Fix delimiter detection in rows_from_file() for ragged and single-column files

When a CSV or TSV file was not perfectly rectangular, `csv.Sniffer` either
raised "Could not determine delimiter" (single-column files) or returned a
nonsense delimiter such as "a" for "id,name\n1,Cleo,extra". The latter
silently turned the header into a single column called "id,n" and broke the
documented extra-field handling.

Treat an alphanumeric sniffed delimiter as a failed sniff and fall back to
whichever of comma or tab appears on the most lines, using comma when
neither appears (a single-column file). Well-formed files are unaffected.

sqlite_utils/utils.py

@@ -285,6 +285,20 @@ def _extra_key_strategy(
             yield row_out
 
 
+def _fallback_dialect(sample: str) -> type[csv.Dialect]:
+    """Return comma or tab for a sample that could not be sniffed.
+
+    Whichever appears on the most non-blank lines wins. Comma is used when the
+    sample contains neither, which is what a single-column file looks like.
+    """
+    lines = [line for line in sample.splitlines() if line.strip()]
+    if not lines:
+        return csv.excel
+    commas = sum(1 for line in lines if "," in line)
+    tabs = sum(1 for line in lines if "\t" in line)
+    return csv.excel_tab if tabs > commas else csv.excel
+
+
 def rows_from_file(
     fp: BinaryIO,
     format: Format | None = None,
@@ -389,9 +403,21 @@ def rows_from_file(
             with buffered:
                 return rows_from_file(buffered, format=Format.JSON)
         else:
-            dialect = csv.Sniffer().sniff(
-                first_bytes.decode(encoding or "utf-8-sig", "ignore")
-            )
+            sample = first_bytes.decode(encoding or "utf-8-sig", "ignore")
+            try:
+                dialect = csv.Sniffer().sniff(sample)
+            except csv.Error:
+                # Sniffing needs a consistent structure, so it fails on
+                # single-column and on ragged files.
+                dialect = _fallback_dialect(sample)
+            else:
+                # For ragged input the sniffer does not fail, it returns a
+                # nonsense delimiter: it reports "a" as the delimiter of
+                # "id,name\n1,Cleo,extra", which silently mangles the column
+                # names. A delimiter is punctuation, never a letter or digit, so
+                # treat an alphanumeric answer as a failed sniff.
+                if dialect.delimiter.isalnum():
+                    dialect = _fallback_dialect(sample)
             rows, _ = rows_from_file(
                 buffered, format=Format.CSV, dialect=dialect, encoding=encoding
             )

tests/test_rows_from_file.py

@@ -131,3 +131,48 @@ def test_detect_format_keeps_streaming_reader_open(
     assert not buffered_readers[0].closed
     assert len(list(rows)) == 2000
     assert buffered_readers[0].closed
+
+
+@pytest.mark.parametrize(
+    "input,expected_format,expected",
+    (
+        # csv.Sniffer raises "Could not determine delimiter" on a single-column
+        # file, so it could not be loaded at all
+        (b"name\nalpha\nbeta", Format.CSV, [{"name": "alpha"}, {"name": "beta"}]),
+        # On ragged input the sniffer does not raise, it reports "a" as the
+        # delimiter, which turned the header into a single column called "id,n"
+        (
+            b"id,name\n1,Cleo\nextra",
+            Format.CSV,
+            [{"id": "1", "name": "Cleo"}, {"id": "extra", "name": None}],
+        ),
+        # The same, for a tab-separated file
+        (
+            b"id\tname\n1\tCleo\nextra",
+            Format.TSV,
+            [{"id": "1", "name": "Cleo"}, {"id": "extra", "name": None}],
+        ),
+    ),
+)
+def test_rows_from_file_falls_back_when_sniffing_fails(
+    input, expected_format, expected
+):
+    # csv.Sniffer either raises "Could not determine delimiter" (single-column) or
+    # returns a nonsense alphanumeric delimiter (ragged), which mangled the column
+    # names of every file that was not perfectly rectangular.
+    rows, format = rows_from_file(BytesIO(input))
+    assert format == expected_format
+    assert list(rows) == expected
+
+
+def test_rows_from_file_single_column_format_is_csv():
+    _rows, format = rows_from_file(BytesIO(b"name\nalpha"))
+    assert format == Format.CSV
+
+
+def test_rows_from_file_ragged_rows_raise_row_error_in_autodetect():
+    # The documented behaviour for rows with more values than headings still holds
+    # once the delimiter is detected correctly.
+    with pytest.raises(RowError):
+        rows, _ = rows_from_file(BytesIO(b"id,name\n1,Cleo,oops"))
+        list(rows)