Essential Standard Library Modules

import os

# Current directory
os.getcwd()          # Current working directory
os.chdir("/tmp")     # Change directory

# File operations
os.listdir(".")      # List directory contents
os.mkdir("new_dir")  # Create directory
os.makedirs("a/b/c", exist_ok=True)  # Create nested directories
os.remove("file.txt")
os.rename("old.txt", "new.txt")

# Environment variables
os.environ.get("HOME", "/default/path")
os.environ["MY_VAR"] = "value"

# Path info
os.path.join("dir", "file.txt")  # Platform-correct path
os.path.exists("file.txt")
os.path.isfile("file.txt")
os.path.isdir("mydir")

import sys

# Command-line arguments
# sys.argv = ["script.py", "--verbose", "input.txt"]
script_name = sys.argv[0]
args = sys.argv[1:]

# Exit program
sys.exit(0)      # Success
sys.exit(1)      # Error

# Python version
print(sys.version)        # 3.12.0 ...
print(sys.version_info)   # sys.version_info(major=3, minor=12, ...)

# Module search path
print(sys.path)   # Directories Python searches for imports

# Standard streams
sys.stdout.write("Hello\n")
sys.stderr.write("Error!\n")
data = sys.stdin.read()

from datetime import datetime, date, time, timedelta, timezone

# Current time
now = datetime.now()
today = date.today()

# Create specific dates
d = date(2024, 12, 25)
t = time(14, 30, 0)
dt = datetime(2024, 12, 25, 14, 30)

# Formatting
now.strftime("%Y-%m-%d %H:%M:%S")      # '2024-12-25 14:30:00'
now.strftime("%A, %B %d")               # 'Wednesday, December 25'

# Parsing
parsed = datetime.strptime("2024-01-15", "%Y-%m-%d")

# Arithmetic
tomorrow = today + timedelta(days=1)
next_week = today + timedelta(weeks=1)
age = today - date(1990, 5, 15)

# Timezone aware
from zoneinfo import ZoneInfo  # Python 3.9+
dt_utc = datetime.now(ZoneInfo("UTC"))
dt_ny = dt_utc.astimezone(ZoneInfo("America/New_York"))

import json

data = {"name": "Alice", "scores": [85, 92, 78], "active": True}

# String conversion
json_str = json.dumps(data, indent=2)
parsed = json.loads(json_str)

# File operations
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

with open("data.json") as f:
    data = json.load(f)

# Custom serialization
from datetime import datetime

def json_serializer(obj):
    if isinstance(obj, datetime):
        return obj.isoformat()
    raise TypeError(f"Type {type(obj)} not serializable")

data = {"time": datetime.now()}
json.dumps(data, default=json_serializer)

import re

# Basic matching
pattern = r"\d+"  # One or more digits
text = "Order 123: $45.99"

match = re.search(pattern, text)
if match:
    print(match.group())  # '123'

# All matches
re.findall(r"\d+", text)        # ['123', '45', '99']
re.findall(r"\d+\.\d+", text)   # ['45.99']

# Substitution
re.sub(r"\d+", "NUM", text)     # 'Order NUM: $NUM.NUM'

# Splitting
re.split(r"[,;\s]+", "a,b;c d")  # ['a', 'b', 'c', 'd']

# Named groups
pattern = r"(?P<area>\d{3})-(?P<num>\d{4})"
m = re.search(pattern, "Call 555-1234")
print(m.group("area"))  # '555'

# Compile for performance
phone_pattern = re.compile(r"\d{3}-\d{4}")
phone_pattern.findall("555-1234, 555-5678")

# Common patterns
EMAIL = r"[\w.+-]+@[\w-]+\.[\w.]+"
URL = r"https?://[\w./-]+"
PHONE = r"\d{3}[-.]?\d{3}[-.]?\d{4}"

from collections import Counter, defaultdict, namedtuple, deque, OrderedDict

# Counter — count occurrences
colors = ["red", "blue", "red", "green", "blue", "blue"]
counts = Counter(colors)
print(counts)             # Counter({'blue': 3, 'red': 2, 'green': 1})
print(counts.most_common(2))  # [('blue', 3), ('red', 2)]

# defaultdict — default values for missing keys
dd = defaultdict(list)
dd["a"].append(1)         # No KeyError — auto-creates list

# namedtuple — lightweight objects
Point = namedtuple("Point", ["x", "y"])
p = Point(3, 4)
print(p.x, p.y)           # 3 4

# deque — double-ended queue
dq = deque([1, 2, 3])
dq.appendleft(0)          # deque([0, 1, 2, 3])
dq.append(4)              # deque([0, 1, 2, 3, 4])
dq.popleft()              # 0

from itertools import chain, product, permutations, combinations, groupby, cycle

# chain — combine iterables
list(chain([1, 2], [3, 4]))  # [1, 2, 3, 4]

# product — Cartesian product
list(product([1, 2], ["a", "b"]))
# [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b')]

# permutations
list(permutations([1, 2, 3], 2))
# [(1, 2), (1, 3), (2, 1), (2, 3), (3, 1), (3, 2)]

# combinations
list(combinations([1, 2, 3], 2))
# [(1, 2), (1, 3), (2, 3)]

# groupby — group sorted data
data = [("a", 1), ("a", 2), ("b", 3)]
for key, group in groupby(data, lambda x: x[0]):
    print(key, list(group))

# cycle — infinite loop
colors = cycle(["red", "green", "blue"])
next(colors)  # 'red'
next(colors)  # 'green'

import math
import random
import statistics

# math
math.pi           # 3.14159...
math.sqrt(16)     # 4.0
math.ceil(3.2)    # 4
math.floor(3.8)   # 3

# random
random.randint(1, 10)     # Random integer 1-10
random.choice(["a", "b", "c"])  # Random element
random.shuffle(items)      # Shuffle in place
random.sample(population, 3)  # 3 unique elements

# statistics
data = [2, 4, 4, 4, 5, 5, 7, 9]
statistics.mean(data)      # 5.0
statistics.median(data)    # 4.5
statistics.mode(data)      # 4
statistics.stdev(data)     # Standard deviation

import hashlib
import secrets

# Hashing
text = "hello".encode()
hashlib.md5(text).hexdigest()
hashlib.sha256(text).hexdigest()

# Secure random tokens
secrets.token_hex(16)       # 32-char hex string
secrets.token_urlsafe(16)   # URL-safe base64
secrets.choice("abcd")      # Cryptographically secure choice