After this lesson, you will be able to:

• Create a dictionary and retrieve a value through its key.
• Add new key-value pairs and update existing values.
• Check whether a key exists, including when its value is falsy.
• Delete a known key safely and inspect the dictionary's size.

A dictionary maps keys to values. It is useful for named settings and records, where settings["region"] is clearer than remembering a numeric position. A literal uses {key: value} entries; an empty dictionary is {}. Keys must be hashable (strings are a common choice), and each key is unique: assigning the same key again replaces its value.

Read a known value using record[key], and assign record[key] = new_value to add or update it. Looking up a missing key with square brackets raises KeyError.

Check key membership with key in record before reading an optional field. This checks whether a key exists, not whether its value is truthy: a stored 0, False, "", or None may still be valid data. Use del record[key] to remove an existing key; check membership first when it may be absent.

1. Create and read a named field

Square brackets look up the value belonging to a key.

account = {"team": "payments", "retries": 3}
print(account["team"])
print(account["retries"])

Output:

payments
3

2. Add and update fields

Assigning a missing key adds it; assigning an existing key changes its value.

config = {"region": "us", "retries": 2}
config["region"] = "eu"
config["enabled"] = True
print(config["region"])
print(config["enabled"])

Output:

eu
True

3. Check keys, even when values are zero

Membership tests the key's presence, not the stored value.

profile = {"user": "Maya", "attempts": 0}
print("attempts" in profile)
print("active" in profile)

Output:

True
False

4. Delete a known key

Deleting removes the entry completely.

settings = {"debug": False, "port": 8000}
del settings["debug"]
print(len(settings))
print("debug" in settings)

Output:

1
False

1. record["missing"] raises KeyError when the key does not exist.
2. key in record checks keys, not values.
3. A stored 0 is still a value for a present key.
4. Assigning an existing key replaces its value, rather than creating a duplicate key.
5. Dictionary .get() and its default behavior are introduced in the next lesson; use key-membership checks here.