>>> mydict = {'key-a': 'value-a', 'key-b': 'value-b'}
>>> mydict['key-a']
'value-a'

All of a sudden, storm clouds appear. Your dictionary variable accidentally gets assigned a list, and life is no longer good!

>>> mydict = {'key-a': 'value-a', 'key-b': 'value-b'}
>>> mydict['key-a']
'value-a'
>>> mydict = []
>>> mydict['key-a']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers

The error message "TypeError: list indices must be integers" is generated when you attempt to use a non-int for a index inside the [] operator.

By the way, almost the same message is used if you do this with a tuple:

TypeError: tuple indices must be integers