You don't even need the locals() function to get into trouble:

    x = [1, 2, 3, 4]
    def foo():
        x[0] += 3  # Okay
    def bar():
        x += [3]   # UnboundLocalError
    def qux():
        x = [5, 6, 7, 8]  # Binds a new `x`.