C:\>python
Python 2.7.5 (default, May 15 2013, 22:44:16) [MSC v.1500 64 bit AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.pack(u'B',1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Struct() argument 1 must be string, not unicode
>>> struct.pack(u'B',1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Struct() argument 1 must be string, not unicode
>>> struct.pack('B',1) # this is ok
'\x01'
>>> struct.pack(u'B',1)
'\x01'