Python Basics — Creating Dictionaries

Creating Dictionaries

A dictionary maps keys to values. Think of it like a real dictionary: you look up a word (key) and get its definition (value).

Literal Syntax

# Curly braces with key: value pairs
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}
print(person)
# {'name': 'Alice', 'age': 30, 'city': 'New York'}

dict() Constructor

# From keyword arguments
person = dict(name="Alice", age=30, city="New York")
print(person)
# {'name': 'Alice', 'age': 30, 'city': 'New York'}

# From list of tuples
pairs = [("name", "Bob"), ("age", 25), ("city", "London")]
person = dict(pairs)
print(person)
# {'name': 'Bob', 'age': 25, 'city': 'London'}

# From zip
keys = ["name", "age", "city"]
values = ["Charlie", 35, "Paris"]
person = dict(zip(keys, values))
print(person)
# {'name': 'Charlie', 'age': 35, 'city': 'Paris'}

dict.fromkeys()

Create a dictionary with specified keys and a shared default value:

keys = ["a", "b", "c"]
d = dict.fromkeys(keys, 0)
print(d)  # {'a': 0, 'b': 0, 'c': 0}

# Without default — values are None
d = dict.fromkeys(keys)
print(d)  # {'a': None, 'b': None, 'c': None}

Warning: If the default is a mutable object, all keys share the same object:

d = dict.fromkeys(["a", "b", "c"], [])
d["a"].append(1)
print(d)  # {'a': [1], 'b': [1], 'c': [1]}  ← all changed!

Dictionary Ordering

Since Python 3.7, dictionaries preserve insertion order:

d = {"c": 3, "a": 1, "b": 2}
for key in d:
    print(key, end=" ")
# c a b  (insertion order preserved)

Valid Key Types

Keys must be hashable — immutable types that have a hash value:

# ✅ Valid keys
d = {
    "name": "Alice",        # string
    42: "answer",            # integer
    3.14: "pi",              # float
    (1, 2): "coordinates",   # tuple
    True: "boolean",         # bool
}

# ❌ Invalid keys (mutable types)
# d = {[1, 2]: "list"}    # TypeError: unhashable type: 'list'
# d = { {}: "dict" }      # TypeError: unhashable type: 'dict'

Empty Dictionaries

empty1 = {}
empty2 = dict()

print(len(empty1))   # 0
print(bool(empty1))  # False (empty = falsy)
print(type(empty1))  # <class 'dict'>

Note: {} creates an empty dict, not an empty set. Use set() for empty sets.

Duplicate Keys

If you define duplicate keys, the last value wins:

d = {"a": 1, "b": 2, "a": 3}
print(d)  # {'a': 3, 'b': 2}

Mixed Value Types

Values can be any type:

profile = {
    "name": "Alice",
    "age": 30,
    "scores": [95, 87, 92],
    "address": {
        "city": "New York",
        "zip": "10001"
    },
    "active": True
}

Summary

• Create with {}, dict(), dict.fromkeys(), or dict(zip(keys, values))
• Keys must be hashable (strings, numbers, tuples)
• Values can be any type
• Insertion order is preserved (Python 3.7+)
• Duplicate keys keep the last value
• {} creates an empty dict, not a set

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Next up: Accessing & Modifying Dictionaries →