Dictionary Methods

The full set of dict methods, and which ones change the dictionary.

The whole list

Method	Does	Returns
`get(key, default)`	look up without raising	the value or the default
`keys()`	a live view of the keys	a view
`values()`	a live view of the values	a view
`items()`	key and value pairs	a view
`setdefault(key, default)`	look up, creating it if missing	the value
`update(other)`	copy pairs in	`None`
`pop(key, default)`	remove and return	the value
`popitem()`	remove and return the last pair	a tuple
`clear()`	remove everything	`None`
`copy()`	a new shallow copy	a dict
`fromkeys(keys, value)`	build from keys	a dict

Reading without raising

Python

person = {"name": "Ada"}

print(person.get("name"))
print(person.get("age"))
print(person.get("age", "unknown"))
print("age" in person)

Output

Ada
None
unknown
False

Use [] when a missing key means a bug, and get() when absence is expected.

Views are live

keys(), values() and items() are windows onto the dictionary, not copies. Change it and they follow:

Python

scores = {"a": 1}
keys = scores.keys()

print(list(keys))
scores["b"] = 2
print(list(keys))

Output

['a']
['a', 'b']

Python

scores = {"a": 1, "b": 2}

for key in list(scores):
    if scores[key] < 2:
        del scores[key]

print(scores)

Output

{'b': 2}

setdefault builds nested structures

Python

groups = {}
for subject, name in [("maths", "ada"), ("maths", "grace"), ("code", "kat")]:
    groups.setdefault(subject, []).append(name)

print(groups)

Output

{'maths': ['ada', 'grace'], 'code': ['kat']}

It returns the existing list, or stores and returns a new one. No key check needed.

Merging

Python

defaults = {"colour": "black", "size": 10}
custom = {"size": 12}

print(defaults | custom)
print(defaults)

merged = dict(defaults)
merged.update(custom)
print(merged)

Output

{'colour': 'black', 'size': 12}
{'colour': 'black', 'size': 10}
{'colour': 'black', 'size': 12}

| builds a new dictionary; update() changes one in place. The right-hand side wins on a clash, which is what you want for overriding defaults.

Removing

Python

person = {"name": "Ada", "born": 1815, "tmp": True}

print(person.pop("tmp"))
print(person.pop("missing", "not there"))
print(person.popitem())
print(person)

Output

True
not there
('born', 1815)
{'name': 'Ada'}

popitem() takes the last inserted pair, which makes a dictionary usable as a stack of pairs.

fromkeys, and removing duplicates

Python

print(dict.fromkeys(["a", "b"], 0))
print(list(dict.fromkeys(["a", "b", "a", "c"])))

Output

{'a': 0, 'b': 0}
['a', 'b', 'c']

The second line is the idiomatic way to drop duplicates and keep the order, which a set cannot do.

Python

d = dict.fromkeys(["a", "b"], [])
d["a"].append(1)
print(d)

Output

{'a': [1], 'b': [1]}

Test yourself

2 questions

What does keys() give you?

Show the answer

A live view that reflects later changes — Because it is live, changing the dictionary while looping over it raises RuntimeError. Loop over list(d) instead.

What is wrong with dict.fromkeys(["a", "b"], [])?

Show the answer

Both keys share one list — fromkeys gives every key the same value object. It is the mutable-default trap in another costume.

Next chapter

Set Methods

Adding, removing, and the four ways to compare two sets.