Dictionaries

Python · 8 entries

Creating Dictionaries

syntax

d = {key: value, ...}
d = dict(key=value, ...)

example

user = {"name": "Alice", "age": 30, "active": True}
from_pairs = dict(host="localhost", port=8080)
print(user)
print(from_pairs)

output

{'name': 'Alice', 'age': 30, 'active': True}
{'host': 'localhost', 'port': 8080}

Note Keys must be hashable (strings, numbers, tuples of hashables). Lists and dicts cannot be keys.

create dictionarynew dictkey value pairinitialize dict

Accessing & Modifying Values

syntax

d[key]
d[key] = value

example

config = {"debug": False, "port": 3000}
print(config["port"])
config["debug"] = True
config["host"] = "0.0.0.0"
print(config)

output

3000
{'debug': True, 'port': 3000, 'host': '0.0.0.0'}

Note Accessing a missing key with d[key] raises KeyError. Use d.get(key) to safely return None instead.

get dict valueset dict valueaccess dictionaryupdate dictionary

get() with Default Value

syntax

d.get(key, default)

example

settings = {"theme": "dark"}
theme = settings.get("theme", "light")
lang = settings.get("language", "en")
print(theme, lang)

output

dark en

Note get() returns the default when the key is missing but does NOT add it to the dict. Use setdefault() if you want to also store the default.

get with defaultsafe access dictmissing key defaultsetdefault

Dictionary Methods

syntax

d.keys() / d.values() / d.items()

example

scores = {"alice": 95, "bob": 82, "carol": 91}
print(list(scores.keys()))
print(list(scores.values()))

for name, score in scores.items():
    print(f"{name}: {score}")

output

['alice', 'bob', 'carol']
[95, 82, 91]
alice: 95
bob: 82
carol: 91

Note keys(), values(), and items() return view objects that reflect changes to the dict in real time.

dict keysdict valuesiterate dictionaryloop over dict

Dictionary Comprehensions

syntax

{key_expr: val_expr for item in iterable if condition}

example

words = ["hello", "world", "python"]
lengths = {w: len(w) for w in words}
print(lengths)

original = {"a": 1, "b": 2, "c": 3}
filtered = {k: v for k, v in original.items() if v >= 2}
print(filtered)

output

{'hello': 5, 'world': 5, 'python': 6}
{'b': 2, 'c': 3}

Note Dict comprehensions are great for transforming or filtering dictionaries in a single expression.

dict comprehensionbuild dictionaryfilter dictionarytransform dict

Merging Dictionaries (| Operator)

syntax

merged = d1 | d2
d1 |= d2  # in-place

example

defaults = {"color": "blue", "size": "medium"}
overrides = {"size": "large", "bold": True}
final = defaults | overrides
print(final)

output

{'color': 'blue', 'size': 'large', 'bold': True}

Note The | operator (Python 3.9+) creates a new dict. Right-hand side wins on duplicate keys. Use |= to merge in place.

merge dictionariescombine dictsunion dictpipe operator dict

defaultdict

syntax

from collections import defaultdict
dd = defaultdict(factory)

example

from collections import defaultdict

word_count = defaultdict(int)
for word in "the cat sat on the mat".split():
    word_count[word] += 1
print(dict(word_count))

output

{'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1}

Note defaultdict auto-creates missing keys using the factory function. Common factories: int (0), list ([]), set (set()).

defaultdictauto default valuecount occurrencesgroup by key

Counter

syntax

from collections import Counter

example

from collections import Counter

letters = Counter("mississippi")
print(letters.most_common(3))

inventory = Counter(apples=5, oranges=3)
inventory.update(apples=2)
print(inventory["apples"])

output

[('s', 4), ('i', 4), ('p', 2)]
7

Note Counter supports arithmetic: Counter('aab') - Counter('ab') gives Counter({'a': 1}). most_common() returns elements in descending frequency.

countercount elementsfrequencymost commontally