File I/O

Overview

Python reads and writes files through file objects returned by open(). Always close files—or prefer context managers—to release system handles. The pathlib module offers a modern path API.

Syntax / Usage

from pathlib import Path

# Text read/write (context manager recommended)
with open("notes.txt", "r", encoding="utf-8") as f:
    content = f.read()

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello\n")

# Line by line
with open("data.csv") as f:
    for line in f:
        print(line.strip())

# pathlib
path = Path("logs/app.log")
path.parent.mkdir(parents=True, exist_ok=True)
path.write_text("started", encoding="utf-8")
text = path.read_text(encoding="utf-8")

# JSON
import json
data = json.loads(path.read_text())
path.write_text(json.dumps(data, indent=2))

Modes: r read, w write (truncate), a append, rb/wb binary.

Examples

Append a timestamped log entry:

from datetime import datetime
from pathlib import Path

log = Path("app.log")
entry = f"{datetime.now().isoformat()} - Server started\n"
with log.open("a", encoding="utf-8") as f:
    f.write(entry)

Load configuration safely:

import json
from pathlib import Path

config_path = Path("config.json")
if config_path.exists():
    config = json.loads(config_path.read_text())
else:
    config = {"debug": False}

Common Mistakes

• Omitting encoding="utf-8" and getting platform-dependent defaults on Windows
• Using read() on huge files—iterate line by line instead
• Forgetting that "w" overwrites existing files without warning
• Not handling FileNotFoundError when paths are user-supplied

See Also

python-exceptions python-context-managers python-modules