Deleting dictionary entries

Deleting dictionary keys

Earlier we saw that we could use list’s .pop() method to remove an element from a list, removing either the last element in the list (the default, when no argument is supplied), or at a specific index (if we supply an argument).

Dictionaries are mutable, and thus, like lists, they can be changed. Dictionaries also support .pop() but it works a little differently than it does with lists. The .pop() method for dictionaries requires a valid key as an argument. This is because dictionaries don’t have the same sense of linear order as a list—everything is based on keys.

So this works:

>>> d = {'foo': 'bar'}
>>> d.pop('foo')
'bar'

but this does not:

>>> d = {'foo': 'bar'}
>>> d.pop()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: pop expected at least 1 argument, got 0

Python also provides the keyword del which can be used to remove a key from a dictionary.

>>> pets = {'fluffy': 'gerbil', 'george': 'turtle', 
...         'oswald': 'goldfish', 'wyatt': 'ferret'}
>>> del pets['oswald']   # RIP oswald :(
>>> pets
{'fluffy': 'gerbil', 'george': 'turtle', 'wyatt': 'ferret'}

But be careful! If you do not specify a key, the entire dictionary will be deleted!

>>> pets = {'fluffy': 'gerbil', 'george' : 'turtle', 
...         'oswald': 'goldfish', 'wyatt': 'ferret'}
>>> del pets  # oops!
>>> pets
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'pets' is not defined

Notice also that .pop() with a key supplied will return the value associated with that key and then remove the key/value pair. del will simply delete the entry.

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).