Python KeyError exception occur when the key is not found in a dictionary.

>>> d = {'1' : 'a', '4' : 'b', '5' : 'c'}
>>> x = d['1']
>>> x

a

>>> x = d['1']
>>> x

b

>>> x = d['2'] #key 2 is not in dictionary d

Traceback (most recent call last):
File "", line 1, in 
KeyError: '2'

>>> try:
... x = d['2']
... except KeyError:
... print ("Key 2 is not exist!")
...

Key 2 is not exist!