It's already easy to write race conditions in Python.

  if x in d:
      del d[x]
  else:
      d[x] = True

Is a classic example -- if two threads execute that, you can't predict the outcome (but a KeyError is quite likely)

The GIL only protects the CPython virtual machine; it doesn't protect user code. Concurrent code with shared mutable state already needs explicit mutexes.