CSV

The csv module provides functionality for reading and writing CSV (Comma-Separated Values) files.

import csv

Warning

Python 3 File Opening In Python 3, open CSV files with newline='' to prevent blank rows on Windows. Use text mode ("r" / "w") instead of binary mode ("rb" / "wb" which was required in Python 2).

Reading as Dictionary

csv.DictReader reads each row into a dictionary where keys are the column headers.

import csv

with open("data.csv", "r", newline='') as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["age"])

Dialect Parameters

You can customise how the CSV is parsed using dialect parameters:

Parameter	Default	Description
delimiter	,	Character that separates fields
doublequote	True	Whether quotechar inside a field is doubled
escapechar	None	Character used to escape the delimiter
lineterminator	\r\n	String used to terminate lines
quotechar	"	Character used to quote fields
quoting	QUOTE_MINIMAL	Controls when quotes are generated
skipinitialspace	False	Whether whitespace after delimiter is ignored

with open("data.csv", "r", newline='') as f:
    reader = csv.DictReader(f, delimiter=";", quotechar="'")
    for row in reader:
        print(row)

Reading as Strings

You can also read CSV files using basic file methods without the csv module:

# Read line by line
with open("data.csv", "r") as f:
    line = f.readline()        # Read one line
    print(line.strip())        # Remove trailing newline

# Read all lines at once
with open("data.csv", "r") as f:
    lines = f.readlines()
    for line in lines:
        fields = line.strip().split(",")
        print(fields)

Writing as List

csv.writer writes rows as lists.

import csv

header = ["name", "age", "city"]
rows = [
    ["Alice", 30, "London"],
    ["Bob", 25, "Paris"],
    ["Charlie", 35, "Berlin"],
]

with open("output.csv", "w", newline='') as f:
    writer = csv.writer(f)
    writer.writerow(header)   # Write a single row
    writer.writerows(rows)    # Write multiple rows

Writing as Dictionary

csv.DictWriter writes rows from dictionaries.

import csv

header = ["name", "age", "city"]
rows = [
    {"name": "Alice", "age": 30, "city": "London"},
    {"name": "Bob", "age": 25, "city": "Paris"},
]

with open("output.csv", "w", newline='') as f:
    writer = csv.DictWriter(f, fieldnames=header)
    writer.writeheader()
    writer.writerows(rows)

Sorting by Column

Read the CSV, sort the data, and write it back:

import csv

# Read data
with open("data.csv", "r", newline='') as f:
    reader = csv.DictReader(f)
    rows = list(reader)
    fieldnames = reader.fieldnames

# Sort by the "age" column (convert to int for numeric sort)
rows.sort(key=lambda row: int(row["age"]))

# Write sorted data
with open("sorted.csv", "w", newline='') as f:
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerows(rows)