Pythonで文字列を結合・繰り返す方法：+、join、f文字列、*の使い方

# 改行や空白の扱い
names = ["Alice", "Bob", "Carol"]
print(", ".join(names))           # 区切り: カンマ+スペース
print("\n".join(names))           # 区切り: 改行
print("A", "B", "C", sep="\t")    # printの区切りをタブに
print("line without newline", end="")  # 末尾改行なし

# + で簡単に連結
first = "Python"
second = "3.12"
message = "Using " + first + " " + second + "!"
print(message)

# 悪い例: + をループで使ってどんどん足す
def concat_plus(n: int) -> str:
    s = ""
    for i in range(n):
        s += str(i)  # 毎回新しい文字列を生成
    return s

# 良い例: 一旦リストに集めて join
def concat_join(n: int) -> str:
    parts = []
    for i in range(n):
        parts.append(str(i))
    return "".join(parts)

print(len(concat_plus(10_000)))
print(len(concat_join(10_000)))

# joinの基本
items = ["spam", "eggs", "ham"]
csv = ",".join(items)
lines = "\n".join(items)
print(csv)
print("---")
print(lines)

# 数値や None を含む場合の対処
data = [1, 2, None, 4, 5]

# 1) すべてを文字列化（None -> "None" になる）
s1 = ", ".join(map(str, data))

# 2) None を除外して結合
s2 = ", ".join(str(x) for x in data if x is not None)

print(s1)
print(s2)

# f文字列の基本
user = "Alice"
points = 1234567
ratio = 0.0789

msg = f"{user} has {points:,} points (ratio={ratio:.2%})"
print(msg)

# 同等の処理を format() で
template = "{user} has {points:,} points (ratio={ratio:.2%})"
msg = template.format(user=user, points=points, ratio=ratio)
print(msg)

# 基本的な繰り返し
print("-" * 10)
print("ab" * 3)

# 罫線やインデントの例
title = "Report"
line = "=" * len(title)
print(title)
print(line)

indent = " " * 4  # スペース4つのインデント
print(indent + "• item 1")
print(indent + "• item 2")

# 幅合わせ（桁埋め）
print("42".zfill(5))         # 0埋め
print("cat".rjust(6, "."))   # 右寄せ・ドット埋め
print("dog".center(7, "-"))  # 中央寄せ・ハイフン埋め

# 簡易ベンチマーク（値は環境で変わります）
import timeit

setup = """
items = [str(i) for i in range(5000)]
"""

t_plus = timeit.timeit(
    stmt='"".join([s for s in items])'  # 参考として join の速さを見る
    , setup=setup, number=50)

t_bad_plus = timeit.timeit(
    stmt='''
s = ""
for x in items:
    s += x
''', setup=setup, number=10)

t_join = timeit.timeit(
    stmt='",".join(items)', setup=setup, number=1000)

t_f = timeit.timeit(
    stmt='f"{items[0]}-{items[1]}-{items[2]}"', setup=setup, number=500000)

t_mul = timeit.timeit(
    stmt='"-" * 1000', number=500000)

print("join (good pattern):", t_join)
print("+ in loop (bad pattern):", t_bad_plus)
print("f-string (small pieces):", t_f)
print("* repeat:", t_mul)

# 前後の空白除去と改行での結合
raw_lines = ["  apple ", "", " banana", "cherry  "]
clean = [s.strip() for s in raw_lines if s.strip() != ""]
text = "\n".join(clean)
print(text)

# printのsep/end
print("A", "B", "C", sep=" | ", end="\n\n")  # 区切りを明示、2重改行
print("done", end="")                         # 最後に改行しない

from typing import Iterable, Optional

def join_safe(sep: str, items: Iterable[Optional[object]]) -> str:
    """
    None はスキップし、他は str() によって文字列化して結合する安全な関数。
    """
    filtered = (str(x) for x in items if x is not None)
    return sep.join(filtered)

print(join_safe(", ", [1, None, "x", 3.14]))

def must_join_strings(sep: str, items: Iterable[str]) -> str:
    for x in items:
        if not isinstance(x, str):
            raise TypeError("all items must be str")
    return sep.join(items)

try:
    print(must_join_strings(", ", ["ok", 1]))  # ここで TypeError
except Exception as e:
    print(repr(e))

pip install wcwidth

# 表示幅に基づくパディング（wcwidthが必要）
# pip install wcwidth
from wcwidth import wcswidth

def pad_display_width(s: str, width: int, fill: str = " ") -> str:
    """
    表示幅がwidthになるよう右側をfillで埋める。
    """
    w = wcswidth(s)
    if w < 0:
        # 測定不能な場合はそのまま返す
        return s
    return s + (fill * max(0, width - w))

items = ["abc", "こんにちは", "🍣", "🇯🇵JP"]
for x in items:
    print(pad_display_width(x, 10, ".") + "|")