Day 10: 文件、路径与编码

count = 0
with open("notes.txt", encoding="utf-8") as f:
for line in f:
if line.strip():
count += 1
print(count, f.closed)
输出：3 True。追踪：循环依次得到 'alpha\n'、'beta\n'、'gamma\n'（若文件没有结尾换行符则为 'gamma'）；strip() 之后每一行都非空，所以 count 达到 3，最后一行的结尾换行符不会产生第四行。print 在 with 块退出之后才执行，此时 f 已关闭，所以 f.closed 为 True。

练习脚本的观察输出（因为夹具是私有的，在每台机器上都相同）：

[project] exit=0
cwd is project: True
exists: True
lines: 3
[outside] exit=1
cwd is project: False
exists: False
FileNotFoundError: [Errno 2] No such file or directory: 'data/notes.txt'

诊断：文件存在（cwd 为 project 时 exists: True），但第二次运行的 cwd 不同（cwd is project: False），所以相对路径解析为并不存在的 outside/data/notes.txt。原因是工作目录，而不是文件缺失。最小修复：用 BASE = Path(__file__).resolve().parent 把路径锚定到脚本，并打开 BASE / "data" / "notes.txt"；计数那一行保持不变。下面的完整参考代码把锚定版 reader 写入一个新夹具，从两个文件夹分别运行，然后删除数据文件以展示另一种原因：当锚定路径本身不存在时，任何代码修改都没用，恢复夹具才能解决。

import subprocess
import sys
import tempfile
from pathlib import Path
FIXED = (
    "from pathlib import Path\n"
    "BASE = Path(__file__).resolve().parent\n"
    "with open(BASE / 'data' / 'notes.txt', encoding='utf-8') as f:\n"
    "    print('lines:', sum(1 for line in f if line.strip()))\n"
)

def run(script, cwd):
    done = subprocess.run([sys.executable, str(script)],
                          cwd=cwd, capture_output=True, text=True)
    if done.returncode == 0:
        return f"exit=0 {done.stdout.strip()}"
    last = done.stderr.strip().splitlines()[-1]
    return f"exit={done.returncode} {last.split(':')[0]}"

with tempfile.TemporaryDirectory() as tmp:
    root = Path(tmp).resolve()
    project = root / "project"
    outside = root / "outside"
    notes = project / "data" / "notes.txt"
    notes.parent.mkdir(parents=True)
    outside.mkdir()
    notes.write_text("alpha\nbeta\ngamma\n", encoding="utf-8")
    reader = project / "reader.py"
    reader.write_text(FIXED, encoding="utf-8")
    print("anchored, from outside:", run(reader, outside))
    print("anchored, from project:", run(reader, project))
    notes.unlink()
    print("fixture deleted, anchored path exists:", notes.exists())
    print("anchored, from project:", run(reader, project))
    notes.write_text("alpha\nbeta\ngamma\n", encoding="utf-8")
    print("fixture restored:", run(reader, outside))

预期输出：

anchored, from outside: exit=0 lines: 3
anchored, from project: exit=0 lines: 3
fixture deleted, anchored path exists: False
anchored, from project: exit=1 FileNotFoundError
fixture restored: exit=0 lines: 3

预测与观察输出：

{'ana': 7, 'cy': 5} [2]

第 2 行（bo,x9）在 try 内抛出 ValueError，所以它的行号进入 skipped，continue 跳过了赋值；第 1 行和第 3 行执行到 scores[name] = score。try 只包住 int(raw)，所以 open() 失败或拆行代码里的 bug 仍会以 traceback 暴露出来，而不会被悄悄算作坏行。enumerate(f, 1) 给物理行编号，所以空行也会让行号前进。解码问题是另一类失败，下面的程序把它与 ValueError 对比：

data = "café".encode("latin-1")
print(data)
try:
    data.decode("utf-8")
except UnicodeDecodeError as exc:
    print(type(exc).__name__ + ":", exc)
print(data.decode("latin-1") == "café")
print(b"plain ascii".decode("utf-8") == b"plain ascii".decode("latin-1"))
both = b"caf\xc3\xa9"
as_utf8 = both.decode("utf-8")
as_latin1 = both.decode("latin-1")
print(as_utf8 == "café", as_latin1 == "café", len(as_utf8), len(as_latin1))

预期输出：

b'caf\xe9'
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 3: unexpected end of data
True
True
True True 4 5

b'caf\xe9' 是 Latin-1 编码的 café，不是有效的 UTF-8，所以按 UTF-8 解码会抛出 UnicodeDecodeError；修复方法是声明真实编码（这里是 encoding="latin-1"）。纯 ASCII 在两种编码下解码结果相同，而 b'caf\xc3\xa9' 在两种编码下都能解码成功，却得到不同的文本（café 是 4 个字符，café 是 5 个字符），所以解码成功并不能证明声明的编码正确。相比之下，ValueError 意味着文本解码正常，但某一行的值无效。

带有自包含夹具测试的参考实现（请在记录了你独立完成的尝试之后，再拿它与自己的版本比较）：

import tempfile
from pathlib import Path

def summarize_scores(path):
    result = {"scores": {}, "skipped": [], "missing": False}
    try:
        f = open(path, encoding="utf-8")
    except FileNotFoundError:
        result["missing"] = True
        return result
    with f:
        for lineno, line in enumerate(f, 1):
            line = line.strip()
            if not line:
                continue
            name, sep, raw = line.rpartition(",")
            try:
                score = int(raw)
            except ValueError:
                score = None
            if not sep or not name or score is None:
                result["skipped"].append(lineno)
                continue
            best = result["scores"].get(name)
            if best is None or score > best:
                result["scores"][name] = score
    return result

with tempfile.TemporaryDirectory() as tmp:
    base = Path(tmp)
    (base / "s1.txt").write_text("ana,7\nbo,9\nana,8\n", encoding="utf-8")
    (base / "s2.txt").write_text("ana,7\nbo,x9\ncy,5\n", encoding="utf-8")
    print(summarize_scores(base / "s1.txt"))
    print(summarize_scores(base / "s2.txt"))
    print(summarize_scores(base / "missing.txt"))

预期输出：

{'scores': {'ana': 8, 'bo': 9}, 'skipped': [], 'missing': False}
{'scores': {'ana': 7, 'cy': 5}, 'skipped': [2], 'missing': False}
{'scores': {}, 'skipped': [], 'missing': True}

为什么这样写：FileNotFoundError 处理器里只有 open()，所以只有文件缺失才会把 missing 设为 True，其他错误照常暴露。with f: 块在每条路径上都会关闭文件。ValueError 处理器里只有 int(raw)；没有逗号、名字为空或分数不是整数的行会被跳过，并记录其从 1 开始的行号。每个名字的最高分通过与已存值比较来保留，所以 ana 最终是 8。