Dictionaries¶

A standard data structure in computer science is the associative array, which is also called a map. In Python, this structure is called a dictionary. Dictionaries are used when you have key-value pairs of data – an input which is mapped to an output, with the requirement that the keys are unique (within the same dictionary of course).

Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type (strings, numbers, booleans, tuples). You can’t use lists as keys, since lists can be modified in place.

Create¶

Suppose we are creating a new social network called “FriendSpace” where any life form or intelligent agent is allowed to join. Let us make our first post to FriendSpace, we will use this post to show you how to create a dictionary.

To begin, we will collect several pieces of data for each post:

>>> # FriendSpace post
>>> # user_id = 209
>>> # message = "I am learning about dictionaries. Yay!"
>>> # language = "English"
>>> # created = "2021-03-02T10:11:06.278326"
>>> # location = (46.766667, 23.583333)
>>>

Using dictionaries we can store all of this data in a single object. A pair of braces create an empty dictionary:

>>> empty_dictionary = {}
>>>

Placing a comma-separated list of key-value pairs withing the braces will add initial key-value pairs to the dictionary. This is also the way dictionaries are printed on console.

>>> post = {"user_id": 209, "message": "I am learning about dictionaries. Yay!", "language": "English", "created": "2021-03-02T10:11:06.278326", "location": (46.766667, 23.583333)}
>>>

We have created a dictionary called post with 5 pieces of data. If you think of a dictionary as a map, there are 5 inputs and 5 outputs. In Python, these inputs are called keys, and the outputs are called values.

post dictionary¶
Keys	Values
user_id	209
message	“I am learning about dictionaries. Yay!”
language	“English”
created	“2021-03-02T10:11:06.278326”
location	(46.766667, 23.583333)

Notice how the values have a variety of data types in this dictionary: an integer, three strings and a tuple of floats. Python is flexible: you can use all kinds of data types for both keys and values. You are free to mix and match data to suit your needs.

>>> type(post)
<class 'dict'>
>>>

If you use the type function, you will see post is an instance of the dict class. This suggests we can use the dict constructor to create a dictionary. Let’s create another FriendSpace post using the dict constructor. This time, we will only provide a few pieces of data…

>>> post2 = dict(message="There is always another way.", language="English")
>>> print(post2)
{'message': 'There is always another way.', 'language': 'English'}
>>>

You can see this made a dictionary. We add additional pieces of data by using brackets. The key name goes inside the brackets, and you use the equals sign to assign the value.

>>> post2["user_id"] = 209
>>> post2["created"] = "2021-03-02T10:11:06.278326"
>>> print(post2)
{'message': 'There is always another way.', 'language': 'English', 'user_id': 209, 'created': '2021-03-02T10:11:06.278326'}
>>>

Notice that in the constructor, you did not have to put quotes around the keyname, but when you add new data using brackets, you do use quotes. If you print the dictionary, you will see all the data is there inside braces.

Access data¶

Just as you use brackets to store new data in a dictionary, you use brackets to access data as well. For example, to see the message from the first post, use the key name “message” inside brackets.

>>> print(post["message"])
I am learning about dictionaries. Yay!
>>>

But do not be careless. Let’s see what happens if you try to access data that is not in the dictionary.

>>> print(post2["location"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'location'
>>>

When we created post2, we did not assign a location. If we try to access this value, we get a KeyError. How do we avoid such catastrophe? Simple. One way is to use the in operator to first check if a key is in the dictionary.

>>> if "location" in post2:
...     print(post2["location"])
... else:
...     print("The post does not have a location")
...
The post does not have a location
>>>

Another way to access data in a dictionary and handle the possibility it does not have a certain key is the get method. This lets you try and get the value for a specific key. If the dictionary does not contain data for that key, you can specify a default value. For our case let’s return an empty tuple.

>>> loc = post2.get("location", tuple())
>>> print(loc)
()
>>>

If you print the value you see the get method did returned an empty tuple.

Now, turn your attention back to the original post.

>>> print(post)
{'user_id': 209, 'message': 'I am learning about dictionaries. Yay!', 'language': 'English', 'created': '2021-03-02T10:11:06.278326', 'location': (46.766667, 23.583333)}
>>>

A common task is to iterate over all the key value pairs in a dictionary. A straightforward way to do this is to loop over all the keys, then get the value:

>>> for key in post:
...     value = post[key]
...     print(key, "=", value)
...
user_id = 209
message = I am learning about dictionaries. Yay!
language = English
created = 2021-03-02T10:11:06.278326
location = (46.766667, 23.583333)
>>>

When you simply loop over a dictionary, this will give you all the keys for that dictionary. The order of the data may be different for you. Do no panic. Dictionaries are not ordered data. As long as you see all the data, everything is under control.

Another way to iterate over all the key value pairs in a dictionary is to use the items method. This will give you both the key and the value in each step of the iteration.

>>> for key, value in post.items():
...     print(key, "=", value)
...
user_id = 209
message = I am learning about dictionaries. Yay!
language = English
created = 2021-03-02T10:11:06.278326
location = (46.766667, 23.583333)
>>>

There are a variety of methods for working with a dictionary. The pop and popitem methods allow you to remove a single item from a dictionary, while the clear method will remove all data. To complete this lesson please take a moment to explore and experiment with these methods.

Exercises¶

Create a dictionary out of the lists below:

keys = ["name", "phone", "email"]
values = ["Mike", "+40791882123", "michael@yahoo.com"]

# Expected output
{'name': 'Mike', 'phone': '+40791882123', 'email': 'michael@yahoo.com'}

Given a dictionary describing a car attributes, print the model and the year of the car or the string N/A if the value is not present.
car = { "brand": "Ford", "model": "Mustang", "year": 1964 }
Print all the key value pairs of any dictionary, but make sure they are shown in the same order every time (order the keys alphabetically).

Count the frequency of all words in the given text:

text = "He ordered his regular breakfast. Two eggs sunnyside up, hash browns, and two strips of bacon. He continued to look at the menu wondering if this would be the day he added something new. This was also part of the routine. A few seconds of hesitation to see if something else would be added to the order before demuring and saying that would be all. It was the same exact meal that he had ordered every day for the past two years."

# expected output
{'the': 6, 'he': 4, 'two': 3, 'of': 3, 'to': 3, 'would': 3, 'be': 3, 'ordered': 2, 'and': 2, 'if': 2, 'this': 2, 'day': 2, 'added': 2, 'something': 2, 'was': 2, 'that': 2, 'his': 1, 'regular': 1, 'breakfast': 1, 'eggs': 1, 'sunnyside': 1, 'up': 1, 'hash': 1, 'browns': 1, 'strips': 1, 'bacon': 1, 'continued': 1, 'look': 1, 'at': 1, 'menu': 1, 'wondering': 1, 'new': 1, 'also': 1, 'part': 1, 'routine': 1, 'a': 1, 'few': 1, 'seconds': 1, 'hesitation': 1, 'see': 1, 'else': 1, 'order': 1, 'before': 1, 'demuring': 1, 'saying': 1, 'all': 1, 'it': 1, 'same': 1, 'exact': 1, 'meal': 1, 'had': 1, 'every': 1, 'for': 1, 'past': 1, 'years': 1}