Python DictReader处理CSV缺列与多列

csv.DictReader能把每行转换为字典，但列数不一致时并不总会报错：多余字段可进入restkey，缺失字段用restval补齐。导入前明确处理这两种情况，才能避免把异常记录当成完整数据。

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先分清缺少字段与字段为空

示例表头是name,age。“Bo”这一行缺少第二个字段；“Cy,”则提供了第二个字段，只是内容为空。两者若都变成空字符串，后续无法区分原始输入结构，因此本例使用独立的missing对象标记缺失。

import csv
from io import StringIO

missing = object()
text = "name,age\nAda,18,extra\nBo\nCy,\n"
rows = list(csv.DictReader(StringIO(text), restkey="__extra__", restval=missing))
assert rows[0]["__extra__"] == ["extra"]
assert rows[1]["age"] is missing
assert rows[2]["age"] == ""
problems = []
for record, row in enumerate(rows, start=1):
    if "__extra__" in row:
        problems.append((record, "extra fields"))
    if any(value is missing for value in row.values()):
        problems.append((record, "missing fields"))
assert problems == [(1, "extra fields"), (2, "missing fields")]
print(problems)