From 6f02782624e0a60d388b2ba56cf7be9cf359582b Mon Sep 17 00:00:00 2001
From: Bert Blommers <info@bertblommers.nl>
Date: Wed, 22 Jan 2020 11:30:17 +0000
Subject: [PATCH] #2627 - Change comparison to differentiate between 0 and None

---
 moto/dynamodb2/comparisons.py         |  8 ++---
 tests/test_dynamodb2/test_dynamodb.py | 42 +++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 4 deletions(-)

diff --git a/moto/dynamodb2/comparisons.py b/moto/dynamodb2/comparisons.py
index 372f612c3..29951d92d 100644
--- a/moto/dynamodb2/comparisons.py
+++ b/moto/dynamodb2/comparisons.py
@@ -977,7 +977,7 @@ class OpLessThan(Op):
         lhs = self.lhs.expr(item)
         rhs = self.rhs.expr(item)
         # In python3 None is not a valid comparator when using < or > so must be handled specially
-        if lhs and rhs:
+        if lhs is not None and rhs is not None:
             return lhs < rhs
         else:
             return False
@@ -990,7 +990,7 @@ class OpGreaterThan(Op):
         lhs = self.lhs.expr(item)
         rhs = self.rhs.expr(item)
         # In python3 None is not a valid comparator when using < or > so must be handled specially
-        if lhs and rhs:
+        if lhs is not None and rhs is not None:
             return lhs > rhs
         else:
             return False
@@ -1021,7 +1021,7 @@ class OpLessThanOrEqual(Op):
         lhs = self.lhs.expr(item)
         rhs = self.rhs.expr(item)
         # In python3 None is not a valid comparator when using < or > so must be handled specially
-        if lhs and rhs:
+        if lhs is not None and rhs is not None:
             return lhs <= rhs
         else:
             return False
@@ -1034,7 +1034,7 @@ class OpGreaterThanOrEqual(Op):
         lhs = self.lhs.expr(item)
         rhs = self.rhs.expr(item)
         # In python3 None is not a valid comparator when using < or > so must be handled specially
-        if lhs and rhs:
+        if lhs is not None and rhs is not None:
             return lhs >= rhs
         else:
             return False
diff --git a/tests/test_dynamodb2/test_dynamodb.py b/tests/test_dynamodb2/test_dynamodb.py
index 333eba135..1a0865ba8 100644
--- a/tests/test_dynamodb2/test_dynamodb.py
+++ b/tests/test_dynamodb2/test_dynamodb.py
@@ -2531,6 +2531,48 @@ def test_condition_expressions():
         )
 
 
+@mock_dynamodb2
+def test_condition_expression_numerical_attribute():
+    dynamodb = boto3.resource("dynamodb")
+    dynamodb.create_table(
+        TableName="my-table",
+        KeySchema=[{"AttributeName": "partitionKey", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "partitionKey", "AttributeType": "S"}],
+    )
+    table = dynamodb.Table("my-table")
+    table.put_item(Item={"partitionKey": "pk-pos", "myAttr": 5})
+    table.put_item(Item={"partitionKey": "pk-neg", "myAttr": -5})
+
+    # try to update the item we put in the table using numerical condition expression
+    # Specifically, verify that we can compare with a zero-value
+    # First verify that > and >= work on positive numbers
+    update_numerical_con_expr(
+        key="pk-pos", con_expr="myAttr > :zero", res="6", table=table
+    )
+    update_numerical_con_expr(
+        key="pk-pos", con_expr="myAttr >= :zero", res="7", table=table
+    )
+    # Second verify that < and <= work on negative numbers
+    update_numerical_con_expr(
+        key="pk-neg", con_expr="myAttr < :zero", res="-4", table=table
+    )
+    update_numerical_con_expr(
+        key="pk-neg", con_expr="myAttr <= :zero", res="-3", table=table
+    )
+
+
+def update_numerical_con_expr(key, con_expr, res, table):
+    table.update_item(
+        Key={"partitionKey": key},
+        UpdateExpression="ADD myAttr :one",
+        ExpressionAttributeValues={":zero": 0, ":one": 1},
+        ConditionExpression=con_expr,
+    )
+    table.get_item(Key={"partitionKey": key})["Item"]["myAttr"].should.equal(
+        Decimal(res)
+    )
+
+
 @mock_dynamodb2
 def test_condition_expression__attr_doesnt_exist():
     client = boto3.client("dynamodb", region_name="us-east-1")