50 Python One-Liners for Files, JSON, CSV & APIs

Python One-Liners You’ll Use Weekly

Developers often underestimate how much Python can do in one line. From parsing JSON and iterating files to hitting APIs and transforming data — here’s a pack of 50 one-liners you’ll actually reuse every week.

Filesystem ops • Text & CSV

open('log.txt').read()                             # read file
open('data.txt', 'w').write('hello\n')             # write text
sum(1 for _ in open('file.txt'))                   # count lines
[x.strip() for x in open('file.txt')]              # list lines
import os; [f for f in os.listdir('.') if f.endswith('.py')]  # list .py files
import glob; files = glob.glob('**/*.csv', recursive=True)

CSV & text in one go:

import csv; rows = list(csv.DictReader(open('data.csv')))

JSON / YAML

import json; data = json.load(open('data.json'))   # read JSON
json.dump(data, open('out.json','w'), indent=2)    # write JSON
import yaml; yaml.safe_load(open('conf.yaml'))     # YAML to dict

Convert between formats:

json.dump(yaml.safe_load(open('conf.yaml')), open('conf.json','w'))

HTTP (requests)

import requests; data = requests.get('https://api.github.com').json()
r = requests.post('https://api.example.com', json={'ok':1}).json()

Quick file download:

import requests; open('img.jpg','wb').write(requests.get(url).content)

CLI (argparse)

import argparse; p=argparse.ArgumentParser(); p.add_argument('--n'); print(p.parse_args().n)

Small data tricks (itertools)

from itertools import groupby, combinations
list(combinations([1,2,3,4], 2))                   # pairs
{k: len(list(g)) for k,g in groupby('aaabbccc')}   # run-length encode

When to prefer one-liner vs function?

Use one-liners for scripts, data cleaning, or ad-hoc debugging. For anything reused — wrap it in a function with clear names and error handling.

Cross-platform file paths?

Always use pathlib.Path:

from pathlib import Path; Path.home() / 'Downloads'

Virtualenv / uv / poetry quickstart?

python -m venv .venv && source .venv/bin/activate
pip install requests
# or
uv venv && uv add requests

💡 See also: Regex Cheat Sheet

Save this post — it’s the perfect reference when you need just one line of Python magic.