Lists and dictionaries — Python for Data

The two containers everything else is built from, and the one question that tells you which you need.

Almost every structure in data work is a list, a dictionary, or a nesting of the two. A CSV row is a dictionary. A CSV file is a list of them. A JSON response from an API is both, several layers deep.

A list is ordered

cities = ["Chennai", "Kochi", "Pune"]

cities[0]        # "Chennai" — counting starts at zero
cities[-1]       # "Pune" — negative counts from the end
len(cities)      # 3
cities.append("Surat")

Use a list when position matters or when you simply have several of the same kind of thing.

A dictionary is labelled

student = {"name": "Asha", "city": "Kochi", "marks": 88}

student["name"]              # "Asha"
student["marks"] = 91        # change it
student.get("email")         # None, rather than an error

Use a dictionary when you want to look something up by name rather than by position.

That .get() is worth remembering now. student["email"] on a missing key raises an error and stops your program; .get() hands back None. Real data is full of missing keys, and which of those two behaviours you want is a decision you should make on purpose.

The question that decides it

Do I want to ask "what is the third one?" or "what is the city one?"

Position → list. Name → dictionary.

Nesting them is normal

students = [
    {"name": "Asha", "city": "Kochi", "marks": 88},
    {"name": "Ravi", "city": "Pune", "marks": 74},
]

students[1]["city"]   # "Pune"

That shape — a list of dictionaries — is what a table looks like in plain Python, and it is exactly what you get back when you read a CSV before you hand it to anything cleverer.

Read students[1]["city"] left to right: take the list, take item 1, take its city. Long chains of brackets are not mysterious; they are just several of those steps in a row.

Try this

Build a list of three dictionaries describing three things you own. Print the name of the second one. Then add a fourth to the list and print how many you have.

It takes two minutes and it is the whole of this lesson.