Simple example:

>>> from telnetlib import Telnet
>>> t = Telnet("google.com", 80)
>>> t.write("GET / HTTP/1.1\r\n")        
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/py3k/Lib/telnetlib.py", line 280, in write
    self.sock.sendall(buffer)
TypeError: sendall() argument 1 must be string or buffer, not str
>>> t.write(b"GET / HTTP/1.1\r\n")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/py3k/Lib/telnetlib.py", line 277, in write
    if IAC in buffer:
TypeError: Type str doesn't support the buffer API