Summary

In this lesson, you learned that:

• A dictionary keeps together values that describe one thing: {"name": "Anna", "age": 21}. It is made of key–value pairs, separated by commas, with a colon between the key and the value.
• print() shows a dictionary with text in single quotes: {'name': 'Anna', 'age': 21}. The pairs stay in the order in which they were added.
• You read a value with its key in square brackets: person["name"]. A key that isn't there ends the program with a KeyError. Uppercase and lowercase letters in keys matter. In an f-string, write the key in single quotes: f"{person['name']}".
• Assigning changes a value (person["age"] = 22) or adds a new pair (person["city"] = "Brno"). Each key is in a dictionary only once. {} is an empty dictionary, and len() gives the number of pairs.
• "phone" in person checks whether a key is there. person.get("phone", "unknown") reads a value without an error: when the key is missing, it gives the default value (without one, None).
• The key in square brackets can be in a variable: menu[item].
• for key in person: goes through the keys, and for key, value in person.items(): goes through the keys and values together.
• A dictionary is great for counting: counts[x] = counts.get(x, 0) + 1.
• A list of dictionaries holds many things of the same kind. movies[1]["title"] is the title of the second movie, and a for loop goes through the list one dictionary at a time.
• A function can take a dictionary or a list of dictionaries as an argument, and it can return a new dictionary.

In the next lesson, the final project, you'll put together everything you've learned in this course (variables, conditions, lists, loops, functions and dictionaries) and build a whole program step by step.

Now some practice. The fruit shop in the editor keeps its stock in a dictionary: each fruit and how many pieces of it are left.