Rows as dictionaries

The problem with positions

In the last lesson, the minutes of a trip were row[4]. You must remember that 4 means minutes. When someone adds a column to the file, 4 means something else. Your code then gives wrong answers, and it shows no error.

DictReader

csv.DictReader reads the header row for you. It gives each other row as a dictionary. The keys are the names of the columns.

import csv

with open("pets.csv", newline="", encoding="utf-8") as file:
    pets = list(csv.DictReader(file))

print(pets[0])           # {'name': 'Biscuit', 'age': '4'}
print(pets[0]["name"])   # Biscuit
print(len(pets))         # 2

The list has no header row. Every item is a row of data. This shape is a list of dictionaries. It is the most common shape of data in Python, and you used it in the grade book project of the Python course.

Convert the numbers once

The values are still text. Convert each column of numbers one time, directly after you read the file. The rest of your code then works with real numbers.

for pet in pets:
    pet["age"] = int(pet["age"])

Do not convert a column that is a label. A trip id such as 1017 looks like a number, but you never add two ids. Keep it as text.

Look at the data first

Before you calculate, look at what you have.

print(len(pets))   # the number of rows
print(pets[0])     # the first row: the keys and the kinds of values

Find the row with the largest value

Keep the best row so far in a variable. Compare each row with it.

oldest = pets[0]
for pet in pets:
    if pet["age"] > oldest["age"]:
        oldest = pet

A function that loads a file

Almost every program starts with a file that it reads. Put the steps in a function. The function takes the name of the file and returns the clean rows.

def load_pets(file_name):
    with open(file_name, newline="", encoding="utf-8") as file:
        rows = list(csv.DictReader(file))
    for row in rows:
        row["age"] = int(row["age"])
    return rows

Your turn

The file trips.csv has 12 trips, and it has a new column, bike. Do the tasks in order.