From 8ce2d3c8bf148b3f86a8a7eff4495b70897f0346 Mon Sep 17 00:00:00 2001
From: Steve Pulec <spulec@gmail.com>
Date: Wed, 4 May 2016 22:10:11 -0400
Subject: [PATCH] Cleanup updating ASGs. Closes #603.

---
 moto/autoscaling/models.py                 | 21 ++++++++++++++-------
 tests/test_autoscaling/test_autoscaling.py |  6 +++++-
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/moto/autoscaling/models.py b/moto/autoscaling/models.py
index bf4c42fd7..e4c6f1081 100644
--- a/moto/autoscaling/models.py
+++ b/moto/autoscaling/models.py
@@ -217,18 +217,25 @@ class FakeAutoScalingGroup(object):
                launch_config_name, vpc_zone_identifier, default_cooldown,
                health_check_period, health_check_type, load_balancers,
                placement_group, termination_policies):
-        self.availability_zones = availability_zones
-        self.max_size = max_size
-        self.min_size = min_size
+        if availability_zones:
+            self.availability_zones = availability_zones
+        if max_size is not None:
+            self.max_size = max_size
+        if min_size is not None:
+            self.min_size = min_size
 
         if launch_config_name:
             self.launch_config = self.autoscaling_backend.launch_configurations[launch_config_name]
             self.launch_config_name = launch_config_name
-        self.vpc_zone_identifier = vpc_zone_identifier
-        self.health_check_period = health_check_period
-        self.health_check_type = health_check_type
+        if vpc_zone_identifier is not None:
+            self.vpc_zone_identifier = vpc_zone_identifier
+        if health_check_period is not None:
+            self.health_check_period = health_check_period
+        if health_check_type is not None:
+            self.health_check_type = health_check_type
 
-        self.set_desired_capacity(desired_capacity)
+        if desired_capacity is not None:
+            self.set_desired_capacity(desired_capacity)
 
     def set_desired_capacity(self, new_capacity):
         if new_capacity is None:
diff --git a/tests/test_autoscaling/test_autoscaling.py b/tests/test_autoscaling/test_autoscaling.py
index 7ee9b30f0..e48750700 100644
--- a/tests/test_autoscaling/test_autoscaling.py
+++ b/tests/test_autoscaling/test_autoscaling.py
@@ -437,5 +437,9 @@ def test_update_autoscaling_group_boto3():
     response = client.update_auto_scaling_group(
         AutoScalingGroupName='test_asg',
         MinSize=1,
-        DesiredCapacity=1,
     )
+
+    response = client.describe_auto_scaling_groups(
+        AutoScalingGroupNames=["test_asg"]
+    )
+    response['AutoScalingGroups'][0]['MinSize'].should.equal(1)