The following code prints out "I L" when run with python 2.5 and current SVN trunk (if you silence the DeprecationWarnings):

"""
import struct

for c in "bBhHiIlLqQdf":
    try:
        struct.pack(c, "foo bar")
    except struct.error:
        pass
    else:
        print c
"""

It works correctly (prints nothing) in Python 2.4.