Working with Dictionaries
=========================
Dictionaries are *mutable*, so we can change the value assigned to a key, add
new key/value pairs, or remove key/value pairs.
Since dictionaries are *unordered*, we have no options for sorting or
rearranging the key/value pairs within the collections.
Change One Value
----------------
To update a single value in a dictionary, use the syntax:
.. sourcecode:: python
dictionary_name[key] = new_value
``dictionary_name[key]`` identifies the value in the collection that we want to
change. The ``=`` operator assigns ``new_value`` to that key.
.. admonition:: Example
.. sourcecode:: python
:linenos:
phone_book = {
'Mom' : '555-5555',
'Work' : '555-5556',
'Home' : '123-456-7890'
}
print(phone_book)
phone_book['Work'] = '314-555-5556' # Link the key 'Work' to the value '314-555-5556'.
print(phone_book)
**Console Output**
::
{'Mom' : '555-5555', 'Work' : '555-5556', 'Home' : '123-456-7890'}
{'Mom' : '555-5555', 'Work' : '314-555-5556', 'Home' : '123-456-7890'}
.. admonition:: Note
We cannot use this method to change the names of the keys.
Add a New Key/Value Pair
------------------------
After defining a dictionary, we can add new new key/value pairs at any time by
using bracket syntax:
.. sourcecode:: python
dictionary_name['new_key'] = new_value
.. admonition:: Example
.. sourcecode:: python
:linenos:
phone_book = {
'Mom' : '555-5555',
'Work' : '555-5556',
'Home' : '123-456-7890'
}
print(phone_book)
phone_book['BFF'] = '555-5557'
print(phone_book)
**Console Output**
::
{'Mom' : '555-5555', 'Work' : '555-5556', 'Home' : '123-456-7890'}
{'Mom' : '555-5555', 'Work' : '555-5556', 'Home' : '123-456-7890', 'BFF' : '555-5557'}
Remove a Key/Value Pair
-----------------------
To remove a key/value pair from a dictionary, use the ``del`` keyword. The
general syntax is:
.. sourcecode:: python
del dictionary_name[key]
.. admonition:: Example
.. sourcecode:: python
:linenos:
phone_book = {
'Mom' : '555-5555',
'Work' : '555-5556',
'Home' : '123-456-7890',
'BFF' : '555-5557'
}
print(phone_book)
del phone_book['Home']
print(phone_book)
**Console Output**
::
{'Mom' : '555-5555', 'Work' : '555-5556', 'Home' : '123-456-7890', 'BFF' : '555-5557'}
{'Mom' : '555-5555', 'Work' : '555-5556', 'BFF' : '555-5557'}
.. admonition:: Note
Once we define a key, it remains in the dictionary unless we use ``del`` to
remove it.
For example, if we wanted to rename the key ``'Mom'`` to ``'Mother'``, we
would have to delete the old key first, then add a new key/value pair.
.. sourcecode:: python
:lineno-start: 9
del phone_book['Mom']
phone_book['Mother'] = '555-5555'
Try It!
-------
.. admonition:: Example
Use a dictionary to keep track of favorite ice cream flavors!
In the editor below:
#. Change the value for ``'Mom'`` to something more exciting than
``'vanilla'``.
#. Add your own name and favorite flavor to the dictionary.
#. Use ``del`` to remove the ``'Odd choice'`` key/value pair.
#. Fix the misspelling in the ``'teachher'`` key.
.. raw:: html
Check Your Understanding
------------------------
.. admonition:: Question
Given the following dictionary:
.. sourcecode:: python
pet_population = {'cats' : 10, 'dogs' : 5, 'elephants' : 25}
What value does ``len(pet_population)`` return? (*Hint*: Try it in the
code editor above).
.. raw:: html
3
6
40
.. Answer = a
.. admonition:: Question
Using the same ``pet_population`` dictionary, what would the following
statement do?
.. sourcecode:: python
pet_population['birds'] = 5
.. raw:: html
Throw an error message because pet_population does not contain a 'birds' key.
Add the 'birds' : 5 key/value pair to the dictionary.
Add five 'birds' keys to the dictionary.
Replace the 'dogs' key with 'birds'.
.. Answer = b
.. admonition:: Question
What is printed by the following statements?
.. sourcecode:: python
:linenos:
pet_population = {'cats' : 10, 'dogs' : 5, 'elephants' : 25}
pet_population['mice'] = pet_population['cats'] + pet_population['dogs']
print(pet_population['mice'])
.. raw:: html
0
5
10
15
.. Answer = d
.. admonition:: Question
Once we added mice to our pet collection, all the elephants ran away! Which
of the following shows the correct syntax for setting the value of the
``'elephants'`` key to zero?
.. raw:: html