DynamoDB: Fix calculation when adding/subtracting decimals (#7365)

moto/dynamodb/models/dynamo_type.py

@@ -1,6 +1,6 @@
 import base64
 import copy
-import decimal
+from decimal import Decimal
 from typing import Any, Dict, List, Optional, Union
 
 from boto3.dynamodb.types import TypeDeserializer, TypeSerializer
@@ -100,9 +100,14 @@ class DynamoType(object):
         if self.type != other.type:
             raise TypeError("Different types of operandi is not allowed.")
         if self.is_number():
-            self_value = float(self.value) if "." in self.value else int(self.value)
-            other_value = float(other.value) if "." in other.value else int(other.value)
-            return DynamoType({DDBType.NUMBER: f"{self_value + other_value}"})
+            self_value: Union[Decimal, int] = (
+                Decimal(self.value) if "." in self.value else int(self.value)
+            )
+            other_value: Union[Decimal, int] = (
+                Decimal(other.value) if "." in other.value else int(other.value)
+            )
+            total = self_value + other_value
+            return DynamoType({DDBType.NUMBER: f"{total}"})
         else:
             raise IncorrectDataType()
 
@@ -385,12 +390,7 @@ class Item(BaseModel):
                 if set(update_action["Value"].keys()) == set(["N"]):
                     existing = self.attrs.get(attribute_name, DynamoType({"N": "0"}))
                     self.attrs[attribute_name] = DynamoType(
-                        {
-                            "N": str(
-                                decimal.Decimal(existing.value)
-                                + decimal.Decimal(new_value)
-                            )
-                        }
+                        {"N": str(Decimal(existing.value) + Decimal(new_value))}
                     )
                 elif set(update_action["Value"].keys()) == set(["SS"]):
                     existing = self.attrs.get(attribute_name, DynamoType({"SS": {}}))

tests/test_dynamodb/test_dynamodb_update_expressions.py

@@ -1,3 +1,5 @@
+from decimal import Decimal
+
 import boto3
 import pytest
 
@@ -40,3 +42,50 @@ def test_update_different_map_elements_in_single_request(table_name=None):
         ExpressionAttributeValues={":MyCount": 5},
     )
     assert table.get_item(Key={"pk": "example_id"})["Item"]["MyTotalCount"] == 5
+
+
+@pytest.mark.aws_verified
+@dynamodb_aws_verified()
+def test_update_item_add_float(table_name=None):
+    table = boto3.resource("dynamodb", "us-east-1").Table(table_name)
+
+    # DECIMAL - DECIMAL
+    table.put_item(Item={"pk": "foo", "amount": Decimal(100), "nr": 5})
+    table.update_item(
+        Key={"pk": "foo"},
+        UpdateExpression="ADD amount :delta",
+        ExpressionAttributeValues={":delta": -Decimal("88.3")},
+    )
+    assert table.scan()["Items"][0]["amount"] == Decimal("11.7")
+
+    # DECIMAL + DECIMAL
+    table.update_item(
+        Key={"pk": "foo"},
+        UpdateExpression="ADD amount :delta",
+        ExpressionAttributeValues={":delta": Decimal("25.41")},
+    )
+    assert table.scan()["Items"][0]["amount"] == Decimal("37.11")
+
+    # DECIMAL + INT
+    table.update_item(
+        Key={"pk": "foo"},
+        UpdateExpression="ADD amount :delta",
+        ExpressionAttributeValues={":delta": 6},
+    )
+    assert table.scan()["Items"][0]["amount"] == Decimal("43.11")
+
+    # INT + INT
+    table.update_item(
+        Key={"pk": "foo"},
+        UpdateExpression="ADD nr :delta",
+        ExpressionAttributeValues={":delta": 1},
+    )
+    assert table.scan()["Items"][0]["nr"] == Decimal("6")
+
+    # INT + DECIMAL
+    table.update_item(
+        Key={"pk": "foo"},
+        UpdateExpression="ADD nr :delta",
+        ExpressionAttributeValues={":delta": Decimal("25.41")},
+    )
+    assert table.scan()["Items"][0]["nr"] == Decimal("31.41")