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Dictionaries in Python

A dict stores values by key instead of by position. Creating one, reading safely with get, adding and removing, looping over keys and values, counting, nesting, and the keys you can use.

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A list finds a value by position: scores[2]. Often you want a name instead: the price of “apple”, the phone number of “Dana”, the setting called “theme”. That’s a dictionary, dict for short: “a set of key: value pairs, with the requirement that the keys are unique”, as the tutorial puts it.

Every example ran with Python 3.13, checked against the dictionaries section of the tutorial and the mapping types reference.

Creating a dictionary

Write key: value pairs between braces. A value is read by putting its key in square brackets:

prices = {"apple": 4.5, "bread": 9.9, "milk": 6.2}
print(prices["bread"])
print(len(prices))
print(prices)

empty = {}
person = dict(name="Dana", age=31)
print(person)

9.9
3
{'apple': 4.5, 'bread': 9.9, 'milk': 6.2}
{'name': 'Dana', 'age': 31}

{} is an empty dictionary, not an empty set. dict(name="Dana") is another way to write string keys. Pairs keep the order you added them.

Reading safely: in and get()

A missing key is an error:

prices = {"apple": 4.5, "bread": 9.9}
print(prices["cheese"])

KeyError: 'cheese'

in checks if a key exists. get() returns the value, or None (or a default you choose) when the key is missing:

print("apple" in prices, "cheese" in prices)
print(prices.get("cheese"))
print(prices.get("cheese", 0))

True False
None
0

in checks the keys, not the values. 4.5 in prices is False.

Adding, changing and removing

Assigning to a key adds it if it’s new, and replaces the value if it exists. Keys are unique, so there’s never a second “apple”:

stock = {"apple": 10}
stock["pear"] = 3
stock["apple"] = 8
stock["apple"] += 1
print(stock)

stock.update({"kiwi": 5, "pear": 4})
print(stock)

{'apple': 9, 'pear': 3}
{'apple': 9, 'pear': 4, 'kiwi': 5}

update() copies every pair from another dictionary, replacing the keys that already exist. To remove a key, del deletes it, and pop() deletes it and returns the value:

del stock["kiwi"]
pears = stock.pop("pear")
print(stock, pears)
print(stock.pop("plum", "not there"))

{'apple': 9} 4
not there

Like get(), pop() takes a default for a missing key. Without one, a missing key raises KeyError.

Looping over a dictionary

A for loop over a dictionary gives its keys. values() gives the values, and items() gives (key, value) pairs, which you unpack in the loop:

prices = {"apple": 4.5, "bread": 9.9, "milk": 6.2}
for name in prices:
    print(name)

print(sum(prices.values()))

for name, price in prices.items():
    print(f"{name:<6} {price:>5.2f}")

apple
bread
milk
20.6
apple   4.50
bread   9.90
milk    6.20

The format specs :<6 and :>5.2f line up the columns, as in the f-strings post. Don’t add or remove keys while looping over a dictionary (RuntimeError: dictionary changed size during iteration); loop over list(prices) if you must.

Counting with a dictionary

To count how many times each thing appears, get(key, 0) gives zero for a key not seen yet:

words = "the cat and the hat and the bat".split()
counts = {}
for word in words:
    counts[word] = counts.get(word, 0) + 1
print(counts)
print(max(counts, key=counts.get))

{'the': 3, 'cat': 1, 'and': 2, 'hat': 1, 'bat': 1}
the

max(counts, key=counts.get) finds the key with the largest value. The standard library’s collections.Counter does the same counting in one line.

Nested data

Values can be anything, including lists and other dictionaries, like a JSON response from a web API:

user = {
    "name": "Dana",
    "langs": ["he", "en"],
    "address": {"city": "Haifa", "zip": "3200003"},
}
print(user["langs"][0])
print(user["address"]["city"])
user["langs"].append("fr")
print(user["langs"])

he
Haifa
['he', 'en', 'fr']

Read nested values one step at a time, from the outside in: user["address"] is a dictionary, and ["city"] reads from it.

What can be a key

Keys must be hashable: in practice, values that can’t change, like strings, numbers and tuples. A list can’t be a key:

grid = {(0, 0): "start", (2, 3): "treasure"}
print(grid[(2, 3)])
bad = {["a", "b"]: 1}

treasure
TypeError: unhashable type: 'list'

A list could change after being used as a key, so it’s refused. Values have no such limit.

Common mistakes

  • Expecting in to search values. "Dana" in user checks keys. For values, use "Dana" in user.values().
  • Copying with =. As with lists, b = a gives a second name for the same dictionary. Use a.copy() for a separate one.

What’s next

Next in the series: functions, which give a name to a block of code so you can run it again with different values.

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