Python Write and Create Files

Python Write and Create Files: Modes, Buffering, and Safe Writes

Writing files in Python works through open() with mode "w" (overwrite/create), "a" (append), or "x" (create only if missing). Python's .write() method writes raw text exactly as provided and does not add a newline character automatically.

When saving configuration or state files in production, writing to a temporary file first and replacing the target prevents corrupted half-written files if a crash occurs mid-write.

Writing, Appending, and Exclusive Creation

Notice how "x" protects existing files by raising FileExistsError if the target path already exists.

from pathlib import Path

report = Path("daily_report.txt")

# Overwrite or create
with report.open("w", encoding="utf-8") as f:
    f.write("Daily Summary
")
    f.writelines(["Orders: 14
", "Errors: 0
"])

# Append a new line without erasing existing text
with report.open("a", encoding="utf-8") as f:
    f.write("Status: Verified
")

print(report.read_text(encoding="utf-8"))

Safe Atomic File Updates

To avoid corrupting a critical JSON or config file if power fails mid-write, write to target.with_suffix(".tmp") and call tmp_path.replace(target)—OS rename operations replace targets atomically.

Common Mistakes to Avoid

• Expecting f.writelines() to insert newlines: writelines(["a", "b"]) outputs "ab" unless each string ends with " ".
• Using "w" when you wanted "a": Opening a log file with "w" erases every previous log entry immediately.
• Writing non-string values directly: f.write(42) raises TypeError; convert numbers with str(42) or use print(42, file=f).

Frequently Asked Questions

What does "x" mode do when creating a file?

"x" creates a new file exclusively and raises FileExistsError if a file already exists at that path, avoiding accidental overwrites.

How do I write UTF-8 text in one line with pathlib?

Call Path("notes.txt").write_text("Hello ", encoding="utf-8"), which opens, writes, and closes the file automatically.

When is buffered output actually written to disk?

Python buffers writes in memory until the buffer fills, f.flush() is called, or the with block exits and closes the file.