Dictionaries — Basics

What a dictionary is

A dictionary stores data as key-value pairs. You look up a key to get its value, like a real dictionary.

Creating dictionaries

You can create dictionaries with braces or with dict().

python
user = {"name": "Lina", "age": 28}
settings = dict(theme="dark", lang="en")

Reading values by key

Use brackets to get a value. The key must exist or you will get a KeyError.

python
print(user["name"])  # Lina

Safe access with get()

get() returns a default value if the key is missing.

python
print(user.get("city", "Unknown"))

Adding and updating keys

Assign a value to a key to add or update it.

python
user["country"] = "Canada"   # add
user["age"] = 29              # update

Removing keys

Use del, pop(), or popitem().

python
del user["country"]
age = user.pop("age")  # removes and returns

Checking if a key exists

Use the in keyword before accessing.

python
print("name" in user)
print("email" in user)

What can be a key

Dictionary keys must be hashable. Strings, numbers, and tuples work. Lists and dictionaries do not.

Copying vs referencing

A copy is independent. A reference points to the same dictionary.

python
original = {"a": 1}
copy = original.copy()
copy["a"] = 2

Merging dictionaries

update() changes the dictionary in place. The | operator creates a new one (Python 3.9+).

python
base = {"a": 1}
extra = {"b": 2}
base.update(extra)
merged = base | extra

Common mistakes to avoid

• Using a list or dict as a key.
• Accessing missing keys without get().
• Forgetting that update() changes the dictionary in place.

What you should understand after this lesson

• How to create, read, add, update, and remove keys.
• How to access keys safely with get().
• What kinds of keys are allowed.
• How to copy and merge dictionaries.