A KeyError is raised when a look-up in a mapping fails.

You can see that here when we look up a non-existent key in a dict:

>>> codes = dict(gb=44, us=1, no=47, fr=33, es=34)
>>> codes['de']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    KeyError: 'de'