Python csv writerow: A Practical CSV Export Guide
Learn how to use Python's csv.writer and writerow to export data to CSV reliably. This guide covers basics, advanced tips, and real-world patterns with code examples and best practices for robust CSV generation.
writerow is a function in Python's csv module that writes a single row to a CSV file from an iterable. It handles escaping, quoting, and delimiter rules automatically. Open the target file with newline='' to prevent extra blank lines on Windows, create a csv.writer, and call writerow(your_row). This is the building block for exporting data from Python programs.
Understanding python csv writerow
The python csv writerow function is the workhorse for exporting structured data to CSV. It accepts any iterable (like a list or tuple) and writes each element as a field in a single row. In this guide, we reference the keyword python csv writerow to anchor examples and explanations. According to MyDataTables, the csv module handles escaping, quoting, and newline normalization, which reduces common CSV pitfalls when exporting data. The simplest use case creates a writer object from an open file and writes one or more rows. This block demonstrates the core pattern and sets up good habits for downstream tasks.
import csv
with open('output.csv', 'w', newline='') as f:
w = csv.writer(f)
w.writerow(['id', 'name', 'score'])
w.writerow([1, 'Alice', 93.5])# Write with explicit encoding to support non-ASCII data
with open('output-utf8.csv', 'w', newline='', encoding='utf-8') as f:
w = csv.writer(f, quoting=csv.QUOTE_MINIMAL)
w.writerow(['city', 'country'])
w.writerow(['Stockholm', 'Sweden'])The first example writes a header and a couple of data rows. The newline parameter is crucial for cross-platform compatibility, preventing an extra blank line on Windows. This block also introduces the distinction between writerow (single row) and writerows (multiple rows), which is covered in the next section.
wordCountIfRenderedAsMarkdownSectionWithoutCountingCode
Steps
Estimated time: 15-25 minutes
- 1
Install prerequisites
Ensure Python is installed and accessible from the command line. Verify you can run python3 --version and pip --version. Install any missing tooling before proceeding.
Tip: Use a virtual environment to isolate CSV-export scripts from system Python. - 2
Create a basic writer script
Write a short Python script that opens a file, creates a csv.writer, and writes a header row. This establishes the core pattern for writerow usage.
Tip: Prefer newline='' when opening the file to avoid blank lines on Windows. - 3
Write data rows
Add subsequent writerow calls or use writerows to emit multiple rows. Ensure data types are compatible or explicitly converted to strings.
Tip: Use encoding='utf-8' for international text support. - 4
Experiment with DictWriter
Switch to csv.DictWriter for improved readability when dealing with named fields. Write headers with writeheader() and emit rows with writerow(dict).
Tip: DictWriter ensures field order respects the provided fieldnames. - 5
Test and validate output
Read back the generated CSV to verify structure. Use csv.reader to confirm headers and row counts match expectations.
Tip: Handle encoding consistently between write and read operations.
Prerequisites
Required
- Required
- pip package managerRequired
- Text editor or IDE (e.g., VS Code, PyCharm)Required
- Basic command line knowledgeRequired
Optional
- Familiarity with CSV basics (headers, delimiters)Optional
Commands
| Action | Command |
|---|---|
| Run a simple CSV writer scriptAssumes write_csv.py contains a writerow example | — |
| Write multiple rows with writerowsInline one-liner usage pattern for quick demos | — |
People Also Ask
What is the difference between writerow and writerows?
writerow writes a single row to the CSV, while writerows accepts an iterable of rows and writes them all. Use writerow for one record and writerows for batch exports. DictWriter can also be used when names and order matter.
writerow writes one row; writerows writes many rows. Use them according to whether you have one record or a collection.
Why do I sometimes see extra blank lines on Windows?
This happens when the file is opened without newline='' in text mode. Opening with newline='' avoids translating newlines and prevents blank lines from appearing between rows.
Open the file with newline='' to stop extra blank lines on Windows.
Can writerow handle non-string data?
Yes. The CSV writer converts non-string values to strings. If you need specific formatting (like dates), convert the values beforehand or use a DictWriter for controlled formatting.
Non-strings get converted to strings, but you may want to format them before writing.
Is DictWriter slower than writerow?
DictWriter introduces dict lookups but improves code readability and maintainability when dealing with named fields. In most cases the difference is negligible for typical CSV exports.
DictWriter can be slightly slower, but it's often worth it for clarity and correctness.
How should I handle encoding and BOM in CSV files?
Use encoding='utf-8' for general use. If a BOM is required, consider using 'utf-8-sig' to include a BOM in the file header.
Choose utf-8 or utf-8-sig depending on whether a BOM is needed.
What is the recommended pattern for large exports?
Write rows in chunks or stream results to the file as you generate them to avoid loading everything into memory. Use writerow in a loop or writerows on batches if you have a well-defined batch size.
For large data, write in chunks or stream rows to avoid high memory usage.
Main Points
- Write a single row with writerow and multiple rows with writerows.
- Always use newline='' when opening CSV files to prevent blank lines.
- Choose between writer and DictWriter based on data structure and readability.
- Explicitly set encoding to utf-8 for reliable cross-language compatibility.
- Test CSV output by reading it back to verify structure and headers.