ファイル I/O

# ファイルに書き込む
file = open("hello.txt", "w", encoding="utf-8")
file.write("Hello, World!")
file.close()

# ファイルから読み取る
file = open("hello.txt", "r", encoding="utf-8")
content = file.read()
file.close()

print(content)              # Hello, World!

# 書き込み
with open("hello.txt", "w", encoding="utf-8") as f:
    f.write("Hello, World!")

# 読み取り
with open("hello.txt", "r", encoding="utf-8") as f:
    content = f.read()

print(content)

# "w" — 書き込み（クリアして書き直し）
with open("test.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")

# "a" — 追記
with open("test.txt", "a", encoding="utf-8") as f:
    f.write("Third line\n")

# "r" — 読み取り
with open("test.txt", "r", encoding="utf-8") as f:
    print(f.read())

First line
Second line
Third line

# ファイルの内容を想定：
# First line
# Second line
# Third line

# 方法 1：read() — 一度にすべて読み取り
with open("test.txt", "r", encoding="utf-8") as f:
    content = f.read()
print(content)

# 方法 2：readlines() — リストとして読み取り
with open("test.txt", "r", encoding="utf-8") as f:
    lines = f.readlines()
print(lines)                # ['First line\n', 'Second line\n', 'Third line\n']

# 方法 3：1 行ずつ（推奨）
with open("test.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())  # strip() で改行文字を除去

def backup_file(filename):
    """ファイルのバックアップコピーを作成"""
    try:
        with open(filename, "r", encoding="utf-8") as f:
            content = f.read()
        
        backup_name = filename + ".bak"
        with open(backup_name, "w", encoding="utf-8") as f:
            f.write(content)
        
        print(f"Backup created: {backup_name}")
    except FileNotFoundError:
        print(f"File {filename} does not exist!")

backup_file("test.txt")

import csv

# CSV 書き込み
data = [
    ["Name", "Age", "City"],
    ["Zhang San", 25, "Beijing"],
    ["Li Si", 30, "Shanghai"],
    ["Wang Wu", 22, "Guangzhou"],
]

with open("students.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(data)

# CSV 読み取り
with open("students.csv", "r", encoding="utf-8") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

import csv

# 書き込み
data = [
    {"name": "Zhang San", "age": 25, "city": "Beijing"},
    {"name": "Li Si", "age": 30, "city": "Shanghai"},
]

with open("students_dict.csv", "w", newline="", encoding="utf-8") as f:
    fieldnames = ["name", "age", "city"]
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()       # ヘッダーを書き込み
    writer.writerows(data)     # データ行を書き込み

# 読み取り
with open("students_dict.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['name']} — {row['age']} years old, from {row['city']}")

import os

# 現在のディレクトリを取得
print(os.getcwd())          # /project

# パスを結合（区切り文字を自動処理）
path = os.path.join("data", "files", "test.txt")
print(path)                 # data/files/test.txt

# パスの存在確認
print(os.path.exists("test.txt"))          # True/False
print(os.path.isfile("test.txt"))          # ファイルか？
print(os.path.isdir("data"))               # ディレクトリか？

# ファイル情報を取得
print(os.path.getsize("test.txt"))         # ファイルサイズ（バイト）

# ディレクトリ内容の一覧
for f in os.listdir("."):
    print(f)

from pathlib import Path

# パスを作成
p = Path("data/files/test.txt")

# パス情報
print(p.name)               # test.txt（ファイル名）
print(p.stem)               # test（拡張子なし）
print(p.suffix)             # .txt（拡張子）
print(p.parent)             # data/files（親ディレクトリ）

# ファイルの読み書き（簡潔）
p = Path("hello.txt")
p.write_text("Hello, World!", encoding="utf-8")
content = p.read_text(encoding="utf-8")
print(content)              # Hello, World!

# 再帰的走査
for py_file in Path(".").rglob("*.py"):
    print(py_file)