Exceptions

Published

2023-07-31

Exceptions

`KeyError`

If you try to read or pop or delete a key from a dictionary which does not exist, a KeyError is raised. This is similar to the IndexError you’ve seen in the cases of lists and tuples.

If you encounter a KeyError it means the specified key does not exist in the dictionary.

>>> furniture = {'living room': ['armchair', 'sofa', 'table'],
...              'bedroom': ['bed', 'nightstand', 'dresser'],
...              'office': ['desk', 'chair', 'cabinet']}
>>> furniture['kitchen']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'kitchen'

`TypeError`

If you try to add to a dictionary a key which is not hashable, Python will raise a type error:

>>> d = {[1, 2, 3]: 'cheese'}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Unlike IndexError which is almost always due to a programming error—and thus we do not wish to handle such exceptions—there are cases where we would wish to handle KeyError should it be raised. This will depend on context.

Original author: Clayton Cafiero < [given name] DOT [surname] AT uvm DOT edu >

No generative AI was used in producing this material. This was written the old-fashioned way.

This material is for free use under either the GNU Free Documentation License or the Creative Commons Attribution-ShareAlike 3.0 United States License (take your pick).