Combining lists and dictionaries

Dealing with lists of dictionaries and dictionaries with lists inside can sometimes be confusing, even if we understand how each work separately. Let’s break them all down.

A little review

Lists

First, we have lists. We can do two major things with lists:

• We can grab specific elements, and
• we can loop through each element of the list

# A list of a few numbers
numbers = [5, 32, -3]

# Get the first number and print it
first_number = numbers[0]
print(first_number)

5

# Use a for loop to print every number
for number in numbers:
  print(number)

5
32
-3

Dictionaries

Then we have dictionaries. We can do a few things with lists

• We can grab a specific value using a key
• We can grab all of the keys
• We can grab all of the values (you probably don’t do this often)

# A dictionary about the film Maid in Manhattan
movie = {
  'name': 'Maid in Manhattan',
  'starring': 'Jennifer Lopez',
  'year': 2002
}

# Print the name of the movie using the 'name' key
print(movie['name'])

Maid in Manhattan

# View all the keys of the dictionary (name, starring, year) 
print(movie.keys())

dict_keys(['year', 'starring', 'name'])

Lists of dictionaries

Now that we’ve looked at the two separately, let’s combine the two.

# A list of three Heath Ledger movies
movies = [
  {
    'name': 'Ten Things I Hate About You',
    'starring': 'Heath Ledger',
    'year': 1999,
    'rating': 61
  },
  {
    'name': 'Brokeback Mountain',
    'starring': 'Heath Ledger',
    'year': 2005,
    'rating': 87
  },
  {
    'name': 'The Dark Knight',
    'starring': 'Heath Ledger',
    'year': 2008,
    'rating': 94
  }
]

Don’t think of movies as a list of dictionaries - just think of it as a list, ignore what’s inside. What can we do with a list?

• Get specific elements (the first, the last, etc)
• Loop through each element of the list

So, well, let’s do those!

# Get the first movie and print it
movie = movies[0]
print(movie)

{'rating': 61, 'year': 1999, 'starring': 'Heath Ledger', 'name': 'Ten Things I Hate About You'}

Okay, so it printed out the entire first dictionary. The same thing will happen if we loop through each element.

# Loop through each movie
for movie in movies:
  print(movie)

{'rating': 61, 'year': 1999, 'starring': 'Heath Ledger', 'name': 'Ten Things I Hate About You'}
{'rating': 87, 'year': 2005, 'starring': 'Heath Ledger', 'name': 'Brokeback Mountain'}
{'rating': 94, 'year': 2008, 'starring': 'Heath Ledger', 'name': 'The Dark Knight'}

This usually isn’t helpful, since we normally only want to deal with one or two aspects of our data (maybe the rating or the name). But think about it - each time we pull out a specific element or each time we’re inside the loop, movie is a dictionary. What are our favorite things to do with a dictionary?

• We can grab a specific value using a key
• We can grab all of the keys

So let’s combine the two steps, first dealing with the list on the outside, then the dictionary on the inside.

1. We have a list, so we can pull out a specific element
2. That specific element is a dictionary, so we can get its keys
3. Which means we can also use those keys to get data!

# MOVIES is a list, so we can get the first movie using [0]
movie = movies[0]
# MOVIE is a dictionary, so we can find the keys using .keys()
print("The movie has the keys", movie.keys())
# MOVIE is a dictionary, so we can get data using a key
print("The movie's name is", movie['name'])

The movie has the keys dict_keys(['rating', 'year', 'starring', 'name'])
The movie's name is Ten Things I Hate About You

But maybe you want to deal with all of the elements, like maybe we want to see the ratings? Let’s combine again.

1. We have a list, so we can loop through all of the elements
2. Each element is a dictionary, so we can get data using a key

# movies is a list, so we can loop through all of the elements
for movie in movies:
    # What's inside the list? a DICTIONARY. So we can use keys to get data!
    print(movie['name'], "had a rating of", movie['rating'])

Ten Things I Hate About You had a rating of 61
Brokeback Mountain had a rating of 87
The Dark Knight had a rating of 94

Dictionaries with lists inside

Another version of this is when you have a dictionary with a list inside.

# Lauren is indecisive and has many favorites
lauren = {
    'age': 12,
    'favorite_numbers': [4, 5, 3, 1, 4, 5],
    'favorite_colors': ['blue', 'green', 'purple']
}

Since lauren is just a dictionary, you can do dictionary things to it.

• We can grab a specific value using a key
• We can grab all of the keys

# This value is boring
print(lauren['age'])

12

# This value is more exciting!
print(lauren['favorite_colors'])

['blue', 'green', 'purple']

# You can even what data type things are using 'type'
print(type(lauren['age']))
print(type(lauren['favorite_colors']))

<class 'int'>
<class 'list'>

Now that we’ve found a list, we can do list things.

• Get specific elements (the first, the last, etc)
• Loop through each element of the list

So we ‘ll have two steps

1. We have a dictionary, so specific data out using a key
2. We then a list, so use [...] indices or for loops to go through each element

# STEP ONE: Use a key to get something out of the dictionary
colors = lauren['favorite_colors']
# STEP TWO: It's a list, so let's get the first one
print(colors[0])

# STEP ONE + STEP TWO all at once
print(lauren['favorite_colors'][0])

blue
blue

# STEP ONE: Use a key to get something out of the dictionary
colors = lauren['favorite_colors']

print(type(colors))

# STEP TWO: It's a list, so let's loop through them
for color in colors:
    print(color)

<class 'list'>
blue
green
purple