Python Basics: Reading and Writing CSV with Headers

CSV files often contain a header row followed by records. Python’s standard-library csv module handles quoted fields, including values that contain commas.

Use Python 3 on Linux. Run the shell commands in order in the same shell, starting in a fresh temporary directory:

cd "$(mktemp -d)"

Read records by column name

Create a practice CSV file and read it with DictReader:

Save as stock.csv:

name,quantity
"Notebook, blue",2
Pencil,5

Save as demo_csv_1.py:

import csv

with open("stock.csv", newline="", encoding="utf-8") as file:
    for row in csv.DictReader(file):
        print(f'{row["name"]}: {row["quantity"]}')

Run:

python3 demo_csv_1.py

Output:

Notebook, blue: 2
Pencil: 5

DictReader uses the first row as dictionary keys. The quoted comma stays inside the notebook’s name. Splitting each line with split(",") would break that field.

Open CSV files with newline="" so the CSV module handles line endings itself.

Convert numeric fields

CSV fields are strings by default. Convert quantities before adding them:

Save as demo_csv_2.py:

import csv

with open("stock.csv", newline="", encoding="utf-8") as file:
    total = sum(int(row["quantity"]) for row in csv.DictReader(file))

print(total)

Run:

python3 demo_csv_2.py

Output:

7

int() raises ValueError for a blank or nonnumeric quantity. For incoming files, check the headers and validate each row before relying on it.

Write a new file

This creates summary.csv. Mode "w" overwrites that file if it already exists; keep this example in the practice directory.

Save as demo_csv_3.py:

import csv

with open("summary.csv", "w", newline="", encoding="utf-8") as file:
    writer = csv.DictWriter(file, fieldnames=["name", "quantity"])
    writer.writeheader()
    writer.writerow({"name": "Notebook, blue", "quantity": 2})

with open("summary.csv", newline="", encoding="utf-8") as file:
    for row in csv.DictReader(file):
        print(row["name"], row["quantity"])

Run:

python3 demo_csv_3.py

Output:

Notebook, blue 2

fieldnames controls column order. The writer adds quoting where needed; you do not need to insert quotes around the notebook’s name yourself. The default format uses commas, so specify a different delimiter when your input format requires one.

Reference

Python csv documentation

Acknowledgements

This post was created with assistance from AI.