5.5 Mapping Data Type

dict

What Is It?

Python’s built-in hash map — a mutable, unordered (technically insertion-ordered since 3.7) collection of key-value pairs.

Why Is It Used?

It’s the natural representation for structured, labeled data — config settings, JSON objects, API responses.

>>> d = {"name": "Alice", "age": 30}
>>> type(d)
<class 'dict'>

Key-Value Pairs

How Is It Used?

Every entry maps a unique, hashable key to a value. Keys can be any immutable type (str, int, tuple); values can be anything.

>>> d["name"]
'Alice'
>>> d.get("missing", "default")    # avoids KeyError for absent keys
'default'

Dictionary Use Cases

• Parsed JSON / API response bodies
• Configuration settings (key → value)
• Counting/grouping (e.g. word → frequency)
• Fast lookups by a unique identifier (e.g. user ID → user record)

Quick Interview Answer

“dict is Python’s built-in hash map — a mutable collection of key-value pairs, insertion-ordered since Python 3.7. Keys must be hashable (immutable types like str, int, tuple); values can be anything. It’s the natural fit for structured, labeled data — config settings, parsed JSON, API responses — and .get(key, default) is the standard way to look up a key without risking a KeyError.”

Common Mistakes

• Using d[key] when the key might not exist, raising an unhandled KeyError — use .get() with a default instead.
• Trying to use a mutable type (like a list) as a dict key, which raises TypeError: unhashable type.
• Assuming dict ordering was always guaranteed — insertion order is only guaranteed from Python 3.7 onward.