Python Write & Create Files

Opening Files for Writing

Use 'w', 'a', or 'x' modes depending on your goal:

• 'w': Overwrite existing file or create a new one.
• 'a': Append to the end; creates the file if it does not exist.
• 'x': Create new file and raise an error if it already exists.

with open("notes.txt", "w", encoding="utf-8") as file:
    file.write("Project ideas\n")

Opening in text mode replaces newline characters with the system-specific sequence when writing.

Writing Strings

file.write() stores a string and returns the number of characters written. Remember to include newline characters manually.

tasks = ["Refactor", "Review", "Deploy"]

with open("tasks.txt", "w", encoding="utf-8") as file:
    for task in tasks:
        file.write(f"{task}\n")

Writing Multiple Lines

Use file.writelines(iterable) to write several strings without automatically adding newline characters.

lines = ["First line\n", "Second line\n", "Third line\n"]

with open("output.txt", "w", encoding="utf-8") as file:
    file.writelines(lines)

Ensure each element already contains newline characters if you want line breaks.

Append Mode

Preserve existing content and add new data to the end of the file with append mode.

with open("events.log", "a", encoding="utf-8") as file:
    file.write("INFO: Service restarted\n")

Append mode moves the file pointer to the end automatically, so no seek() call is required.

Binary Mode Writing

Write binary data (images, audio, serialized bytes) with 'wb' or 'ab'. Data must be bytes-like.

data = bytes([0x89, 0x50, 0x4E, 0x47])

with open("header.bin", "wb") as file:
    file.write(data)

When converting strings to bytes, specify the encoding: text.encode('utf-8').

Structured Data Formats

Leverage dedicated libraries to serialize dictionaries, lists, and other objects.

• json.dump(obj, file, indent=2) for JSON.
• csv.writer(file) or csv.DictWriter for CSV.
• pickle.dump(obj, file) for Python-specific binary serialization (trusted input only).

import json

payload = {"email": "info.studygrid@gmail.com", "active": True}

with open("user.json", "w", encoding="utf-8") as file:
    json.dump(payload, file, indent=2)

File Creation Safeguards

Use the exclusive creation mode to avoid overwriting important data.

try:
    with open("archive.zip", "x") as file:
        file.write("binary data")
except FileExistsError:
    print("Archive already exists")

Alternatively, check with Path.exists() before writing.

Flushing and Buffering

Python buffers writes for performance. Call file.flush() to force writing buffered data to disk, or open the file with buffering=1 for line buffering.

with open("live.log", "a", encoding="utf-8") as file:
    file.write("User signed in\n")
    file.flush()

Error Handling

Intercept PermissionError, FileExistsError, or OSError to deliver actionable feedback.

from pathlib import Path

path = Path("/restricted/log.txt")

try:
    with path.open("w", encoding="utf-8") as file:
        file.write("Audit trail\n")
except PermissionError:
    print("Cannot write to the target directory.")

Best Practices

• Specify encoding="utf-8" for text output.
• Write to temporary files first and atomically replace the original to avoid corruption.
• Lock critical files when writing from multiple processes (use filelock or platform APIs).
• Validate user-supplied filenames to prevent directory traversal.

Next Steps

Learn how to remove obsolete files, clear directories, and guard against accidental deletions in the next chapter.

write vs writelines, and Newlines

write() does not add a newline — you must include \n yourself. writelines() writes a list of strings (also without adding newlines).

lines = ["apple\n", "banana\n", "cherry\n"]

with open("fruits.txt", "w", encoding="utf-8") as f:
    f.writelines(lines)

# print() to a file adds newlines for you
with open("fruits.txt", "a", encoding="utf-8") as f:
    print("date", file=f)

Write vs Append — Don't Lose Data

with open("log.txt", "a", encoding="utf-8") as f:   # append, keep history
    f.write("2026-08-18: started\n")

Key Takeaways

• write() needs explicit \n; print(..., file=f) adds it.
• "w" truncates; "a" appends.
• Use with and encoding="utf-8".