付録A — 旧 Python コードをモダンスタイルに移す

print "hello"
print "x =", x

print("hello")
print("x =", x)

"hi %s, you are %d" % (name, age)

"hi {}, you are {}".format(name, age)
"hi {name}, you are {age}".format(name=name, age=age)

f"hi {name}, you are {age}"
f"{name = }, {age = }"   # デバッグ用

5 / 2     # 2  (整数同士の除算が整数)
5.0 / 2   # 2.5

5 / 2     # 2.5  (常に float)
5 // 2    # 2    (整数除算は // で明示)

if d.has_key("user"):
    ...

if "user" in d:
    ...

try:
    do_work()
except Exception, e:
    print "failed:", e

try:
    do_work()
except Exception as e:
    print(f"failed: {e}")

for i in xrange(10):
    ...

for i in range(10):
    ...

s = u"한글"     # u プレフィックスが必要
isinstance(s, unicode)

s = "한글"      # str がそのまま Unicode
isinstance(s, str)

if type(x) == int:
    ...

if isinstance(x, int):
    ...

class User(object):
    def __init__(self, id, name, email):
        self.id = id
        self.name = name
        self.email = email

    def __repr__(self):
        return "User(id=%r, name=%r, email=%r)" % (self.id, self.name, self.email)

    def __eq__(self, other):
        if not isinstance(other, User):
            return False
        return (self.id, self.name, self.email) == (other.id, other.name, other.email)

from dataclasses import dataclass

@dataclass
class User:
    id: int
    name: str
    email: str

def add(a, b):
    """
    :param a: 一つ目の整数
    :type a: int
    :param b: 二つ目の整数
    :type b: int
    :rtype: int
    """
    return a + b

def add(a: int, b: int) -> int:
    """一つ目と二つ目を足して返します。"""
    return a + b

from typing import List, Dict, Tuple, Set

names: List[str] = []
ages: Dict[str, int] = {}
point: Tuple[float, float] = (0.0, 0.0)
unique: Set[str] = set()

names: list[str] = []
ages: dict[str, int] = {}
point: tuple[float, float] = (0.0, 0.0)
unique: set[str] = set()

from typing import Optional, Union

def find(id: int) -> Optional[str]:
    ...

def parse(s: str) -> Union[int, float, None]:
    ...

def find(id: int) -> str | None:
    ...

def parse(s: str) -> int | float | None:
    ...

import os
config_path = os.path.join(os.path.dirname(__file__), "config", "app.toml")
with open(config_path, "r") as f:
    data = f.read()

from pathlib import Path
config_path = Path(__file__).parent / "config" / "app.toml"
data = config_path.read_text()

import os
os.system("ls -la /tmp")          # 結果を受け取れない、セキュリティリスク

from subprocess import Popen, PIPE
proc = Popen(["ls", "-la", "/tmp"], stdout=PIPE, stderr=PIPE)
out, err = proc.communicate()

import subprocess
result = subprocess.run(
    ["ls", "-la", "/tmp"],
    capture_output=True,
    text=True,
    check=True,
)
print(result.stdout)

import asyncio

@asyncio.coroutine
def fetch():
    response = yield from get_url("https://example.com")
    return response

async def fetch():
    response = await get_url("https://example.com")
    return response

uv tool install pyupgrade
pyupgrade --py312-plus path/to/file.py

[tool.ruff.lint]
select = ["UP"]    # pyupgrade ルール有効化

uv run ruff check . --select UP --fix

import os

class User(object):
    def __init__(self, id, name, email):
        self.id = id
        self.name = name
        self.email = email

    def __repr__(self):
        return "User(id=%r, name=%r)" % (self.id, self.name)

def load_users(path):
    """
    :param path: users.txt のパス
    :type path: str
    :rtype: List[User]
    """
    full = os.path.join(os.path.dirname(__file__), path)
    users = []
    f = open(full, "r")
    try:
        for line in f:
            parts = line.strip().split(",")
            if len(parts) != 3:
                continue
            id_str, name, email = parts
            users.append(User(int(id_str), name, email))
    finally:
        f.close()
    return users