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Yes, the dictionary will always contain "a" afterwards.
However, a cleaner way would be to use locals().copy(), or, if you don't
need current locals in the exec'd string, just a fresh dict.
In scopes not using the optimized locals, there may be some other
effects when using the dict returned by locals() directly:
* in module scopes, the dictionary returned by locals() is the same as
the globals() one
* in class scopes, the locals() dict is actually the local scope of the
class