JavaScript length of a string (Python)

jslen.py

# jslen -- find the JavaScript length of a string (Python 3)
# 
# Note to Python developers: since you know it as of Python 3.3, it'd
# be nice to have a way to find out in O(1) if a string contains any
# wide characters.

# find the preferred (i.e. faster) byte order
if ''.encode('utf-16') == b'\xFE\xFF':
    preferred_encoding = 'utf-16le'
else:
    preferred_encoding = 'utf-16be'

def jslen(str):
    return len(str.encode(preferred_encoding)) >> 1