fix(conversions): reject a Decimal whose scale is negative

pyiceberg/conversions.py

@@ -310,9 +310,11 @@ def _(primitive_type: DecimalType, value: Decimal) -> bytes:
         bytes: The byte representation of `value`.
     """
     _, digits, exponent = value.as_tuple()
-    exponent = abs(int(exponent))
-    if exponent != primitive_type.scale:
-        raise ValueError(f"Cannot serialize value, scale of value does not match type {primitive_type}: {exponent}")
+    # A Decimal carries the negated scale as its exponent, so a value with a negative
+    # scale (1E+2) must not be read as if it had the matching positive one
+    value_scale = -int(exponent)
+    if value_scale != primitive_type.scale:
+        raise ValueError(f"Cannot serialize value, scale of value does not match type {primitive_type}: {value_scale}")
     elif len(digits) > primitive_type.precision:
         raise ValueError(
             f"Cannot serialize value, precision of value is greater than precision of type {primitive_type}: {len(digits)}"

tests/test_conversions.py

@@ -510,6 +510,13 @@ def __repr__(self) -> str:
     "primitive_type, value, expected_error_message",
     [
         (DecimalType(7, 3), Decimal("123.4567"), "Cannot serialize value, scale of value does not match type decimal(7, 3): 4"),
+        # A Decimal with a negative scale must not pass as its positive counterpart
+        (DecimalType(10, 2), Decimal("1E+2"), "Cannot serialize value, scale of value does not match type decimal(10, 2): -2"),
+        (
+            DecimalType(10, 2),
+            Decimal("100").normalize(),
+            "Cannot serialize value, scale of value does not match type decimal(10, 2): -2",
+        ),
         (
             DecimalType(18, 8),
             Decimal("123456789.123456789"),