Python variable range problem

I've been programming for years, and recently started learning Python. The code below works normally on Python 2.5 and 3.0 as I expected :

a, b, c = (1, 2, 3)

print(a, b, c)

def test():
    print(a)
    print(b)
    print(c)    # (A)
    #c+=1       # (B)
test()

If you unannotate the currently annotated line (B), the error UnboundLocalError: 'c' not assigned occurs on line (A). The variables a and b are output normally. This led to two questions:

The only reason I can come up with is that the region variable c is generated by c+=1, and even before the region variable is created, the region variable takes precedence over the global variable c. Of course, it doesn't seem to make sense to take away the range of variables before the local variable even exists.

Please give me an explanation.