Python Programming: Dictionaries

Learning Objectives

After this lesson, you will be able to:

Perform common dictionary actions.
Build more complex dictionaries.

Kicking Off Unit 3

In Unit 2, we ended by changing what our movie app printed depending on the value of a variable.

Unit 3 is about objects in programming.

Objects are different kinds of things variables can hold.
Objects help give our programs more structure and functionality.
You already have one object down! Lists are an object with built-in functionality like append() and pop().

In Unit 3, we’re going to add many more objects. By the end, your movie app will have the same functionality, but it will be structured in a totally different way.

Ready? Let’s go!

Introducing Dictionaries

Think about dictionaries — they’re filled with words and definitions that are paired together.

Programming has a dictionary object just like this!

Dictionaries hold keys (words) and values (the definitions).
In a real dictionary, you can look up a word and find the definition. In a Python dictionary, you can look up a key and find the value.

Introducing Dictionaries

Declaring a Dictionary

Dictionaries in programming are made of key-value pairs.

# Here's the syntax:
name_of_dictionary = {"Key1": "Value", "Key2": "Value", "Key3": "Value"}
print(name_of_dictionary[key_to_look_up])
# Prints the value

# And in action...
my_dictionary = {"Puppy": "Furry, energetic animal", "Pineapple": "Acidic tropical fruit", "Tea": "Herb-infused drink"}

print(my_dictionary)
# Prints the whole dictionary
print(my_dictionary["Puppy"])
# => Prints Puppy's value: "Furry, energetic animal"

We Do: Dictionaries and Quick Tips

The order of keys you see printed may differ from how you entered them. That’s fine!

You can’t have the same key twice. Imagine having two “puppies” in a real dictionary! If you try, the last value will be the one that’s kept.

What’s more, printing a key that doesn’t exist gives an error.

Let’s create a dictionary together.

Teaching Tips:

Create a dictionary; print it out; print out a few keys.
Make your dictionary anything you’d like! Get input from the students on what it should be.
Dictionaries can be tough, especially with all the syntax. The more practice, the better! Make sure they follow along.
Try other cases: Use an integer for a key. Try printing a value and show that it fails — just like a real dictionary.

Talking Points:

Keys can be a string or integer. (Show this.)
You can look up any key — but not the value (just like a real dictionary!).
Use meaningful keys: my_zip_code is better than some_numbers.
The items that are returned when you access a dictionary come in any order they please.
- This doesn’t really matter that much, however, because the typical use case for a dictionary is when you know the exact key for the value you’re looking for.
- But, you should never count on the contents of a dictionary being in any order at all.

We Do: Dictionary Syntax

What if a value changes? We can reassign a key’s value: my_dictionary["Puppy"] = "Cheerful".

What if we have new things to add? It’s the same syntax as changing the value, just with a new key: my_dictionary["Yoga"] = "Peaceful".

Changing values is case sensitive — be careful not to add a new key!

Teaching Tips:

Be sure students are doing this with you. The more practice, the better.
Demonstrate how to change and add new keys. Print out the dictionary after each one.
Show that capitalization matters!

Repl.it note: This repl.it has:

my_dictionary = {"Puppy": "Furry, energetic animal", "Pineapple": "Acidic tropical fruit", "Tea": "Herb-infused drink"}

print(my_dictionary)

Quick Review: Dictionaries

We can:

Make a dictionary.
Print a dictionary.
Print one key’s value.
Change a key’s value.

Here’s a best practice: Declare your dictionary across multiple lines for readability. Which is better?

# This works but is not proper style.
my_dictionary = {"Puppy": "Furry, energetic animal", "Pineapple": "Acidic tropical fruit", "Tea": "Herb-infused drink"}

# Do this instead!
my_dictionary = {
  "Puppy": "Furry, energetic animal",
  "Pineapple": "Acidic tropical fruit",
  "Tea": "Herb-infused drink"
}

Discussion: Collection Identification Practice

What are a and b below?:

# What object is this?

collection_1 = [3, 5, 7, "nine"]

# What object is this?
collection_2 = {"three": 3, "five": 5, 9: "nine"}

Looping Through Dictionaries

We can print a dictionary with print(my_dictionary), but, like a list, we can also loop through the items with a for loop:

Teaching Tips:

This is just a quick demo, not an exercise. Walk through this to show it working; describe what each line does.

