Dictionaries and records

A list is the right shape when the things in it are the same kind of thing and their order matters. A dictionary is the right shape when you need to look something up by name.

student = {"id": "u1234567", "name": "A. Visitor", "marks": [72, 68]}

Three things to notice:

• Keys are usually strings, and student["name"] is a lookup, not a position.
• The values do not all have to be the same type. marks is a list inside a dictionary, and that is normal.
• student["email"] on a missing key raises KeyError. Use student.get("email") when absence is expected, and let it raise when absence is a bug.

Records versus tables

One dictionary is a record. A list of dictionaries is a table:

enrolled = [
    {"id": "u1234567", "name": "A. Visitor"},
    {"id": "u7654321", "name": "B. Visitor"},
]

Almost every data exercise in the rest of this course is a loop over a table that builds either a smaller table or a single accumulated value. Once you see that shape, week 10 onwards is mostly recognising it.

Exercise

Given a table of enrolments, write by_id(rows) that returns a dictionary mapping each id to its whole record. Decide what should happen if two rows share an id, and say so in a comment before you write the loop.