13.1 Dictionaries

Before you start

Work through the earlier fundamentals chapters. Use a Python 3 interpreter and run each example locally.

Core concepts

A dictionary maps unique, hashable keys to values. Assignment replaces an existing value for the same key. Dictionaries preserve insertion order; lookups are typically constant time on average.

Use direct indexing when a key is required and get() when a missing key is expected. A missing key accessed with brackets raises KeyError. Values can be mutable, so copying a dictionary does not recursively copy nested objects.

Worked example

server = {"name": "web-01", "state": "running"}
server["owner"] = "platform"
print(server.get("region", "unknown"))
for key, value in server.items():
    print(key, value)

Mini lab

Build a dictionary that counts occurrences in ["running", "stopped", "running"]. Expected result: {"running": 2, "stopped": 1}.

counts = {}
for state in ["running", "stopped", "running"]:
    counts[state] = counts.get(state, 0) + 1
print(counts)

Knowledge check

What happens when you assign to an existing dictionary key?

Common mistake

Never use a mutable list as a dictionary key. Use a tuple of hashable values if you need a composite key.

Completion checklist

• Run the example and explain its output.
• Complete the mini lab without copying the solution.
• Test an edge case and explain how the code handles it.
• Answer the knowledge check in your own words.

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