpass.getpass('Password: ', open('/dev/stdout', 'w'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/getpass.py", line 72, in unix_getpass
    passwd = _raw_input(prompt, stream, input=input)
  File "/home/serhiy/py/cpython/Lib/getpass.py", line 143, in _raw_input
    stream.write(bytes(prompt, tty_encoding))
TypeError: must be str, not bytes

It seems that you are moving in the wrong direction. No need to test explicitly for stdout/stderr/etc, the code should work with arbitrary text stream.