Pythonのpartition()で区切り文字を残して3分割する方法

# 基本動作のデモ

s1 = "a=b=c"
print(s1.partition("="))      # 最初の "=" で3分割

s2 = "abc"
print(s2.partition(","))      # 区切りがない

s3 = "scheme://host/path"
print(s3.partition("://"))    # 区切りは複数文字でもOK

# 空の区切り文字はエラー
try:
    print("x".partition(""))
except ValueError as e:
    print("エラー:", type(e).__name__, str(e))

text = "name: Alice: admin"
left, sep, right = text.partition(":")  # 最初の ":" で3分割
print(f"left={left!r}, sep={sep!r}, right={right!r}")

config_line = "timeout = 30"

key, eq, value = config_line.partition("=")
# 前後の空白を取り除きつつ、valueを数値にキャスト
key = key.strip()
value = value.strip()

print(key, eq, value, type(value))
value_int = int(value)  # "30" -> 30
print("value_int + 1 =", value_int + 1)

filename = "archive.tar.gz"

base, dot, ext = filename.rpartition(".")  # 末尾側の "." で3分割
print(base, dot, ext)

# dot が空の場合は拡張子なし
if dot:
    print(f"拡張子: .{ext}")
else:
    print("拡張子なし")

url = "https://example.com/path/to/page"
scheme, sep, rest = url.partition("://")
print("scheme:", scheme, "sep:", sep, "rest:", rest)

path = "users/alice/doc.txt"
head, slash, tail = path.partition("/")  # 最初の "/" で3分割
print("head:", head, "slash:", slash, "tail:", tail)

s = "key=value=extra"

# split + maxsplit=1 は2要素(区切りは消える)
parts = s.split("=", maxsplit=1)
print("split:", parts)

# partition は3要素(区切りが保持される)
before, sep, after = s.partition("=")
print("partition:", (before, sep, after))

log = "2025-09-10 12:00:00 INFO module.sub: message"
left, sep, right = log.rpartition(":")  # 右から最初の ":" で分割
print(left)
print(sep)
print(right)

s = "a=b=c=d"
sep = "="

parts = []
rest = s
while True:
    left, mid, right = rest.partition(sep)
    parts.append(left)
    if not mid:  # 区切りが見つからない
        break
    parts.append(mid)  # 区切りを保持
    rest = right

print(parts)

row = "name , Alice"
key, comma, value = row.partition(",")
key = key.strip()
value = value.strip()
print(key, comma, value)

text = "Header: Value"
sep = "header:"  # 大文字小文字を無視して分けたい

lower_text = text.lower()
lower_sep = sep.lower()

idx = lower_text.find(lower_sep)
if idx != -1:
    # 元の文字列から実際の区切り(元の大文字小文字を保持)を抜き出す
    real_sep = text[idx:idx + len(sep)]
    before = text[:idx]
    after = text[idx + len(sep):]
    print((before, real_sep, after))
else:
    print((text, "", ""))

value = 12345
try:
    # 数値には partition が存在しない
    value.partition("3")  # AttributeError
except AttributeError as e:
    print("AttributeError:", e)

# 文字列化すれば使える
s = str(value)
print(s.partition("3"))