Preserve binary decimals outside the current decimal context range

src-python/amazon/ion/reader_binary.py

@@ -13,7 +13,7 @@
 # License.
 from collections import deque
 from datetime import timedelta
-from decimal import Decimal, localcontext
+from decimal import Decimal
 from enum import IntEnum
 from functools import partial
 from io import BytesIO
@@ -185,18 +185,8 @@ def _parse_decimal(buf):
     exponent = _parse_var_int(buf, signed=True)
     sign_bit, coefficient = _parse_signed_int_components(buf)
 
-    if coefficient == 0:
-        # Handle the zero cases--especially negative zero
-        value = Decimal((sign_bit, (0,), exponent))
-    else:
-        coefficient *= sign_bit and -1 or 1
-        with localcontext() as context:
-            # Adjusting precision for taking into account arbitrarily
-            # large/small numbers
-            context.prec = len(str(coefficient))
-            value = Decimal(coefficient).scaleb(exponent)
-
-    return value
+    # Construct the value without applying the caller's arithmetic context.
+    return Decimal((sign_bit, Decimal(coefficient).as_tuple().digits, exponent))
 
 
 def _parse_sid_iter(data):

tests/test_simpleion.py

@@ -15,7 +15,7 @@
 from functools import partial
 from io import BytesIO, StringIO
 
-from decimal import Decimal
+from decimal import Decimal, localcontext
 from itertools import chain
 from math import isnan
 
@@ -948,6 +948,19 @@ def wrap_annot(content, annot_bytes):
     return ivm + content
 
 
+@mark.parametrize('text', ['1e3', '-1e3', '1e-3', '-1e-3', '1.2300', '0.00', '-0.00'])
+@mark.parametrize('parse_eagerly', [True, False])
+def test_binary_decimal_preserves_value_outside_context(text, parse_eagerly):
+    expected = Decimal(text)
+    data = dumps(expected, imports=[])
+    with localcontext() as context:
+        context.prec = 2
+        context.Emax = 2
+        context.Emin = -2
+        actual = loads(data, catalog=SymbolTableCatalog(), parse_eagerly=parse_eagerly)
+    assert actual.as_tuple() == expected.as_tuple()
+
+
 def test_load_deeply_nested_annotations_binary_raises_ion_exception():
     data = _build_nested_annotation_binary(3000)
     buf = BytesIO(data)