Dictionaries

Python dictionaries are a collection of a key-value stores. In Python 3.6+, the keys of the dictionaries are preserved in insertion order. The keys of the dictionary must be unique but the values do not have to be. We use a dictionary when we want to associate a specific key with a value and allow for fast lookup later one (a real life analogy would be a phonebook).

Basics

You can instantiate an empty dictionary in two ways:

my_dict = {}
my_dct = dict()

We can add to a dictionary in two ways:

my_dict['bob'] = 10
my_dict.update({'alice' : 11})

# `my_dict` is now:
# { 'bob': 10, 'alice': 11 }

We can retrieve information from a dictionary in two ways:

print(my_dict['bob']) # prints out 10.
print(my_dict.get('alice')) # prints out 11.

The dictionary keeps track of the last value added to the key:

my_dict['bob'] = 10
my_dict['bob'] = 12

print(my_dict.get('bob')) # prints out 12.

Now let's take a look at an example problem to understand why dictionaries are a helfpul data structure.

Example Usage

We are given an array of earnings in an array like so:

earnings = [('Bob', 1), ('Alice', 2), ('Bob', 5), ('Bob', 10), ('Alice', 11) ...]

And now we want to find the total earnings per individual.

A solution without dictionaries might look like:

alice = 0
bob = 0
for person, earning in earnings:
    if person == 'Alice':
        alice += earning
    elif person == 'Bob':
        bob += earning

print(alice)
print(bob)

This might be fine if our array only had values from Bob and Alice. However, what if we added new people? We would be forced to add a new conditional for every single person (which obviously isn't a scalable solution).

Let's look at the solution now using dictionaries.

earning_tracker = {}

for person, earning in earnings:
    earning_tracker[person] = earning_tracker.get(person, 0) + earning

print(earning_tracker)

With the dictionary, we don't need to worry about each individual person; the dictionary does the tracking and updating for us.

Note:

When using get, the second parameter is a default value. The default value is returned if the key (first parameter) does not exist in the dictionary.

Iteration

The default iterator for a dictionary returns the keys.

my_dict = {'Alice': 10, 'Bob': 5}

for key in my_dict:
    print(key) # Prints the key (person name)
    print(my_dict[key]) # Prints the value (earning)

If we want both the key and the value, we can use the items function. items yields the key and value as a tuple.

my_dict = {'Alice': 10, 'Bob': 5}

for key, value in my_dict.items():
    print(key) # Prints the key (person name)
    print(value) # Prints the value (earning)

There are other ways to iterate through a dictionary (for example using the keys and values function), but the above should be sufficient for this course.

Methods

Be familiar with the following set functions:

• update: Updates the dictionary based on the dictionary passed in.
• get: Gets a specific value by the passed in key (or a default value if the key does not exist), O(1) time complexity.
• pop: Removes the key-value pair based on the key passed in, O(1) time complexity

To check if a key is present in a dictionary, we can use in:

my_dict = {'Alice': 10, 'Bob': 5}

print('Alice' in my_dict) # prints True
print('John' in my_dict) # prints False

For more information, see: documentation.