1.4 · 文件处理

目标： 读、写、追加、修改文本文件。必备操作：删、插、追加、修改记录。

打开模式

模式	含义
"r"	读（默认；文件须存在）
"w"	写 —— 截断 / 创建
"a"	追加 —— 不存在则创建
"r+"	读 + 写
"x"	独占创建 —— 存在则失败
"b" 后缀	二进制模式（"rb"、"wb"）

读

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

with 确保文件自动关 —— 总优先用。

批量读：

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

写

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 处理

import csv

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

记录更新 —— 四项必备操作

设 CSV students.csv 表头 id,name,score。

把所有行读入内存

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)

删除

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

错误处理

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

学生常见错误

• 忘 with → 文件留打开。
• 模式错（"r" 去写、"w" 去读）。
• Windows 上换行混乱 —— csv writer 用 newline=""。
• 不指定 encoding="utf-8" → 中文变乱码。

考试式题目

题（5 分）： 写 Python 读 marks.csv（列 id、name、score），给每个低于 50 的 score 加 5 分奖励（上限 100），并写回同文件。

参考答案：

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)

关键要点

• 总用 with open(... , encoding="utf-8")。
• 4 项必备操作：插、追加、修改、删。
• 表格数据用 csv 模组。

➡️ 下一节：1.5 线性与二分查找