Explain Python dictionaries and their characteristics.

A dictionary is a mutable mapping from unique keys to values. It is built on a hash table, so looking up a value by its key takes O(1) time on average. Its main characteristics are:

• Keys must be unique and hashable, which means their hash value never changes. Strings, numbers and tuples of hashable items work as keys, but lists do not.
• Values can be any object.
• Since Python 3.7, a dictionary keeps its items in insertion order.
• d[key] raises KeyError for a missing key, while d.get(key) returns None or a default you pass.

student = {
    "name": "John",
    "age": 25,
    "grades": [85, 90, 78]
}

# Accessing values
print(student["name"])  # John
print(student.get("age"))  # 25

# Adding/updating
student["email"] = "john@email.com"