Dictionaries
A real dictionary doesn't make you read every page. You look up a word, and its meaning is right there.
Lists find things by position: item 0, item 1, item 2. That's perfect when order is what matters. But a lot of information isn't about order at all. What's Maria's score? What's the price of adobo? What's the capital of Japan? You don't want "item 7"; you want to look it up by name.
For that, Python has the dictionary, often called a dict.
Looking up a word
In a printed dictionary, every entry has two parts: the word you look up, and its meaning. You never read the book from the start to find "umbrella." You jump straight to it, because every word is unique and the book is organized for looking things up.
A Python dictionary works the same way. It stores pairs:
- a key, like the word you look up;
- a value, like its meaning.
Give it a key, and it hands back the value, instantly, however big the dictionary is.
Making a dictionary
Write the pairs between curly braces, with a colon between each key and its value, and commas between pairs:
prices = {"adobo": 65, "rice": 15, "juice": 20}
student = {"name": "Maria", "grade": 11, "honor_roll": True}
print(prices)
print(len(student)){'adobo': 65, 'rice': 15, 'juice': 20}
3len counts the pairs. Keys are usually strings, and values can be anything: numbers, strings, booleans, even lists.
Looking things up
Put a key in square brackets to get its value:
prices = {"adobo": 65, "rice": 15, "juice": 20}
print(prices["adobo"])
print(prices["rice"] * 2)65
30Ask for a key that isn't there, and Python stops with a KeyError:
prices = {"adobo": 65, "rice": 15, "juice": 20}
print(prices["pizza"]) # error: KeyError: 'pizza'When a key might be missing, use the get method instead. It gives back None if the key isn't there, or a default value you choose:
prices = {"adobo": 65, "rice": 15, "juice": 20}
print(prices.get("pizza"))
print(prices.get("pizza", 0))
print(prices.get("juice", 0))None
0
20You can also check first with in, which looks at the keys:
prices = {"adobo": 65, "rice": 15}
if "pizza" in prices:
print("We sell pizza!")
else:
print("No pizza here.")No pizza here.Adding, changing, and removing
Assigning to a key adds it if it's new, or replaces its value if it already exists:
prices = {"adobo": 65, "rice": 15}
prices["juice"] = 20
prices["adobo"] = 70
print(prices){'adobo': 70, 'rice': 15, 'juice': 20}Each key can appear only once, so there's never any confusion about which value you get.
To remove a pair, use del, or pop, which also gives you the removed value:
prices = {"adobo": 70, "rice": 15, "juice": 20}
del prices["rice"]
old = prices.pop("juice")
print(prices, "and juice cost", old){'adobo': 70} and juice cost 20Dictionaries keep their pairs in the order you added them, so printing one always shows them in that order.
Looping through a dictionary
A for loop over a dictionary gives you its keys. More often, you'll want the keys and values together, which is what .items() is for. Each item is a (key, value) tuple, which you can unpack like in Tuples:
prices = {"adobo": 65, "rice": 15, "juice": 20}
for item in prices:
print("Item:", item)
for item, price in prices.items():
print(f"{item}: {price} pesos")
print(list(prices.values()))
print(sum(prices.values()))Item: adobo
Item: rice
Item: juice
adobo: 65 pesos
rice: 15 pesos
juice: 20 pesos
[65, 15, 20]
100.keys()gives the keys..values()gives the values..items()gives both, as pairs.
Counting things
One of the most useful dictionary patterns is counting: how many times does each word appear, how many votes did each candidate get? Start with an empty dictionary, and add one to each key's count as you go. get with a default of 0 handles keys you haven't seen yet:
votes = ["Ana", "Ben", "Ana", "Carlo", "Ana", "Ben"]
counts = {}
for name in votes:
counts[name] = counts.get(name, 0) + 1
print(counts){'Ana': 3, 'Ben': 2, 'Carlo': 1}Read the middle line as: "this name's count becomes its current count (or 0, if it's new) plus one."
What can be a key?
Keys must be values that can never change: strings, numbers, and tuples all work. Lists can't be keys, because a list could change after being stored, and the dictionary would lose track of it:
seats = {("A", 1): "Maria", ("A", 2): "Ben"}
print(seats[("A", 2)])Benseats = {["A", 1]: "Maria"} # error: TypeError: cannot use 'list' as a dict key (unhashable type: 'list')Values have no such rule. Any value at all can go on the right of the colon.
Try it
This program runs a cafeteria order. It looks up prices, handles an item that isn't on the menu, counts how many of each item were ordered, and totals the bill. Predict every line.
max(counts, key=counts.get) finds the key with the biggest value: "compare the keys using each one's count."
Running Python here
Running Python right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have Python installed on your computer (see Setting Up), save it as main.py and run it with python main.py (or python3 main.py on macOS and Linux).
Check your prediction
Sorry, no pizza today.
Order: {'rice': 3, 'adobo': 2, 'juice': 1}
3 x rice 45
2 x adobo 130
1 x juice 20
Total: 195 pesos
Most ordered: rice
Menu now has 5 itemscounts lists items in the order each was first ordered: rice, then adobo, then juice. Pizza was skipped with continue because it wasn't on the menu, until the last lines added it.
Try it yourself
- Add
"halo-halo"twice to the order. How do the counts and the total change? - Change the program so an item that isn't on the menu costs 0 instead of being skipped, using
menu.get(item, 0). - Write a program that counts how many times each letter appears in the word
"mississippi", and prints the counts.
Check your understanding
Up next
Dictionaries make sure each key appears only once. Sometimes that's all you need: a collection where every item is unique, and you don't care about order or values. That's a Set.