Dictionaries

Syntax

{key1: value1, key2: value2}

Examples

student = {
    "name": "John",
    "age": 21,
    "course": "Software Engineering"
}

print(student["name"])          # John
print(student.get("age"))       # 21
print(student.get("gpa", 0.0))  # 0.0 (default, since 'gpa' doesn't exist)

student = {"name": "John", "age": 21}

student["gpa"] = 3.8            # add a new key
student["age"] = 22              # update an existing key
print(student)

del student["gpa"]               # remove a key
print(student)

age = student.pop("age")         # remove and return the value
print(age, student)

scores = {"Alice": 85, "Bob": 92, "Charlie": 78}

print(list(scores.keys()))    # ['Alice', 'Bob', 'Charlie']
print(list(scores.values()))  # [85, 92, 78]
print(list(scores.items()))   # [('Alice', 85), ('Bob', 92), ('Charlie', 78)]

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

defaults = {"theme": "light", "font_size": 12}
overrides = {"font_size": 16, "language": "en"}

# Python 3.9+ merge operator
merged = defaults | overrides
print(merged)  # {'theme': 'light', 'font_size': 16, 'language': 'en'}

# Older, equally valid approach
merged2 = {**defaults, **overrides}
print(merged2)

Best practices

• Use .get(key, default) instead of square brackets when a key might not exist, to avoid a KeyError
• Use the | merge operator (Python 3.9+) or {**a, **b} for combining dictionaries without a loop
• Iterate with .items() when you need both keys and values, rather than looking up dict[key] inside a loop over keys
• Use dictionaries for structured records with named fields, and lists of dictionaries for collections of records
• Remember dictionary keys must be hashable - lists cannot be used as keys, but tuples can

Runtime & versions

Python interpreter. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

Related Python documentation

Specifications & further reading

• Python language reference