local variable 'a' referenced b

[contro opinion]

code1
>>> def foo():
...     a = 1
...     def bar():
...         b=2
...         print a + b
...     bar()
...
...
>>> foo()
3

code2
>>> def foo():
...     a = 1
...     def bar():
...         b=2
...         a = a + b
...         print a
...     bar()
...
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in foo
  File "<stdin>", line 5, in bar
UnboundLocalError: local variable 'a' referenced b

why code2 can not get output of 3?
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