Python - locals() and closure

I can not find an adequate explanation for this behavior.

>>> def a():
...     foo = 0
...     print locals()
...     def b():
...         print locals()
...     b()

>>> a()
{'foo': 0}
{}

But:

>>> def a():
...     foo = 0
...     print locals()
...     def b():
            foo
...         print locals()
...     b()

>>> a()
{'foo': 0}
{'foo': 0}

I understand that in the second case there is a closure, but I can not find a detailed description of what actually is and under what conditions should return the function locals().