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Writing CSV Files in Python

Python's csv.writer and csv.DictWriter handle writing tabular data to CSV files, including proper quoting and escaping of special characters.

Basic Writing with csv.writer

import cs
                    
import csv

# Write rows as lists
with open("output.csv", "w", newline="") as f:
    writer = csv.writer(f)

    # Write header
    writer.writerow(["name", "age", "city"])

    # Write individual rows
    writer.writerow(["Alice", 30, "New York"])
    writer.writerow(["Bob", 25, "London"])

    # Write multiple rows at once
    more_data = [
        ["Charlie", 35, "Tokyo"],
        ["Diana", 28, "Paris"],
    ]
    writer.writerows(more_data)

# IMPORTANT: newline="" prevents double blank lines on Windows
Writing with csv.DictWriter
import csv

# Write from dictionaries — clearer and less error-prone
users = [
    {"name": "Alice", "email": "alice@example.com", "role": "admin"},
    {"name": "Bob", "email": "bob@example.com", "role": "user"},
    {"name": "Charlie", "email": "charlie@example.com", "role": "user"},
]

with open("users.csv", "w", newline="") as f:
    fieldnames = ["name", "email", "role"]
    writer = csv.DictWriter(f, fieldnames=fieldnames)

    writer.writeheader()    # writes the header row
    writer.writerows(users) # writes all data rows

# Handling missing keys with restval and extrasaction
with open("partial.csv", "w", newline="") as f:
    writer = csv.DictWriter(
        f,
        fieldnames=["name", "email", "phone"],
        restval="N/A",          # value for missing keys
        extrasaction="ignore"   # ignore extra keys in dicts
    )
    writer.writeheader()
    writer.writerow({"name": "Alice", "email": "alice@test.com"})
    # phone column will show "N/A"
Customizing Output Format
import csv

data = [
    ["Product", "Price", "Description"],
    ["Widget", 19.99, "A useful widget, very popular"],
    ["Gadget", 49.99, "High-tech gadget (\"premium\")"],
]

# Tab-separated output
with open("output.tsv", "w", newline="") as f:
    writer = csv.writer(f, delimiter="\t")
    writer.writerows(data)

# Quote all fields
with open("quoted.csv", "w", newline="") as f:
    writer = csv.writer(f, quoting=csv.QUOTE_ALL)
    writer.writerows(data)

# Quote only non-numeric fields
with open("smart_quoted.csv", "w", newline="") as f:
    writer = csv.writer(f, quoting=csv.QUOTE_NONNUMERIC)
    writer.writerows(data)

# Quoting options:
# csv.QUOTE_MINIMAL  — only quote when needed (default)
# csv.QUOTE_ALL      — quote every field
# csv.QUOTE_NONNUMERIC — quote strings, leave numbers unquoted
# csv.QUOTE_NONE     — never quote (use escapechar instead)
Appending to Existing CSV
import csv
import os

filename = "log.csv"

# Check if file exists to decide whether to write header
file_exists = os.path.exists(filename)

with open(filename, "a", newline="") as f:  # "a" for append
    writer = csv.writer(f)

    if not file_exists:
        writer.writerow(["timestamp", "event", "user"])

    writer.writerow(["2024-01-15 10:30", "login", "alice"])
    writer.writerow(["2024-01-15 10:35", "purchase", "alice"])
Practical: Export Database/API Results
import csv

# Convert a list of objects/dicts to CSV
def export_to_csv(data, filename, fields=None):
    """Export list of dicts to a CSV file."""
    if not data:
        print("No data to export")
        return

    # Auto-detect fields from first record if not provided
    if fields is None:
        fields = list(data[0].keys())

    with open(filename, "w", newline="") as f:
        writer = csv.DictWriter(f, fieldnames=fields)
        writer.writeheader()
        writer.writerows(data)

    print(f"Exported {len(data)} records to {filename}")

# Usage
orders = [
    {"id": 1, "product": "Widget", "qty": 5, "total": 99.95},
    {"id": 2, "product": "Gadget", "qty": 2, "total": 99.98},
    {"id": 3, "product": "Tool", "qty": 10, "total": 149.90},
]

export_to_csv(orders, "orders_export.csv")

# Export only specific columns
export_to_csv(orders, "summary.csv", fields=["product", "total"])
Always use newline="" when opening files for CSV writing to prevent blank rows.
Use csv.DictWriter for clarity — column order is explicit and data is self-documenting.
The csv module auto-handles quoting of values containing commas, quotes, or newlines.
Use append mode ("a") to add rows without overwriting existing data.
Set restval and extrasaction to handle incomplete or extra dictionary keys.