python CSV file data standard-library 30 apr 2026

The CSV Library in Python

Python’s csv library provides tools for working with CSV (Comma Separated Values) files, making it simple and effective to read, write and manipulate tabular data.

The library is included in Python’s standard library: nothing needs to be installed.

import csv

The examples in this guide use a file students.csv with columns: id, surname, name, subject, grade.

Reading with `csv.reader`

csv.reader returns an iterator that yields a list for each row of the file.

import csv

with open('students.csv', mode='r', newline='') as file:
    reader = csv.reader(file)
    header = next(reader)  # skip the header row
    for row in reader:
        print(row)
        # row is a list: ['12345', 'Rossi', 'Mario', 'Maths', '28']

next(reader) reads and discards the first row (header)
Each subsequent row is returned as a list of strings

Writing with `csv.writer`

csv.writer lets you write rows to a CSV file.

import csv

student_data = [
    ['12345', 'Rossi',   'Mario', 'Maths',   '28'],
    ['67890', 'Bianchi', 'Luca',  'Physics', '30'],
    ['54321', 'Verdi',   'Anna',  'Chemistry','27'],
]

with open('students_out.csv', mode='w', newline='') as file:
    writer = csv.writer(file)
    writer.writerow(['id', 'surname', 'name', 'subject', 'grade'])
    writer.writerows(student_data)

Method	Description
`writerow(list)`	Writes a single row
`writerows(list_of_lists)`	Writes multiple rows in a single call

Reading as a dictionary with `DictReader`

DictReader reads each row as a dictionary, using the first row of the file as keys.

import csv

with open('students.csv', mode='r', newline='') as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row['id'], row['surname'], row['name'],
              row['subject'], row['grade'])

Each row is a dictionary such as:

{'id': '12345', 'surname': 'Rossi', 'name': 'Mario', 'subject': 'Maths', 'grade': '28'}

Reading a CSV without a header

If the file has no header, specify the column names manually with fieldnames:

import csv

fields = ['id', 'surname', 'name', 'subject', 'grade']

with open('students_no_header.csv', mode='r', newline='') as file:
    reader = csv.DictReader(file, fieldnames=fields)
    for row in reader:
        print(row['id'], row['surname'], row['name'])

Writing from dictionaries with `DictWriter`

DictWriter writes rows from dictionaries.

import csv

student_data = [
    {'id': '12345', 'surname': 'Rossi',   'name': 'Mario', 'subject': 'Maths',    'grade': '28'},
    {'id': '67890', 'surname': 'Bianchi', 'name': 'Luca',  'subject': 'Physics',  'grade': '30'},
    {'id': '54321', 'surname': 'Verdi',   'name': 'Anna',  'subject': 'Chemistry','grade': '27'},
]

fields = ['id', 'surname', 'name', 'subject', 'grade']

with open('students_dict_out.csv', mode='w', newline='') as file:
    writer = csv.DictWriter(file, fieldnames=fields)
    writer.writeheader()          # writes the header row
    writer.writerows(student_data)

Class and method reference

Class / Function	Use	Data type
`csv.reader(file)`	Row-by-row reading	List per row
`csv.writer(file)`	Row-by-row writing	List per row
`csv.DictReader(file)`	Reading with column keys	Dict per row
`csv.DictWriter(file, fieldnames)`	Writing from dicts	Dict per row

Writer methods

Method	Description
`writerow(row)`	Writes one row (list or dictionary)
`writerows(rows)`	Writes multiple rows
`writeheader()`	Writes the header row (`DictWriter`)

Practical notes

Always use newline='' when opening files to avoid blank lines on Windows.
The 'w' mode overwrites the file; use 'a' to append.
Values are always strings: convert explicitly with int() or float() as needed.

for row in reader:
    grade = int(row['grade'])
    print(f"{row['name']} {row['surname']}: {grade}/30")

Reading with csv.reader

Writing with csv.writer

Reading as a dictionary with DictReader