19. The standard library you will use every week

Python ships "batteries included". Before you uv add a package, check whether the standard library already does it.

19.1 Dates, times, and time zones

from datetime import datetime, timedelta, timezone
from zoneinfo import ZoneInfo

now = datetime.now(timezone.utc)                     # always store and compare in UTC
blr = now.astimezone(ZoneInfo("Asia/Kolkata"))       # convert only for display
print(now.isoformat(timespec="seconds"), blr.strftime("%Y-%m-%d %H:%M %Z"))
deadline = now + timedelta(days=7)
print((deadline - now).total_seconds())
print(datetime.fromisoformat("2026-10-04T10:15:00+00:00"))
datetime.now() < now
# TypeError: can't compare offset-naive and offset-aware datetimes

Never use naive datetimes (no time zone) for anything that crosses a machine boundary. For measuring durations, use time.perf_counter() or time.monotonic(), never wall-clock time, which jumps when NTP corrects it.

19.2 logging

import logging

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s %(levelname)s %(name)s %(message)s",
)
log = logging.getLogger("billing")        # one logger per module: getLogger(__name__)

log.info("charging %s cents to %s", 1500, "acct-7")   # lazy %-formatting, not f-strings
try:
    1 / 0
except ZeroDivisionError:
    log.exception("charge failed")        # ERROR level + full traceback

Libraries call logging.getLogger(__name__) and never configure logging; applications configure it once at startup. Chapter 68 turns this into structured JSON logs with request IDs.

19.3 subprocess, shutil, tempfile, os

import shutil
import subprocess
import tempfile
from pathlib import Path

result = subprocess.run(
    ["git", "--version"], capture_output=True, text=True, check=False, timeout=10
)
print(result.returncode, result.stdout.strip())

with tempfile.TemporaryDirectory() as tmp:
    src = Path(tmp) / "a.txt"
    src.write_text("x")
    shutil.copy2(src, Path(tmp) / "b.txt")          # copy with metadata
    print(sorted(p.name for p in Path(tmp).iterdir()))
print(shutil.which("python3"), shutil.disk_usage("/").free // 2**30, "GiB free")

Chapter 34 covers subprocess properly, including why shell=True is dangerous.

19.4 re: regular expressions

import re

ipv4 = re.compile(r"\b(?:\d{1,3}\.){3}\d{1,3}\b")
text = "client 10.0.0.7 -> 203.0.113.9 port 443"
print(ipv4.findall(text))                      # ['10.0.0.7', '203.0.113.9']
m = re.fullmatch(r"(?P<user>[a-z_][a-z0-9_-]{0,31})", "deploy_bot")
print(m["user"] if m else "invalid")
print(re.sub(r"(?i)password=\S+", "password=***", "login user=ada password=hunter2"))

Always use raw strings (r"...") for patterns. Beware of catastrophic backtracking on untrusted input ((a+)+$), a denial-of-service class covered in Ch 47.

19.5 The rest of the essentials