1.4 · 文件處理

目標： 讀、寫、追加、修改文本文件。必備操作：刪、插、追加、修改記錄。

打開模式

模式	含義
"r"	讀（默認；文件須存在）
"w"	寫 —— 截斷 / 創建
"a"	追加 —— 不存在則創建
"r+"	讀 + 寫
"x"	獨佔創建 —— 存在則失敗
"b" 後綴	二進制模式（"rb"、"wb"）

讀

with open("students.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

with 確保文件自動關 —— 總優先用。

批量讀：

with open("students.txt", encoding="utf-8") as f:
    all_text = f.read()
    print(all_text)

寫

with open("output.txt", "w", encoding="utf-8") as f:
    f.write("Hello\n")
    f.write("World\n")

追加

with open("log.txt", "a", encoding="utf-8") as f:
    f.write(f"{datetime.now()}: event\n")

CSV 處理

import csv

with open("students.csv", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

記錄更新 —— 四項必備操作

設 CSV students.csv 表頭 id,name,score。

把所有行讀入記憶體

import csv

def read_all(path):
    with open(path, encoding="utf-8") as f:
        return list(csv.DictReader(f))

def write_all(path, rows):
    if not rows: return
    with open(path, "w", encoding="utf-8", newline="") as f:
        w = csv.DictWriter(f, fieldnames=rows[0].keys())
        w.writeheader()
        w.writerows(rows)

追加

rows = read_all("students.csv")
rows.append({"id": "1006", "name": "Frank", "score": "75"})
write_all("students.csv", rows)

插到特定位置

rows = read_all("students.csv")
rows.insert(2, {"id": "1007", "name": "Grace", "score": "80"})
write_all("students.csv", rows)

修改 (Amend)

rows = read_all("students.csv")
for r in rows:
    if r["id"] == "1001":
        r["score"] = "90"
write_all("students.csv", rows)

刪除

rows = read_all("students.csv")
rows = [r for r in rows if r["id"] != "1003"]
write_all("students.csv", rows)

錯誤處理

try:
    with open("missing.txt") as f:
        ...
except FileNotFoundError:
    print("File not found")

學生常見錯誤

• 忘 with → 文件留打開。
• 模式錯（"r" 去寫、"w" 去讀）。
• Windows 上換行混亂 —— csv writer 用 newline=""。
• 不指定 encoding="utf-8" → 中文變亂碼。

考試式題目

題（5 分）： 寫 Python 讀 marks.csv（列 id、name、score），給每個低於 50 的 score 加 5 分獎勵（上限 100），並寫回同文件。

參考答案：

import csv

with open("marks.csv", encoding="utf-8") as f:
    rows = list(csv.DictReader(f))

for r in rows:
    s = int(r["score"])
    if s < 50:
        r["score"] = str(min(100, s + 5))

with open("marks.csv", "w", encoding="utf-8", newline="") as f:
    w = csv.DictWriter(f, fieldnames=["id","name","score"])
    w.writeheader()
    w.writerows(rows)

關鍵要點

• 總用 with open(... , encoding="utf-8")。
• 4 項必備操作：插、追加、修改、刪。
• 表格資料用 csv 模組。

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