How do I read a CSV file in Python?

Use the csv module from the standard library. DictReader reads the header line for you and gives back each row as a dictionary, so you write row["city"] instead of remembering that the city was column 2.

# Make a small file to read. Skip this part if you already have one.
with open("people.csv", "w", newline="", encoding="utf-8") as f:
    f.write("name,city,age\nAda,London,36\nLinus,Helsinki,28\nGrace,New York,45\n")

import csv

with open("people.csv", newline="", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        print(row["name"], "lives in", row["city"])

Ada lives in London
Linus lives in Helsinki
Grace lives in New York

Every value is a string

The CSV format has no idea what a number is. "36" comes back as text, and adding to it fails or, worse, quietly does string things.

import csv, io

data = io.StringIO("name,age\nAda,36\nLinus,28\n")   # behaves like an open file
ages = [int(row["age"]) for row in csv.DictReader(data)]
print(sum(ages) / len(ages))

32.0

Convert at the moment you read, once, and the rest of the program can trust the types.

With pandas, for anything you'll analyse

If the next thing you'll do is filter, group or average, read it straight into a DataFrame. pandas also works out the column types for you. (The first Run here takes a few seconds while pandas downloads.)

import io
import pandas as pd

data = io.StringIO("name,city,age\nAda,London,36\nLinus,Helsinki,28\nGrace,New York,45\n")
df = pd.read_csv(data)          # or pd.read_csv("people.csv")
print(df["age"].mean())
print(df[df["age"] > 30]["name"].tolist())

36.333333333333336
['Ada', 'Grace']

The two things that bite

• A file saved from Excel often starts with an invisible byte-order mark, and your first column turns into '\ufeffname', so row["name"] raises KeyError. Open it with encoding="utf-8-sig" and the mark is dropped.
• UnicodeDecodeError means the file isn't UTF-8 at all, usually an old Windows export. Try encoding="cp1252"; the error page explains how to tell.

Always pass newline="" when you open a CSV. The csv module handles line endings itself, and without it a quoted field containing a line break gets split in two.