That's because the compiler doesn't know the locals() dictionary will be modifie...

That's because the compiler doesn't know the locals() dictionary will be modified at runtime, so it treats b as a global variable (which happens to be undefined).

To show that not even changes to existing local variables work, try

  def main():
      a = 1
      d = locals()
      print d
      b = None
      d['b'] = 123
      print b
      print d
      print locals()
      print d is locals()

which prints

  {'a': 1}
  None
  {'a': 1, 'b': 123}
  {'a': 1, 'b': None, 'd': {...}}
  True

I.e. changes to the dictionary returned by locals() are ignored, and calling locals() again overrides the previous values.