We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Single-subnet ELB detection
*cfn-lint version: 0.10.3*

*Description of issue.*

When configuring `AWS::ElasticLoadBalancingV2::LoadBalancer`, if you specify only 1 subnet, you get back from AWS:

> 2019-02-05 12:22:39 +1100 Elb AWS::ElasticLoadBalancingV2::LoadBalancer CREATE_FAILED At least two subnets in two different Availability Zones must be specified (Service: AmazonElasticLoadBalancingV2; Status Code: 400; Error Code: ValidationError; Request ID: ...)

This could be covered by a cfn-lint check.

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `src/cfnlint/rules/resources/elb/Elb.py`
Content:
```
1 """
2   Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
3 
4   Permission is hereby granted, free of charge, to any person obtaining a copy of this
5   software and associated documentation files (the "Software"), to deal in the Software
6   without restriction, including without limitation the rights to use, copy, modify,
7   merge, publish, distribute, sublicense, and/or sell copies of the Software, and to
8   permit persons to whom the Software is furnished to do so.
9 
10   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
11   INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
12   PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
13   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
14   OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
15   SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
16 """
17 import six
18 from cfnlint import CloudFormationLintRule
19 from cfnlint import RuleMatch
20 
21 
22 class Elb(CloudFormationLintRule):
23     """Check if Elb Resource Properties"""
24     id = 'E2503'
25     shortdesc = 'Resource ELB Properties'
26     description = 'See if Elb Resource Properties are set correctly \
27 HTTPS has certificate HTTP has no certificate'
28     source_url = 'https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-elb-listener.html'
29     tags = ['properties', 'elb']
30 
31     def check_protocol_value(self, value, path, **kwargs):
32         """
33             Check Protocol Value
34         """
35         matches = []
36         if isinstance(value, six.string_types):
37             if value.upper() not in kwargs['accepted_protocols']:
38                 message = 'Protocol must be {0} is invalid at {1}'
39                 matches.append(RuleMatch(path, message.format((', '.join(kwargs['accepted_protocols'])), ('/'.join(map(str, path))))))
40             elif value.upper() in kwargs['certificate_protocols']:
41                 if not kwargs['certificates']:
42                     message = 'Certificates should be specified when using HTTPS for {0}'
43                     matches.append(RuleMatch(path, message.format(('/'.join(map(str, path))))))
44 
45         return matches
46 
47     def match(self, cfn):
48         """Check ELB Resource Parameters"""
49 
50         matches = []
51 
52         results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::Listener'])
53         for result in results:
54             matches.extend(
55                 cfn.check_value(
56                     result['Value'], 'Protocol', result['Path'],
57                     check_value=self.check_protocol_value,
58                     accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'TLS'],
59                     certificate_protocols=['HTTPS', 'TLS'],
60                     certificates=result['Value'].get('Certificates')))
61 
62         results = cfn.get_resource_properties(['AWS::ElasticLoadBalancing::LoadBalancer', 'Listeners'])
63         for result in results:
64             if isinstance(result['Value'], list):
65                 for index, listener in enumerate(result['Value']):
66                     matches.extend(
67                         cfn.check_value(
68                             listener, 'Protocol', result['Path'] + [index],
69                             check_value=self.check_protocol_value,
70                             accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'SSL'],
71                             certificate_protocols=['HTTPS', 'SSL'],
72                             certificates=listener.get('SSLCertificateId')))
73 
74         results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::LoadBalancer'])
75         for result in results:
76             properties = result['Value']
77             if 'Type' in properties and properties['Type'] == 'network':
78                 if 'SecurityGroups' in properties:
79                     path = result['Path'] + ['SecurityGroups']
80                     matches.append(RuleMatch(path, 'Security groups are not supported for load balancers with type "network"'))
81 
82         return matches
83
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/src/cfnlint/rules/resources/elb/Elb.py b/src/cfnlint/rules/resources/elb/Elb.py
--- a/src/cfnlint/rules/resources/elb/Elb.py
+++ b/src/cfnlint/rules/resources/elb/Elb.py
@@ -44,6 +44,24 @@
 
         return matches
 
+    def check_alb_subnets(self, props, path):
+        """ Validate at least two subnets with ALBs"""
+        matches = []
+        elb_type = props.get('Type')
+        if elb_type == 'application':
+            subnets = props.get('Subnets')
+            if isinstance(subnets, list):
+                if len(subnets) < 2:
+                    path = path + ['Subnets']
+                    matches.append(RuleMatch(path, 'You must specify at least two Subnets for load balancers with type "application"'))
