Dictionaries: Labeled Storage

Python for Absolute Beginners · Lesson 13

What you’ll learn

• What a dictionary is, and how it differs from a list
• How to make one with keys and values
• How to read, change, and add entries
• When to choose a dictionary over a list
• How to loop through keys and values

Labels instead of positions

• A list finds things by position: item 0, item 1, item 2
• A dictionary finds things by label
• Each entry has two parts: a key (the label) and a value (what is stored under it)
• Entries sit inside curly brackets, written key: value and separated by commas
• A list answers “what is item number 2?” — a dictionary answers “what is the value labeled name?”

Make a dictionary

student = {"name": "Mia", "age": 9, "grade": "4th"}
print(student)

{'name': 'Mia', 'age': 9, 'grade': '4th'}

Python shows string keys in single quotes — that is just how it displays them, and it means the same thing.

Reading an entry

student = {"name": "Mia", "age": 9, "grade": "4th"}
print(student["name"])
print(student["age"])

Mia
9

Square brackets, just like a list — but with the key instead of a position. A missing key gives a KeyError.

Changing and adding entries

• Changing an entry and adding one look exactly alike
• The only difference is whether the key already exists
• Assigning to an existing key replaces it
• Assigning to a new key adds a brand-new entry
• Same syntax, two jobs — Python decides by whether the key is already there

Change and add

student = {"name": "Mia", "age": 9, "grade": "4th"}
student["age"] = 10
print(student)
student["pet"] = "Biscuit"
print(student)

{'name': 'Mia', 'age': 10, 'grade': '4th'}
{'name': 'Mia', 'age': 10, 'grade': '4th', 'pet': 'Biscuit'}

The age entry was replaced, because “age” already existed; the pet entry was added, because it did not.

List or dictionary?

• List when order matters and items are alike: a week's temperatures, a shopping list
• Dictionary when each item has a name: a student's details, a product's price
• Storing labeled data in a list means memorizing that position 0 is the name and position 1 is the age
• A dictionary removes that memorizing: the label travels with the value

Looping through a dictionary

• A for loop over a dictionary walks its keys
• If you also want the values, .items() gives you both at once
• for key, value in … unpacks each pair into two loop variables at once
• str(value) converts a number to text, because + cannot join text and a number directly

Loop with .items()

student = {"name": "Mia", "age": 10, "grade": "4th"}
for key in student:
    print(key)
for key, value in student.items():
    print(key + ": " + str(value))

name
age
grade
name: Mia
age: 10
grade: 4th

The first loop prints keys only; the second prints each key and value on its own line.

Hands-on exercise

• Make a dictionary for yourself with keys “name”, “age”, and “city”, and print the whole dictionary
• Print just the value labeled “city”
• Change your age entry to next year's age and print the dictionary again
• Add a new entry for your favorite color and print the dictionary
• Loop through your dictionary with .items() and print each key and value on its own line

Key takeaways

• A dictionary stores entries as key: value pairs inside curly brackets.
• Read with dict[key]; a missing key gives a KeyError.
• Assigning to an existing key replaces it; assigning to a new key adds it.
• Choose a list when order matters, a dictionary when items have labels.
• .items() loops through key-value pairs together; str() converts a number to text for joining.

Take the quiz

Take the Lesson 13 quiz — 5 quick questions to check what you learned.

Next lesson

Debugging: Reading Error Messages and Fixing Bugs