Dictionary basics

Looking up by name

A dictionary stores pairs: a key, and the value it points at. Written with braces:

person = {"name": "Ada", "age": 36, "city": "London"}

You look things up by key rather than position:

person["name"]        # "Ada"
person["age"] = 37    # change it
person["job"] = "Mathematician"   # add a new pair

Asking for a key that is not there raises KeyError. .get() is the safe version:

person.get("job")             # None if missing
person.get("job", "unknown")  # your own fallback

Keys are usually strings, but any immutable value works. Values can be anything at all, including other dictionaries.

Your turn: add a "country" of "UK", change the age to 37, and safely read a "job" that may not be there, defaulting to "unknown".

Counting with a dictionary

Counting occurrences is the classic dictionary job:

counts = {}
for word in words:
    counts[word] = counts.get(word, 0) + 1

counts.get(word, 0) gives the count so far, or zero the first time — which avoids having to check whether the key exists.

Your turn: write count_letters(text) returning a dictionary of how many times each character appears.