Talking Points:

Now, how do we loop through a dictionary? We can use the same for loop structure, but, instead of it iterating over each element in the collection, it will iterate over each key. We can then use the key to access the value.
The lines in our for loop work like this:
- The first line says, “For every key in the friendzips dictionary…”
- The second line says, “… print the value in friendzips associated with the provided key.”

Repl.it note: This repl.it has:

my_dictionary = {
  "Puppy": "Furry, energetic animal",
  "Pineapple": "Acidic tropical fruit",
  "Tea": "Herb-infused drink"
}

for key in my_dictionary:
  print(my_dictionary[key])

for key in my_dictionary:
  print(key, ":", my_dictionary[key])

Partner Exercise: Dictionary Practice

You know the drill: Grab a partner and pick a driver!

Create a new local file, dictionary_practice.py. Write a script that declares a dictionary called my_name.

Add a key for each letter in your name with a value of how many times that letter appears.

As an example, here is the dictionary you’d make for "Callee":

my_name = {"c": 1, "a": 1, "l": 2, "e": 2}

Write a loop that prints the dictionary, but formatted.

# The letter l appears in my name 2 times.

Bonus (if you have time): If it’s only one time, instead print The letter l appears in my name once. If it’s only two times, instead print The letter l appears in my name twice.

Partner Exercise: Most Popular Word

With the same partner, switch who’s driving.

Write a function, mostPopularWord(), that takes a list of strings and returns the string that appears the most.

For example:

words = [
    "hello",
    "water",
    "hello"
  ]

print(mostPopularWord(words))
# Prints "hello"

Hints:

Create a dictionary with words as keys and the count as the values.
Check if a key already exists with if "key_to_check" in my_dictionary:.

Other Values

We’re almost there! Let’s make this more complex.

In a list or a dictionary, anything can be a value. - This is a good reason to split dictionary declarations over multiple lines!

Open this repl.it in a new window so you can see it all (the button in the top-right corner).

Teaching Tips:

This slide can be skipped in classes with struggling students; it’s good information but not crucial to know at a beginner’s level.
This is a demo, not an exercise.
This can be tough. Spend some time here.
This repl.it is huge! Open it in a new window so students can see it all.
All the code is there, you just need to walk through it.

Repl.it note: This code is:

other_values_in_a_dictionary = {
  "CA": {"key1" : "value 1",
        "another_key" : "a value",
        "Joe" : "Even more dictionary!"
        },
  "WA": ["Trevor", "Courtney", "Brianna", "Kai"],
  "NY": "Just Tatyana"
}

print("Here's a dictionary and list in a dictionary:", other_values_in_a_dictionary)

print("----------")

other_values_in_a_list = [
  "a value",
  {"key1" : "value 1", "key2" : "value 2"},
  ["now", "a", "list"]
  ]
print("Here's a list and dictionary in a list:", other_values_in_a_list)

Summary and Q&A

Dictionaries:

Are another kind of collection, instead of a list.
Use keys to access values, not indices!
Should be used instead of lists when:
- You don’t care about the order of the items.
- You’d prefer more meaningful keys than just index numbers.

my_dictionary = {
  "Puppy": "Furry, energetic animal",
  "Pineapple": "Acidic tropical fruit",
  "Tea": "Herb-infused drink"
}

You Do: Reverse Lookup

Finding the value from a key is easy: my_dictionary[key]. But, what if you only have the value and want to find the key?

You task is to write a function, reverse_lookup(), that takes a dictionary and a value and returns the corresponding key.

For example:

state_capitals = {
  "Alaska" : "Juneau",
  "Colorado" : "Denver",
  "Oregon" : "Salem",
  "Texas" : "Austin"
  }

print(reverse_lookup("Denver"))
# Prints Colorado