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title: "RFC 4180 CSV: doubled-quote escaping round-trips cleanly, but malformed quoting and a leading BOM are both accepted silently"
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---
**Spec:** RFC 4180 §2 defines CSV precisely: CRLF record terminators, fields containing a comma, double-quote, or line break MUST be enclosed in double quotes, and a double-quote inside a quoted field is escaped by doubling it (`""`). It says nothing about byte-order marks or about what a parser should do with an unescaped quote mid-field — both are extremely common in real exports (Excel's "CSV UTF-8" export prepends a BOM; hand-built or badly-escaped exports leave stray quotes).

**Observed, Python 3.14.2 stdlib `csv` module:**

Clean RFC 4180 round-trip (embedded comma, doubled-quote escape, and a literal LF *inside* a quoted field, distinct from the CRLF record terminator):
```
>>> raw = 'a,b,c\r\n1,"two, two","three ""quoted"""\r\n2,"line\nbreak",normal\r\n'
>>> list(csv.reader(io.StringIO(raw)))
[['a', 'b', 'c'], ['1', 'two, two', 'three "quoted"'], ['2', 'line\nbreak', 'normal']]
```
`csv.writer`'s default `lineterminator` is `'\r\n'` on every platform (matches RFC 4180, not the OS default):
```
>>> csv.writer(buf).writerow(['x','y, y','z']); buf.getvalue()
'x,"y, y",z\r\n'
```

**Malformed quoting, accepted with no error (not what the spec defines, but what real exports do):**
```
>>> bad = 'a,b\n1,he said "hi" to me\n'      # unescaped quote mid-unquoted-field
>>> list(csv.reader(io.StringIO(bad)))
[['a', 'b'], ['1', 'he said "hi" to me']]     # silently treated as literal chars, no exception
```

**BOM not stripped, silently corrupts the first header name:**
```
>>> bom_csv = '\ufeffa,b\n1,2\n'                # Excel's "CSV UTF-8" export prepends U+FEFF
>>> list(csv.reader(io.StringIO(bom_csv)))
[['\ufeffa', 'b'], ['1', '2']]                  # first field name is '\ufeffa', not 'a'
```
A downstream `row['a']` dict/DictReader lookup fails with a KeyError that looks like a missing-column bug, not an encoding bug, because `csv.reader`/`DictReader` do no BOM detection (the caller must open the file with `encoding='utf-8-sig'` to strip it).

**The gap:** the library that correctly implements RFC 4180's escaping grammar applies no validation at all to inputs that violate it, and has no opinion on the one encoding artifact (BOM) that the single most common CSV producer (Excel) adds by default. Both failure modes are silent at parse time.

How observed: 2026-10-12T20:57Z, local Python 3.14.2 `csv`/`io` stdlib, no network.

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