Standard Library Quick Reference

  import os, sys

os.environ["HOME"]           # environment variables
os.getcwd(), os.chdir(path)
os.listdir(path)
os.path.join("a", "b", "c")
os.makedirs("dir/sub", exist_ok=True)
os.remove("file"), os.rename("old", "new")

sys.argv                     # command-line arguments
sys.exit(1)                  # exit with code
sys.path                     # module search paths
sys.platform                 # 'darwin', 'linux', 'win32'
  

  from pathlib import Path

p = Path("data/file.txt")
p.exists(), p.is_file(), p.is_dir()
p.read_text(), p.write_text("content")
p.read_bytes(), p.write_bytes(b"data")
p.parent, p.name, p.stem, p.suffix
p.glob("*.py"), p.rglob("**/*.py")
Path.home(), Path.cwd()
  

  import json

json.dumps(obj, indent=2)    # Python → string
json.loads(string)           # string → Python
json.dump(obj, file)
json.load(file)
  

  from datetime import datetime, date, timedelta, timezone

now = datetime.now()
today = date.today()
utc = datetime.now(timezone.utc)

delta = timedelta(days=7, hours=3)
future = now + delta

now.strftime("%Y-%m-%d %H:%M:%S")
datetime.fromisoformat("2024-06-15T10:30:00")
  

  from collections import (
    defaultdict, Counter, deque,
    namedtuple, OrderedDict, ChainMap,
)

dd = defaultdict(list)
dd["key"].append(val)

counts = Counter("abracadabra")
counts.most_common(3)

queue = deque([1, 2, 3])
queue.append(4), queue.popleft()

Point = namedtuple("Point", ["x", "y"])
p = Point(1, 2)
  

  from itertools import (
    chain, combinations, permutations,
    groupby, islice, cycle, repeat,
    zip_longest,
)

chain([1, 2], [3, 4])              # 1, 2, 3, 4
combinations([1,2,3], 2)           # (1,2), (1,3), (2,3)
groupby(sorted(data), key=func)
islice(range(100), 5, 10)          # 5..9
  

  from functools import lru_cache, partial, wraps, reduce

@lru_cache(maxsize=128)
def fib(n): ...

add_five = partial(add, 5)
reduce(lambda a, b: a + b, [1, 2, 3])
  

  import re

re.search(r"\d+", text)          # first match
re.findall(r"\w+", text)          # all matches
re.sub(r"\d+", "X", text)         # replace
re.split(r"\s+", text)           # split
re.compile(r"pattern")            # compile once
  

  import argparse

parser = argparse.ArgumentParser()
parser.add_argument("name")
parser.add_argument("-v", "--verbose", action="store_true")
parser.add_argument("-n", type=int, default=1)
args = parser.parse_args()
  

  import logging

logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)
logger.debug/info/warning/error/critical("message")
logger.exception("with traceback")
  

  import unittest

class TestMath(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(1, 2), 3)

# pytest style
def test_add():
    assert add(1, 2) == 3
  

  import hashlib, secrets

hashlib.sha256(b"data").hexdigest()
secrets.token_hex(16)            # random token
secrets.compare_digest(a, b)     # timing-safe compare
  

  from urllib.parse import urlparse, urlencode, quote
from urllib.request import urlopen

parsed = urlparse("https://example.com/path?q=1")
params = urlencode({"key": "value"})
  

  import sqlite3

conn = sqlite3.connect("app.db")
cursor = conn.cursor()
cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))
rows = cursor.fetchall()
conn.commit()
conn.close()
  

  import threading, multiprocessing
from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor

with ThreadPoolExecutor(max_workers=4) as ex:
    results = list(ex.map(func, items))