[Tutor] "local variable 'l1' referenced before assignment"

[Dick Moores]

I'm puzzled by the below error msg. If I change the line in a2()
from
l1 = [1,2,3]*100
to
l1 = [1,2,3]
There is no problem. 
Why? And why isn't that line a problem for a1()?
=========================================
def a1():
        return
l1.extend(l2)
if __name__=='__main__':
        l1 =
[1,2,3]*100
        l2 =
[4,5,6]
        from
timeit import Timer
        t =
Timer("a1()", "from __main__ import a1")
        t1 =
t.timeit(number=10)
        
def a2():
        l1 +=
l2
if __name__=='__main__':
        l1 =
[1,2,3]*100
        l2 =
[4,5,6]
        from
timeit import Timer
        t =
Timer("a2()", "from __main__ import a2")
        t2 =
t.timeit(number=10)
print "t1:", t1
print "t2:", t2
print "t2/t1:", t2/t1
Error msg:
E:\PythonWork>timing_2_stupidsV2.py
Traceback (most recent call last):
  File "E:\PythonWork\timing_2_stupidsV2.py", line 21, in
<module>
    t2 = t.timeit(number=10)
  File "E:\Python25\lib\timeit.py", line 161, in
timeit
    timing = self.inner(it, self.timer)
  File "<timeit-src>", line 6, in inner
  File "E:\PythonWork\timing_2_stupidsV2.py", line 15, in
a2
    l1 += l2
UnboundLocalError: local variable 'l1' referenced before assignment
===================================================
Thanks,
Dick Moores


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