File I/O in Python

Read and write files safely with context managers, explicit encodings, and line-oriented streaming.

Quick read/write

The built-in open returns a file object. Always specify the mode and encoding for text.

text = Path("notes.txt").read_text(encoding="utf-8")
Path("out.txt").write_text("hello\n", encoding="utf-8")

Or with open:

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

Modes and encodings

Common modes: "r" read, "w" write (truncate), "a" append, "x" create, "b" binary, "t" text (default). Always set encoding for text files.

with open("data.bin", "wb") as f:
    f.write(b"\x00\x01")

Streaming line by line

Iterate files to avoid loading everything into memory.

with open("big.csv", encoding="utf-8") as f:
    for line in f:
        cols = line.rstrip("\n").split(",")

newline="" preserves newlines exactly (important for csv module on Windows).

Pathlib (recommended)

Use pathlib.Path for portable path operations.

from pathlib import Path
p = Path("/tmp") / "file.txt"
if p.exists():
    data = p.read_bytes()

JSON and CSV

Use stdlib helpers for structured text.

import json, csv
obj = json.loads('{"a":1}')
with open("data.json", "w", encoding="utf-8") as f:
    json.dump(obj, f, ensure_ascii=False, indent=2)

with open("rows.csv", "w", newline="", encoding="utf-8") as f:
    w = csv.writer(f)
    w.writerow(["a", 1])

Temporary files and directories

Use tempfile to create safe temporary files.

import tempfile
with tempfile.NamedTemporaryFile(mode="w+", delete=False) as tmp:
    tmp.write("data")
    name = tmp.name

Errors and cleanup

with closes files even if an exception occurs. Handle FileNotFoundError, PermissionError, etc., where appropriate.

Best practices

Always use context managers (with)
Specify encoding and, for CSV, newline=""
Stream large files; avoid read() on huge inputs
Prefer pathlib APIs for paths

Summary

Use open/with safely; be explicit with modes and encodings
Stream lines for scale; leverage stdlib (json, csv, pathlib, tempfile)