The openhook for fileinput currently will not be called when the input
is from sys.stdin.  However, if the input contains invalid UTF-8
sequences, a program with a hook that specifies errors='replace' will
not behave as expected:

  $ cat x.py
  import fileinput
  import sys
  
  def hook(filename, mode):
      print('hook called')
      return open(filename, mode, errors='replace')
  
  for line in fileinput.input(openhook=hook):
      sys.stdout.write(line)


  $ echo -e "foo\x80bar" >in.txt

  $ python3 x.py in.txt
  hook called
  foo�bar

Good.  Hook is called, and replacement character is observed.

  $ python3 x.py <in.txt
  Traceback (most recent call last):
    File "x.py", line 8, in <module>
      for line in fileinput.input(openhook=hook):
    File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/fileinput.py", line 263, in __next__
      line = self.readline()
    File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/fileinput.py", line 363, in readline
      self._buffer = self._file.readlines(self._bufsize)
    File "/usr/local/Cellar/python3/3.4.3/Frameworks/Python.framework/Versions/3.4/lib/python3.4/codecs.py", line 319, in decode
      (result, consumed) = self._buffer_decode(data, self.errors, final)
  UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 3: invalid start byte

Hook was not called, and so we get the UnicodeDecodeError.

Should fileinput attempt to apply the hook code to stdin?