fix(excel): keep exact Decimal in CSV/spreadsheet export (#216)

The query export coerced Decimal cells with float() before writing, so CSV — a
text format that can carry full precision — lost it on high-precision values
(e.g. a 28-digit conversion result rounded to 88.57142857142857). Keep the
Decimal: csv.writer str()s it losslessly, and pyexcel accepts it for xlsx/ods
(those cells are float64 by the format regardless, so no change there).

test_to_csv_preserves_exact_decimal fails without the fix.

Author: robcohen

src/rustfava/util/excel.py

@@ -96,7 +96,10 @@ def _row_to_pyexcel(row: ResultRow, header: list[Column]) -> list[str]:
             continue
         type_ = column.datatype
         if type_ is Decimal:
-            result.append(float(value))
+            # Keep the exact Decimal: CSV writes str(Decimal) losslessly, and
+            # pyexcel accepts it for xlsx/ods (those cells are float64 by the
+            # format anyway). float() here would corrupt CSV precision.
+            result.append(value)
         elif type_ is int:
             result.append(value)
         elif type_ in (set, frozenset) or isinstance(value, (set, frozenset)):

tests/test_util_excel.py

@@ -1,10 +1,12 @@
 from __future__ import annotations
 
+from decimal import Decimal
 from typing import Any
 from typing import TYPE_CHECKING
 
 import pytest
 
+from rustfava.rustledger.query import ColumnDescription
 from rustfava.rustledger.query import connect
 from rustfava.util import excel
 
@@ -42,6 +44,18 @@ def test_to_csv(example_ledger: RustfavaLedger) -> None:
     assert excel.to_csv(types, rows)
 
 
+def test_to_csv_preserves_exact_decimal() -> None:
+    """CSV export must keep a Decimal exact — no lossy float() (R8-adjacent).
+
+    CSV is text, so it can carry full precision; the old float() rounded
+    high-precision values on the way out.
+    """
+    exact = Decimal("88.571428571428571428571428571")
+    types = [ColumnDescription("amount", Decimal)]
+    out = excel.to_csv(types, [(exact,)]).getvalue().decode("utf-8")
+    assert "88.571428571428571428571428571" in out
+
+
 @pytest.mark.skipif(not excel.HAVE_EXCEL, reason="pyexcel not installed")
 def test_to_excel(example_ledger: RustfavaLedger) -> None:
     types, rows = _run_query(example_ledger, "balances")