PythonでCSVを読み込む・書き出す(csvモジュールの使い方)

# csvモジュールのインポート
import csv

# よく使うクラスや定数を個別に参照することもできます
from csv import reader, writer, DictReader, DictWriter, QUOTE_ALL

# people.csv を作成するサンプル。初回だけ実行してください。
import csv

rows = [
    ["id", "name", "age"],  # ヘッダー
    [1, "Alice", 30],
    [2, "Bob", 25],
    [3, "Carol", 27],
]

# newline="" は必須。csvモジュールに改行制御を任せます
with open("people.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

print("people.csv を作成しました")

id,name,age
1,Alice,30
2,Bob,25
3,Carol,27

import csv

with open("people.csv", "r", newline="", encoding="utf-8") as f:
    reader = csv.reader(f)  # デフォルトはカンマ区切り
    for row in reader:
        # row は ['1', 'Alice', '30'] のようなリスト
        print(row)

import csv

with open("people.csv", "r", newline="", encoding="utf-8") as f:
    rows = list(csv.reader(f))

print(rows)  # ネストしたリストの形

import csv

with open("people.csv", "r", newline="", encoding="utf-8") as f:
    reader = csv.reader(f)
    header = next(reader)  # 1行目を取得してスキップ
    print("ヘッダー:", header)
    for row in reader:
        # ここでは id と age を整数に変換してみます
        person_id = int(row[0])
        age = int(row[2])
        print(f"id={person_id}, name={row[1]}, age={age}")

import csv

with open("people.csv", "r", newline="", encoding="utf-8") as f:
    reader = csv.DictReader(f)  # 1行目がフィールド名とみなされる
    for row in reader:
        # row は {"id": "1", "name": "Alice", "age": "30"} のような辞書
        name = row["name"]
        age = int(row["age"])
        print(f"{name} さんは {age} 歳です")

import csv

# まずはタブ区切りのファイルを作って読み書きしてみます
rows = [
    ["id", "name", "score"],
    [1, "Alice", 88],
    [2, "Bob", 92],
]

# 書き出し(タブ区切り)
with open("people.tsv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f, delimiter="\t")
    writer.writerows(rows)

# 読み込み(タブ区切り)
with open("people.tsv", "r", newline="", encoding="utf-8") as f:
    reader = csv.reader(f, delimiter="\t")
    for row in reader:
        print(row)

import csv

data = [
    [1, "Orange", 120],
    [2, "Apple", 80],
]

with open("fruits.csv", "w", newline="", encoding="utf-8") as f:
    w = csv.writer(f)
    w.writerow(["id", "name", "price"])  # 1行だけ書く
    w.writerows(data)                     # 複数行をまとめて書く

print("fruits.csv を出力しました")

id,name,price
1,Orange,120
2,Apple,80

# 1) writerで手動ヘッダー + 行データ
import csv

headers = ["id", "fruit", "price"]
rows = [
    [1, "Orange", 120],
    [2, "Apple", 80],
]

with open("fruits_with_header.csv", "w", newline="", encoding="utf-8") as f:
    w = csv.writer(f)
    w.writerow(headers)   # ヘッダー
    w.writerows(rows)     # 本体

# 2) DictWriterで辞書から書く(列名の順序は fieldnames で明示)
from csv import DictWriter

items = [
    {"id": 1, "fruit": "Orange", "price": 120},
    {"id": 2, "fruit": "Apple", "price": 80},
]

with open("fruits_dict.csv", "w", newline="", encoding="utf-8") as f:
    w = DictWriter(f, fieldnames=["id", "fruit", "price"])
    w.writeheader()       # ヘッダーを自動生成して書く
    w.writerows(items)    # 辞書のリストをまとめて書く

import csv

# 既存の fruits_with_header.csv に1行だけ追加
with open("fruits_with_header.csv", "a", newline="", encoding="utf-8") as f:
    w = csv.writer(f)
    w.writerow([3, "Banana", 60])

# 追記されたか確認して末尾3行を表示
with open("fruits_with_header.csv", "r", newline="", encoding="utf-8") as f:
    tail = list(csv.reader(f))[-3:]
    for row in tail:
        print(row)

import csv

rows = [
    ["id", "text"],
    [1, "カンマ,入りのテキスト"],         # カンマを含む
    [2, "複数行\nのメモです"],             # 改行を含む
    [3, 'ダブルクォート " を含む'],        # 二重引用符を含む
]

# デフォルト設定で書き出す(必要なときだけ引用符を付ける QUOTE_MINIMAL)
with open("special.csv", "w", newline="", encoding="utf-8") as f:
    w = csv.writer(f)  # quotingの既定値は csv.QUOTE_MINIMAL
    w.writerows(rows)

# 書かれた内容を確認(人間が見るためにファイル内容をそのまま表示)
with open("special.csv", "r", encoding="utf-8") as f:
    print(f.read())

# 読み戻しも正しく復元されることを確認
with open("special.csv", "r", newline="", encoding="utf-8") as f:
    r = csv.reader(f)
    for row in r:
        print(row)