Python Dictionary Methods

In the last lesson, you learned how to create and use dictionaries. Now it's time to meet the tools that come with them. Python dictionary methods are functions built into every dictionary that help you read, change, merge and copy data with very little code. By the end, you'll also know the classic "count the words" pattern and how to write a dictionary comprehension.

Quick reference: all common dictionary methods

keys(), values() and items()

These three return views: live windows into the dictionary. If the dictionary changes, the view shows the change too. Wrap one in list() when you need a real list.

menu = {"pasta": 12, "pizza": 10, "salad": 8}

print(list(menu.keys()))
print(list(menu.values()))
print(list(menu.items()))

prices = menu.values()
menu["soup"] = 6
print(list(prices))     # the view sees the new item

['pasta', 'pizza', 'salad']
[12, 10, 8]
[('pasta', 12), ('pizza', 10), ('salad', 8)]
[12, 10, 8, 6]

Views work great with built-in functions too, for example sum(menu.values()) or max(menu.values()).

get() and setdefault(): reading with a fallback

get() with a default value

get() reads a value without crashing when the key is missing. The optional second argument is what you get back instead of None:

inventory = {"apples": 5}
print(inventory.get("apples", 0))
print(inventory.get("kiwis", 0))
print(inventory)   # get() never changes the dict

5
0
{'apples': 5}

setdefault(): get, or insert if missing

setdefault() is like get(), but if the key is missing it also stores the default. It's ideal for grouping items into lists:

songs = [("Pop", "Flowers"), ("Rock", "Creep"), ("Pop", "Anti-Hero")]
by_genre = {}

for genre, title in songs:
    by_genre.setdefault(genre, []).append(title)

print(by_genre)

{'Pop': ['Flowers', 'Anti-Hero'], 'Rock': ['Creep']}

The first time "Pop" appears, setdefault() creates an empty list for it and returns that list, so append() has somewhere to go.

update(): merge one dictionary into another

update() copies every pair from another dictionary. New keys are added and existing keys are overwritten:

profile = {"name": "Leo", "city": "Berlin"}
changes = {"city": "Lisbon", "job": "designer"}

profile.update(changes)
print(profile)

{'name': 'Leo', 'city': 'Lisbon', 'job': 'designer'}

Python 3.9+ also has the | operator, which merges two dictionaries into a new one: merged = profile | changes.

pop(), popitem() and clear()

cart = {"book": 15.0, "pen": 2.0, "mug": 9.5}

price = cart.pop("pen")
print("Removed pen:", price)
print(cart.pop("lamp", "not in cart"))

last = cart.popitem()
print("Last added:", last)
print(cart)

cart.clear()
print(cart)

Removed pen: 2.0
not in cart
Last added: ('mug', 9.5)
{'book': 15.0}
{}

popitem() always removes the most recently added pair, which makes a dictionary handy as an "undo" history. On an empty dictionary it raises KeyError.

copy(): why = is not a copy

Writing b = a doesn't copy anything; both names point to the same dictionary, just like the labels you saw in Python variables. Use copy() to get an independent dictionary:

original = {"wifi": "on", "sound": "off"}
same = original
backup = original.copy()

original["sound"] = "on"

print(same)
print(backup)

{'wifi': 'on', 'sound': 'on'}
{'wifi': 'on', 'sound': 'off'}

fromkeys(): start many keys with the same value

dict.fromkeys() is called on dict itself and builds a fresh dictionary from a list of keys:

players = ["Ana", "Ben", "Cai"]
scores = dict.fromkeys(players, 0)
print(scores)

{'Ana': 0, 'Ben': 0, 'Cai': 0}

Stick to immutable starting values like 0 or "". With dict.fromkeys(players, []), every key would share one list.

How do you count words with a dictionary?

Counting how often each item appears is one of the most common dictionary jobs. The trick is get(word, 0) + 1: start at 0 for new words, add 1 for words you've seen.

review = "great coffee great service slow wifi great coffee"
counts = {}

for word in review.split():
    counts[word] = counts.get(word, 0) + 1

print(counts)
print("Most common:", max(counts, key=counts.get))

{'great': 3, 'coffee': 2, 'service': 1, 'slow': 1, 'wifi': 1}
Most common: great

split() breaks the text into words (see string methods), and max(counts, key=counts.get) finds the key with the biggest value.

Dictionary comprehension

Just like a list comprehension, a dictionary comprehension builds a dictionary in one line. The pattern is {key_expr: value_expr for item in collection}, with an optional if filter:

prices_usd = {"latte": 4.0, "muffin": 3.0, "bagel": 2.5}

prices_eur = {item: round(price * 0.92, 2) for item, price in prices_usd.items()}
print(prices_eur)

cheap = {item: price for item, price in prices_usd.items() if price < 3.5}
print(cheap)

squares = {n: n * n for n in range(1, 5)}
print(squares)

{'latte': 3.68, 'muffin': 2.76, 'bagel': 2.3}
{'muffin': 3.0, 'bagel': 2.5}
{1: 1, 2: 4, 3: 9, 4: 16}

Common mistakes with dictionary methods

1. Saving the result of update()

settings = {"theme": "light"}
settings = settings.update({"theme": "dark"})
print(settings)

None

update() changes the dictionary in place and returns None. Just call settings.update(...) on its own line.

2. Changing a dictionary while looping over it

stock = {"milk": 0, "eggs": 12, "bread": 0}
for item in stock:
    if stock[item] == 0:
        del stock[item]

RuntimeError: dictionary changed size during iteration

Loop over a copy of the keys instead, for item in list(stock):, or build a new dictionary with a comprehension: {k: v for k, v in stock.items() if v > 0}.

3. Indexing a view

menu.keys()[0] raises TypeError: 'dict_keys' object is not subscriptable. Convert first: list(menu.keys())[0].

Practice exercise

visits = ["sam", "ria", "sam", "tom", "ria", "sam"]
counts = {}

for member in visits:
    counts[member] = counts.get(member, 0) + 1

regulars = {name: n for name, n in counts.items() if n >= 2}

print(counts)
print(regulars)

{'sam': 3, 'ria': 2, 'tom': 1}
{'sam': 3, 'ria': 2}

What's next?

You've now met all four of Python's core collections. Next, put them side by side in Python list vs tuple vs set vs dictionary and learn exactly when to use each one.