Dictionary Views in Python

First published: 2020-10-07

Last Edited: 2020-10-07

Number of edits: 9

In Python, dictionary views actually change their data if the underlying dictionary changes. Let's remove some keys:

>>> data = {'a': 1, 'b': 2, 'c': 3}
>>> keys = data.keys()
>>> keys
dict_keys(['a', 'b', 'c'])
>>> del data['a']
>>> keys
dict_keys(['b', 'c'])
>>>

It also works for the items:

>>> items = data.items()
>>> items
dict_items([('b', 2), ('c', 4)])
>>> items_list = list(items)
>>> items_list
[('b', 2), ('c', 4)]
>>> data['c'] = 0
>>> items
dict_items([('b', 2), ('c', 0)])
>>> items_list
[('b', 2), ('c', 4)]

Note that once the dict_items object is cast into a list, it loses the reference to the data item.

And, just out of curiosity and, as expected, it is not serializable:

>>> with open('test.dat', 'w') as f:
...  pickle.dump(items, f)
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: cannot pickle 'dict_items' object

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Aquiles Carattino

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