Looping and Updating Mappings

Scenario: A report must print both every service name and its port.

New words, in plain English

• items view - the key-value pairs returned by .items()
• Membership - key in mapping checks keys
• Merge - combining mappings

Loop over .keys(), .values(), or .items(). update merges values; setdefault supplies a value only when the key is absent. Modern Python also supports a | b for a new merged dict.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

ports = {"api": 80, "db": 5432}
for name, port in ports.items():
    print(f"{name}:{port}")

Tip: Do not add or remove dictionary keys while directly iterating over it; iterate over list(mapping) if mutation is necessary.

Goal: Practice this idea in the py-dict-basics challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.