Resolved: Why local variables referenced before assignment?

Question:

I made the two functions below in Python 3. The first function test_list works fine with the list a without error. I can modify the list element in that function.
However, the second funciton test_int will pop an error local variable 'b' referenced before assignment. Why can’t I do this to the variable b?

Answer:

b += 1 is equivalent to b = b.__iadd__(1); since the name b gets bound by that statement (l.h.s.), the compiler infers that b is a local variable; however, the r.h.s. expression contains the name b as well and thus you get the error “local variable 'b' referenced before assignment” (the local name b doesn’t refer to anything at the time when the expression is evaluated).
a[0] = 2 on the other hand is equivalent to a.__setitem__(0, 2) and thus involves no name binding.
If you wish to modify a global name binding within a function, you can use the keyword global; there’s also the equivalent nonlocal for names contained in an outer, but not the global, scope.

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