Add and Change Dictionary Entries

Dictionaries are mutable. Assignment with a key can either change an entry or add one.

Change an existing value

Start with a small record:

profile = {
    "name": "Mina",
    "city": "Lisbon",
}

Assign through the existing "city" key:

profile["city"] = "Porto"

The key already exists, so its value changes:

print(profile)
# {'name': 'Mina', 'city': 'Porto'}

The dictionary does not gain a second "city" entry. Keys are unique.

Add a new key-value pair

Use the same syntax with a key that is not present:

profile["language"] = "Portuguese"

Now the dictionary has three entries:

{
    "name": "Mina",
    "city": "Porto",
    "language": "Portuguese",
}

The syntax is identical. Whether it adds or changes depends on whether the key already exists.

Count entries with len()

len() reports how many key-value pairs a dictionary contains:

print(len(profile))
# 3

It does not count the characters in the keys or values.

Order is not the identity

Modern Python remembers insertion order when it displays or loops through a dictionary. Even so, keys are the meaningful way to find fields.

These dictionaries compare equal:

first = {"name": "Mina", "city": "Porto"}
second = {"city": "Porto", "name": "Mina"}

print(first == second)
# True

They hold the same key-value pairs, so their different insertion order does not make them different records.

item = {"name": "Map", "price": 4}
item["price"] = 5
item["stock"] = 12
print(len(item))