Python cheat sheet

import json
from dataclasses import asdict, dataclass, field
 
 
@dataclass(order=True)
class Book:
    title: str
    author: str = field(compare=False)
    pages: int = field(default=0, compare=False)
    tags: list[str] = field(default_factory=list, compare=False)
 
    def __post_init__(self):
        if self.pages < 0:
            raise ValueError(f"pages must be 0 or more, got {self.pages}")
 
    @property
    def is_long(self) -> bool:
        return self.pages > 500
 
 
shelf = [
    Book("Dune", "Frank Herbert", 688, ["sci-fi"]),
    Book("Beloved", "Toni Morrison", 324),
    Book("Circe", "Madeline Miller", 393, ["myth"]),
]
 
print(shelf[1])
# Book(title='Beloved', author='Toni Morrison', pages=324, tags=[])
 
print([b.title for b in sorted(shelf)])  # ['Beloved', 'Circe', 'Dune']
print([b.title for b in shelf if b.is_long])  # ['Dune']
print(Book("Dune", "Someone else") == shelf[0])  # True: only title is compared
 
print(json.dumps(asdict(shelf[2])))
# {"title": "Circe", "author": "Madeline Miller", "pages": 393, "tags": ["myth"]}
 
try:
    Book("Oops", "Nobody", -1)
except ValueError as e:
    print(e)  # pages must be 0 or more, got -1

import functools
import time
 
 
def describe(first, *args, sep=", ", **kwargs):
    print(f"first={first!r} args={args} sep={sep!r} kwargs={kwargs}")
 
 
describe(1, 2, 3, colour="red")
# first=1 args=(2, 3) sep=', ' kwargs={'colour': 'red'}
 
numbers = [4, 5, 6]
options = {"sep": " | ", "debug": True}
describe(*numbers, **options)
# first=4 args=(5, 6) sep=' | ' kwargs={'debug': True}
 
 
def timed(func):
    @functools.wraps(func)  # keeps the wrapped function's name and docstring
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        try:
            return func(*args, **kwargs)
        finally:
            elapsed = time.perf_counter() - start
            print(f"{func.__name__} took {elapsed:.3f}s")
 
    return wrapper
 
 
@timed
def slow_add(a, b, *, delay=0.1):
    time.sleep(delay)
    return a + b
 
 
print(slow_add(2, 3, delay=0.2))
# slow_add took 0.200s
# 5

import csv
import json
from collections import defaultdict
from pathlib import Path
 
data = Path("data")
data.mkdir(exist_ok=True)
 
sales_csv = data / "sales.csv"
sales_csv.write_text(
    "region,product,amount\n"
    "north,tea,12.50\n"
    "south,tea,8.00\n"
    "north,coffee,20.25\n",
    encoding="utf-8",
)
 
totals = defaultdict(float)
with sales_csv.open(newline="", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        totals[row["region"]] += float(row["amount"])  # CSV values are strings
 
report = data / "totals.json"
report.write_text(json.dumps(totals, indent=2), encoding="utf-8")
 
print(dict(totals))  # {'north': 32.75, 'south': 8.0}
print(report.name, report.stat().st_size, "bytes")  # totals.json 36 bytes
 
for path in sorted(data.glob("*")):
    print(path.suffix, path.stem)
# .csv sales
# .json totals

from dataclasses import dataclass
 
 
@dataclass
class Point:
    x: int
    y: int
 
 
def handle(event):
    match event:
        case {"type": "click", "pos": Point(x=0, y=0)}:
            return "click at the origin"
        case {"type": "click", "pos": Point(x=x, y=y)}:
            return f"click at {x}, {y}"
        case {"type": "key", "key": "q" | "escape"}:
            return "quit"
        case {"type": "key", "key": str(key)} if len(key) == 1:
            return f"typed {key}"
        case [first, *rest]:
            return f"batch of {1 + len(rest)}, starting with {handle(first)}"
        case _:
            return "unknown event"
 
 
print(handle({"type": "click", "pos": Point(0, 0)}))  # click at the origin
print(handle({"type": "click", "pos": Point(3, 4)}))  # click at 3, 4
print(handle({"type": "key", "key": "escape"}))  # quit
print(handle({"type": "key", "key": "a", "shift": True}))  # typed a
print(handle([{"type": "key", "key": "q"}, {}]))  # batch of 2, starting with quit
print(handle("hello"))  # unknown event

mkdir weather && cd weather
python3 -m venv .venv
source .venv/bin/activate
 
python -m pip install requests
python -m pip freeze > requirements.txt
 
echo ".venv/" >> .gitignore
git init && git add . && git commit -m "Start weather project"
 
deactivate

python3 -m venv .venv
source .venv/bin/activate
python -m pip install -r requirements.txt