Dictionary Methods and Comprehensions

Iterating with .items()

Dictionary view is a live window on a dictionary returned by keys(), values() or items(). It shows the current entries without copying them, and you can loop over it or pass it to sum() and len().

Use .items() to unpack both the key and the value simultaneously in a for loop.

Iterating over salaries directly (or salaries.keys()) loops only through keys.

Merging dictionaries: the | operator

Dictionary merge is joining two dictionaries into one with the | operator. When both have the same key, the value from the right-hand dictionary wins.

Python 3.9 (PEP 584) introduced the clean union operator | for merging dictionaries.

Values from the right-hand dictionary (custom) override matching keys in defaults, setting port to 5000.

Dictionary comprehensions

Dictionary comprehension is a one-line way to build a new dictionary from a loop, written as {key: value for item in items}. An optional if at the end keeps only the items you want.

Construct a new dictionary concisely: {key_expr: value_expr for item in iterable if condition}.

Produces {'alice': 5, 'bob': 3, 'charlie': 7}.

Changing entries: update() and setdefault()

setdefault is a dictionary method that returns the value of a key. If the key is missing, it first stores a default value under that key, then returns it. An existing value is never changed.

update() copies the pairs of another dictionary into this one, changing the dictionary in place. setdefault() fills a key only when it is still missing.

update replaced the name and added the city. setdefault("plan", ...) added a new key, but setdefault("city", ...) left Pune alone because the key existed.