Fast references

squares = [x*x for x in range(10)]
evens   = [x for x in nums if x % 2 == 0]
pairs   = {k: v for k, v in items}
unique  = {x % 3 for x in nums}
gen     = (x*x for x in range(10))   # lazy generator

d.get(key, default)         # safe lookup
d.setdefault(key, [])       # get-or-create
d |= other                  # merge (3.9+)
for k, v in d.items(): ...
from collections import Counter, defaultdict

import asyncio

async def main():
    async with asyncio.TaskGroup() as tg:   # 3.11+
        tg.create_task(work(1))
        tg.create_task(work(2))

asyncio.run(main())

import pytest

@pytest.fixture
def client(): ...

@pytest.mark.parametrize("a,b,exp", [(1,2,3),(2,2,4)])
def test_add(a, b, exp):
    assert add(a, b) == exp

def test_raises():
    with pytest.raises(ValueError):
        parse("nope")