What You Need to Know:

  • Basic Python syntax

What You Will Learn:

  • Understand the syntax of dictionaries in Python
  • Compare the differences of dictionaries and lists

Why This is Important:

Lists let us group data in order. Dictionaries hold a set of paired keys and values - another way to keep data. Programmers use dictionaries all the time, because lots of problems are easier to solve when pieces of data are associated with each other.

Dictionaries

When you open up a real-world dictionary, what do you see? Words and their definitions. Python dictionaries are like that too - they contain pairs of keys (words) and values (definitions). They make it easy to look up a value (definition) by its key (word).

Dictionaries are like lists - they store data - but instead of having numbers as indexes, the indexes are the keys you define - usually strings. Check it out:

In this example, we associate each of the items on our grocery list with its price. We don’t really care about the order of the grocery list - it is way more helpful to know how much each item will cost!

Think of keys like the words in a dictionary and the values as the definitions. A dictionary holds pairs of keys and values. A key in a dictionary must be unique and associated with a value.

How do we use them? The python syntax for declaring a dictionary is with {curly brackets}. To create an empty dictionary, just set a variable equal to curly braces:

empty_dict = {}

To create a dictionary with values in it, set key-value pairs separated by colons and commas:

my_cart = {'Eggs': 2.59,
'Milk': 3.19,
'Cheese': 4.80,
'Yogurt': 1.35,
'Butter': 2.59,
'More Cheese':6.19 }

Accessing values in dictionary

We get the values out of the dictionary much like we got values out of lists. In order to access the values of a dictionary, you want to pass the key to the dictionary and it will return the value.

cartoon_species = {'bugs': 'rabbit',
           	       'elmer': 'human',
                   'wiley e': 'coyote',
       	           'tom': 'cat',
      	           'jerry': 'mouse'}

print cartoon_species['elmer']

Updating a dictionary

To modify the values inside a dictionary, the same thing applies - you want to use the assignment operator and send it the key value.

cartoon_species = {'bugs': 'rabbit',
           	       'elmer': 'human',
                   'wiley e': 'coyote',
       	           'tom': 'cat',
      	           'jerry': 'mouse'}

cartoon_species['bugs'] = 'bunny' # reassigns rabbit to bunny

You can also use this technique to add key-value pairs to a dictionary that don’t already exist. For instance, with my_dict, I could do the following:

cartoon_species['tweety'] = ‘bird’
print cartoon_species

{'tom': 'cat', 'wiley e': 'coyote', 'elmer': 'human', 'bugs': 'bunny', 'jerry': 'mouse', 'tweety': 'bird'}

The {“tweety”: “bird”} key value pair has now been added to my_dict.

Common Dictionary Operations

len(my_dict)
5

my_dict.clear()
{}

del my_dict['bugs']
{‘elmer’: ‘human’,
‘wiley e’: ‘coyote’,
‘tom’: ‘cat’,
‘jerry’: ‘mouse’}

Key-Value Pairs

Key-value pairs are separated by commas

cartoon_species = {'bugs': 'rabbit',
           	       'elmer': 'human',
                   'wiley e': 'coyote',
       	           'tom': 'cat',
      	           'jerry': 'mouse'}

Keys are on the left of the colons (‘bugs’, ‘elmer’, ‘wiley’, ‘tomas’, ‘jerry’) Values are on the right of the colons (‘rabbit’,‘human’,‘coyote’,‘cat’, ‘mouse’)

Keys must be unique, otherwise the program will throw an error. The same word doesn’t appear in a dictionary more than once!

my_dict = {'bugs': 'rabbit',
      	'bugs': 'non-rabbit' # Doesn't make sense
      }

It doesn’t make sense to have two different values for the same key - one will get overwritten.

Keys are usually strings, because they are easier to read, but they can be other data types too. Values can be any data type - strings, numbers, lists, or even other dictionaries!

With strings as values:

cartoon_species = {'bugs': 'rabbit',
           	       'elmer': 'human',
                   'wiley e': 'coyote',
       	           'tom': 'cat',
      	           'jerry': 'mouse'}

A dictionary with list values:

cartoon_friends = {'bugs': ['daffy','porky', 'foghorn leghorn'],
                   'mickey': ['minnie', 'goofy', 'donald'],
                   'woody': ['buzz', 'rex', 'slinky'],}

dictionary with other dictionaries as values:

cartoon_relationships = {'friends' : cartoon_friends,
                         'species' : cartoon_species }

Searching for a Key in a Dictionary

What if you wanted to check if the key ‘jerry’ was in my_dict? Use the in operator.

if 'jerry' in cartoon_species:
 print 'Hi Jerry!'

This only works for keys in a dictionary, not the values.

Looping through a dictionary

It is often useful to be able to loop over the contents of a dictionary. Using the for-in loop we would write:

for cartoon in cartoon_species:
 print cartoon

What do you notice? What is it printing? It’s printing the keys!

bugs
elmer
wiley e
tom
jerry

How would you print the values? Combine the technique of accessing the dictionary through the key:

for cartoon in cartoon_species:
 print cartoon_species[cartoon]

This prints:

rabbit
human
coyote
cat
mouse

Conclusion:

Dictionaries are phenomenally useful tools for representing all kinds of data. They are similar to lists but rather than having and index (0,1,2,3) you have keys for each value!