Exceptions

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'

If you try to remove an element from a set with .remove() and the element does not exist within the set, you’ll get a KeyError.

>>> silly_words = {'foo', 'bar', 'baz', 'quux'}
>>> silly_words.remove('garply')
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    silly_words.remove('garply')
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'garply'

If you want to remove an element from a set, and you aren’t sure it exists, and don’t want to use in to check first or a try / except, then you can use .discard().

>>> silly_words = {'foo', 'bar', 'baz', 'quux'}
>>> silly_words.remove('garply')
>>> silly_words
{'quux', 'foo', 'baz', 'bar'}

TypeError

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

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

If you try to add an element to a set which is not hashable, Python will raise a TypeError:

>>> s = set()
>>> s.add(["I", "am", "not", "hashable!"])
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    s.add(["I", "am", "not", "hashable!"])
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'list'

If you try to create a named tuple, supplying the wrong number of arguments to the constructor, again, you’ll get a TypeError.

>>> from collections import namedtuple
>>> Xyz = namedtuple('Xyz', ['x', 'y', 'z'])
>>> point = Xyz(5.2, 3.9)
Traceback (most recent call last):
  File ".../interactiveshell.py", line 3553, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  ...
TypeError: Xyz.__new__() 
     missing 1 required positional argument: 'z'

If you try to JSON encode a type which cannot be encoded, Python will raise a TypeError:

>>> import json
>>> json.dumps(enumerate([1, 2, 3]))
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
    json.dumps(enumerate([1, 2, 3]))
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  ...
TypeError: Object of type enumerate is not JSON serializable

AttributeError

Named tuples have fixed fields and field names. So if you try to access a field using a name that does not exist for the given type, you’ll get an AttributeError.

>>> from collections import namedtuple
>>> Car = namedtuple('Car', ['make', 'model', 'year', 'color'])
>>> lesters_car = Car('Pontiac', 'Firebird', 1970, 'red')
>>> lesters_car.hp
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    lesters_car.hp
AttributeError: 'Car' object has no attribute 'hp'

JSONDecodeError

A JSONDecodeError occurs when we try to use json.loads() or json.load() to decode a string which is not a valid JSON-encoded string.

>>> import json
>>> json.loads("Dolittle: I do not understand the human race!")
Traceback (most recent call last):
  File "<python-input-29>", line 1, in <module>
    json.loads("Dolittle: I do not understand the human race!")
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...
json.decoder.JSONDecodeError: 
     Expecting value: line 1 column 1 (char 0)

Copyright © 2023–2025 Clayton Cafiero

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