format() mis understaning.
please pay attention to [0], [x].

>>> d = {0: "a", "0": "A"}
>>> 'd[0] = {0[0]}'.format(d)
'd[0] = a'
>>> 'd[0] = {0["0"]}'.format(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: '"0"'
>>> d2 = {"x": 100}
>>> 'd2[x] = {0[x]}'.format(d2)
'd2[x] = 100'

This is working as defined in PEP 3101: it calls PySequence_GetItem() or PyObject_GetItem() on the value inside the brackets. There is indeed no item in d that is the string "0" (including the quotes).