读写文本文件

用 `with` 打开文件

变量在程序结束时消失，文件不会。open(path, mode) 返回一个文件对象，mode 说明你要做什么："r" 读取（默认）、"w" 写入、"a" 追加。始终用 with 块打开文件：with open(path) as file:。块结束时，Python 会替你关闭文件，即使里面出了错，这样数据不会丢失，也不会只写一半。传入 encoding="utf-8"，让文本在任何电脑上以相同方式读写。

# Write a file, then read it back
with open("fills.txt", "w", encoding="utf-8") as file:
    file.write("AAPL,184.25,200\n")
    file.write("MSFT,410.20,50\n")

with open("fills.txt", encoding="utf-8") as file:
    text = file.read()
print(repr(text))  # 'AAPL,184.25,200\nMSFT,410.20,50\n'

逐行读取

file.read() 把整个文件作为一个字符串返回。更常见的做法是直接遍历文件对象，它每次给出一行，对于内存装不下的大文件也适用：for line in file:。每一行末尾仍带有 \n，所以用 line.strip() 清理。空行清理后变成空字符串 ""，它为假，所以 if text: 会跳过它。需要一次拿到所有行时，可用 file.readlines() 和 text.splitlines()，它们返回行的列表。

# Read line by line, skipping blank lines
with open("fills.txt", "a", encoding="utf-8") as file:
    file.write("\n")                # a blank line

total = 0.0
with open("fills.txt", encoding="utf-8") as file:
    for line in file:
        text = line.strip()
        if not text:
            continue
        symbol, price, quantity = text.split(",")
        total += float(price) * int(quantity)
print(round(total, 2))              # 57360.0

写入与追加

file.write(text) 原样写入你给的字符串，并返回字符数。它不会自动加换行，所以每行末尾要自己加 "\n"。模式 "w" 会先清空文件（不存在则创建），因此用 "w" 打开已有文件会丢掉原来的内容。模式 "a" 保留原有内容并在末尾追加。只能写入字符串：先转换数字，例如用 f"{price:.2f}" 这样的 f-string。

# "w" starts over, "a" adds to the end
with open("log.txt", "w", encoding="utf-8") as file:
    file.write("start\n")
with open("log.txt", "a", encoding="utf-8") as file:
    file.write("order 1 filled\n")
    file.write(f"pnl {12.5:+.2f}\n")

with open("log.txt", encoding="utf-8") as file:
    print(file.read().splitlines())  # ['start', 'order 1 filled', 'pnl +12.50']

with open("log.txt", "w", encoding="utf-8") as file:
    pass                             # opening with "w" emptied it
with open("log.txt", encoding="utf-8") as file:
    print(repr(file.read()))         # ''

路径与缺失的文件

像 "fills.txt" 这样的相对路径会在程序的当前文件夹中查找；用 os.path.join(folder, name) 构建路径，就不必操心分隔符。以读取方式打开不存在的文件会引发 FileNotFoundError，当文件不存在是预料之中的情况时，可用 try/except 捕获。对于小文件，pathlib.Path(path).read_text() 和 .write_text(text) 一次调用就完成打开、读写和关闭。

import os
from pathlib import Path

# A missing file is an exception you can handle
def count_lines(path):
    try:
        with open(path, encoding="utf-8") as file:
            return len(file.read().splitlines())
    except FileNotFoundError:
        return 0

print(count_lines("fills.txt"))      # 3
print(count_lines("nope.txt"))       # 0

# pathlib does open + read + close in one call
path = Path(os.path.join(".", "fills.txt"))
print(path.read_text(encoding="utf-8").splitlines()[0])  # AAPL,184.25,200

• with open(path, mode, encoding="utf-8") as file: 打开文件并确保关闭。
• for line in file: 每次读一行；line.strip() 去掉换行符。
• "w" 会清空文件，"a" 追加；write 不会加换行。

下一节：CSV 文件，存储价格和成交表格的标准方式。

实验：评分内容

• 用 read_symbols 从文件加载自选股列表25 分
• 用 parse_trades 把成交文件转换为元组25 分
• 用 write_report 保存盈亏报告25 分
• 用 append_lines 向日志追加内容而不丢失原有内容25 分