Add DynamoDB tests for update_item() with UpdateExpression, support spaces in the UpdateExpression. (#758)

Fixes #745.

moto/dynamodb2/responses.py

@@ -419,6 +419,12 @@ class DynamoHandler(BaseResponse):
         expression_attribute_names = self.body.get('ExpressionAttributeNames', {})
         expression_attribute_values = self.body.get('ExpressionAttributeValues', {})
         existing_item = dynamodb_backend2.get_item(name, key)
+
+        # Support spaces between operators in an update expression
+        # E.g. `a = b + c` -> `a=b+c`
+        if update_expression:
+            update_expression = re.sub('\s*([=\+-])\s*', '\\1', update_expression)
+
         item = dynamodb_backend2.update_item(name, key, update_expression, attribute_updates, expression_attribute_names, expression_attribute_values)
 
         item_dict = item.to_json()

tests/test_dynamodb2/test_dynamodb_table_with_range_key.py

@@ -1267,6 +1267,39 @@ def test_update_item_add_value_does_not_exist_is_created():
     })
 
 
+@mock_dynamodb2
+def test_update_item_with_expression():
+    table = _create_table_with_range_key()
+
+    table.put_item(Item={
+        'forum_name': 'the-key',
+        'subject': '123',
+        'field': '1'
+    })
+
+    item_key = {'forum_name': 'the-key', 'subject': '123'}
+
+    table.update_item(
+        Key=item_key,
+        UpdateExpression='SET field=2',
+    )
+    dict(table.get_item(Key=item_key)['Item']).should.equal({
+        'field': '2',
+        'forum_name': 'the-key',
+        'subject': '123',
+    })
+
+    table.update_item(
+        Key=item_key,
+        UpdateExpression='SET field  = 3',
+    )
+    dict(table.get_item(Key=item_key)['Item']).should.equal({
+        'field': '3',
+        'forum_name': 'the-key',
+        'subject': '123',
+    })
+
+
 @mock_dynamodb2
 def test_boto3_query_gsi_range_comparison():
     table = _create_table_with_range_key()