Changing Dictionaries In-Place

Let’s continue with our interactive session. Dictionaries, like lists, are mutable, so you can change, expand, and shrink them in-place without making new dictionaries: simply assign a value to a key to change or create an entry. The del statement works here, too; it deletes the entry associated with the key specified as an index. Notice also the nesting of a list inside a dictionary in this example (the value of the key 'ham'). All collection data types in Python can nest inside each other arbitrarily:

>>> D
{'eggs': 3, 'ham': 1, 'spam': 2}

>>> D['ham'] = ['grill', 'bake', 'fry']           # Change entry
>>> D
{'eggs': 3, 'ham': ['grill', 'bake', 'fry'], 'spam': 2}

>>> del D['eggs']                                 # Delete entry
>>> D
{'ham': ['grill', 'bake', 'fry'], 'spam': 2}

>>> D['brunch'] = 'Bacon'                         # Add new entry
>>> D
{'brunch': 'Bacon', 'ham': ['grill', 'bake', 'fry'], 'spam': 2}

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As with lists, assigning to an existing index in a dictionary changes its associated value. Unlike with lists, however, whenever you assign a new dictionary key (one that hasn’t been assigned before) you create a new entry in the dictionary, as was done in the previous example for the key 'brunch'. This doesn’t work for lists because Python considers an offset beyond the end of a list out of bounds and throws an error. To expand a list, you need to use tools such as the append method or slice assignment instead.