Dictionaries

Dictionaries#

Learning Objectives#

At the end of this lesson you will be able to:

  • Identify and explain what a dictionary is.

  • Explain what makes a dictionary different to a list.

  • Understand the key: value relationship.

  • Create a dictionary containing simple values.

  • Update values in a dictionary.

Key points#

  • “A dictionary stores key-value pairs.”

  • “Dictionaries are unordered.”

  • “Dictionaries are immutable.”

Introduction#

Lists and arrays are useful, but don’t cover every use case. One of Python’s best features is its “dictionary” data structure. Whereas a list or array is simply a collection of elements, a dictionary supports key-value storage of data. Put into plain english, it lets you store and retrieve data values by name.

Create a dictionary#

Let’s create and use a simple dictionary of scientist’s birth years to illustrate this:

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833}
print(birth_years)

{'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833}

Here, ‘Newton’, ‘Darwin’, ‘Einstein’ and ‘Nobel’ are keys, and the years are values. Keys and values form a pairwise mapping, allowing the retrieval of a value via the value’s key. We can retrieve a value from a dictionary by entering the correct key for this value. We do this using the following syntax:

value_we_want = dictionary[‘key_of_value_we_want’]

This is similar to accessing a value stored in a list. However, the key does not have to be an integer.

Retrieve a value#

Retrieve the birth year for Isaac Newton from the dictionary of birth years.

Solution

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833}
birth_years['Newton']

Altering a dictionary#

Similar to lists, dictionaries are mutable. We can add a value to a dictionary, using a key:

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833}
birth_years['Turing'] = 1612
print(birth_years)

{'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833, 'Turing': 1612}

Oops, we made a mistake there. Turing was actually born in 1912. We can update a value using a key. Turing’s birthdate above is actually incorrect. Values may be overwritten by re-assignment:

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833, 'Turing': 1612}
birth_years['Turing'] = 1912
print(birth_years)

{'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833, 'Turing': 1912}

Add your own scientists to a dictionary#

Add two more scientist’s birth years to our dictionary of birth years.

Solution

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833, 'Turing': 1612}
birth_years['Curie'] = 1867
birth_years['Franklin'] = 1920
print(birth_years)

Retrieve all keys and values#

Dictionaries contain a number of in-built methods that allow you to easily access the data contained within. These methods are automatically attached to every dictionary.

birth_years = {'Newton': 1642, 'Darwin': 1809, 'Einstein': 1979, 'Nobel': 1833, 'Turing': 1612, 'Curie': 1867, 'Franklin': 1920}
birth_years.keys()

dict_keys(['Newton', 'Darwin', 'Einstein', 'Nobel', 'Turing', 'Curie', 'Franklin'])

Similarly for values:

birth_years.values()

dict_values([1642, 1809, 1979, 1833, 1612, 1867, 1920])

We can convert these to lists:

list(birth_years.values())

[1642, 1809, 1979, 1833, 1612, 1867, 1920]

And finally, retrieve the key, value pairs together, forming a list of tuples:

list(birth_years.items())

[('Newton', 1642),
 ('Darwin', 1809),
 ('Einstein', 1979),
 ('Nobel', 1833),
 ('Turing', 1612),
 ('Curie', 1867),
 ('Franklin', 1920)]

Summary Quiz#