csv - CSV File Reading and Writing

CSV file reading and writing

The csv module implements classes to read and write tabular data in CSV format.

Module Import

import csv

Reading CSV Files

csv.reader()

import csv

with open('data.csv', 'r') as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)  # List of values

DictReader

import csv

with open('data.csv', 'r') as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row)  # OrderedDict with column names as keys
        print(row['name'], row['age'])

Writing CSV Files

csv.writer()

import csv

data = [
    ['Name', 'Age', 'City'],
    ['Alice', 30, 'NYC'],
    ['Bob', 25, 'LA'],
]

with open('output.csv', 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(data)

DictWriter

import csv

data = [
    {'name': 'Alice', 'age': 30, 'city': 'NYC'},
    {'name': 'Bob', 'age': 25, 'city': 'LA'},
]

with open('output.csv', 'w', newline='') as f:
    fieldnames = ['name', 'age', 'city']
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    
    writer.writeheader()
    writer.writerows(data)

Dialects and Formatting

import csv

# Custom delimiter
with open('data.tsv', 'r') as f:
    reader = csv.reader(f, delimiter='\t')
    for row in reader:
        print(row)

# Quote all fields
with open('output.csv', 'w', newline='') as f:
    writer = csv.writer(f, quoting=csv.QUOTE_ALL)
    writer.writerow(['Name', 'Description'])
    writer.writerow(['Product', 'This is a "quoted" field'])

Practical Example

import csv

# Process CSV data
with open('sales.csv', 'r') as f:
    reader = csv.DictReader(f)
    total = sum(float(row['amount']) for row in reader)
print(f"Total sales: ${total}")

# Export data to CSV
data = [
    {'product': 'Widget', 'price': 10.50, 'quantity': 5},
    {'product': 'Gadget', 'price': 25.00, 'quantity': 3},
]

with open('export.csv', 'w', newline='') as f:
    writer = csv.DictWriter(f, fieldnames=['product', 'price', 'quantity'])
    writer.writeheader()
    writer.writerows(data)

json

JSON encoding/decoding

io

Core I/O operations