bb.py

# GistID: 4636013


def grpkey():
    idx = 0
    def _key(e):
        print idx
        if e:
            idx += 1
        return idx
    return _key


kf = grpkey()
print kf(True)

# OUTPUT:
# Traceback (most recent call last):
#   File "bb.py", line 15, in <module>
#     print kf(True)
#   File "bb.py", line 7, in _key
#     print idx
# UnboundLocalError: local variable 'idx' referenced before assignment