+            subnet_mappings = props.get('SubnetMappings')
+            if isinstance(subnet_mappings, list):
+                if len(subnet_mappings) < 2:
+                    path = path + ['SubnetMappings']
+                    matches.append(RuleMatch(path, 'You must specify at least two SubnetMappings for load balancers with type "application"'))
+
+        return matches
+
     def match(self, cfn):
         """Check ELB Resource Parameters"""
 
@@ -74,9 +92,12 @@
         results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::LoadBalancer'])
         for result in results:
             properties = result['Value']
-            if 'Type' in properties and properties['Type'] == 'network':
+            elb_type = properties.get('Type')
+            if elb_type == 'network':
                 if 'SecurityGroups' in properties:
                     path = result['Path'] + ['SecurityGroups']
                     matches.append(RuleMatch(path, 'Security groups are not supported for load balancers with type "network"'))
 
+            matches.extend(self.check_alb_subnets(properties, result['Path']))
+
         return matches

{"golden_diff": "diff --git a/src/cfnlint/rules/resources/elb/Elb.py b/src/cfnlint/rules/resources/elb/Elb.py\n--- a/src/cfnlint/rules/resources/elb/Elb.py\n+++ b/src/cfnlint/rules/resources/elb/Elb.py\n@@ -44,6 +44,24 @@\n \n         return matches\n \n+    def check_alb_subnets(self, props, path):\n+        \"\"\" Validate at least two subnets with ALBs\"\"\"\n+        matches = []\n+        elb_type = props.get('Type')\n+        if elb_type == 'application':\n+            subnets = props.get('Subnets')\n+            if isinstance(subnets, list):\n+                if len(subnets) < 2:\n+                    path = path + ['Subnets']\n+                    matches.append(RuleMatch(path, 'You must specify at least two Subnets for load balancers with type \"application\"'))\n+            subnet_mappings = props.get('SubnetMappings')\n+            if isinstance(subnet_mappings, list):\n+                if len(subnet_mappings) < 2:\n+                    path = path + ['SubnetMappings']\n+                    matches.append(RuleMatch(path, 'You must specify at least two SubnetMappings for load balancers with type \"application\"'))\n+\n+        return matches\n+\n     def match(self, cfn):\n         \"\"\"Check ELB Resource Parameters\"\"\"\n \n@@ -74,9 +92,12 @@\n         results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::LoadBalancer'])\n         for result in results:\n             properties = result['Value']\n-            if 'Type' in properties and properties['Type'] == 'network':\n+            elb_type = properties.get('Type')\n+            if elb_type == 'network':\n                 if 'SecurityGroups' in properties:\n                     path = result['Path'] + ['SecurityGroups']\n                     matches.append(RuleMatch(path, 'Security groups are not supported for load balancers with type \"network\"'))\n \n+            matches.extend(self.check_alb_subnets(properties, result['Path']))\n+\n         return matches\n", "issue": "Single-subnet ELB detection\n*cfn-lint version: 0.10.3*\r\n\r\n*Description of issue.*\r\n\r\nWhen configuring `AWS::ElasticLoadBalancingV2::LoadBalancer`, if you specify only 1 subnet, you get back from AWS:\r\n\r\n> 2019-02-05 12:22:39 +1100 Elb AWS::ElasticLoadBalancingV2::LoadBalancer CREATE_FAILED At least two subnets in two different Availability Zones must be specified (Service: AmazonElasticLoadBalancingV2; Status Code: 400; Error Code: ValidationError; Request ID: ...)\r\n\r\nThis could be covered by a cfn-lint check.\n", "before_files": [{"content": "\"\"\"\n  Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.\n\n  Permission is hereby granted, free of charge, to any person obtaining a copy of this\n  software and associated documentation files (the \"Software\"), to deal in the Software\n  without restriction, including without limitation the rights to use, copy, modify,\n  merge, publish, distribute, sublicense, and/or sell copies of the Software, and to\n  permit persons to whom the Software is furnished to do so.\n\n  THE SOFTWARE IS PROVIDED \"AS IS\", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,\n  INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A\n  PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT\n  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION\n  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE\n  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.\n\"\"\"\nimport six\nfrom cfnlint import CloudFormationLintRule\nfrom cfnlint import RuleMatch\n\n\nclass Elb(CloudFormationLintRule):\n    \"\"\"Check if Elb Resource Properties\"\"\"\n    id = 'E2503'\n    shortdesc = 'Resource ELB Properties'\n    description = 'See if Elb Resource Properties are set correctly \\\nHTTPS has certificate HTTP has no certificate'\n    source_url = 'https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-elb-listener.html'\n    tags = ['properties', 'elb']\n\n    def check_protocol_value(self, value, path, **kwargs):\n        \"\"\"\n            Check Protocol Value\n        \"\"\"\n        matches = []\n        if isinstance(value, six.string_types):\n            if value.upper() not in kwargs['accepted_protocols']:\n                message = 'Protocol must be {0} is invalid at {1}'\n                matches.append(RuleMatch(path, message.format((', '.join(kwargs['accepted_protocols'])), ('/'.join(map(str, path))))))\n            elif value.upper() in kwargs['certificate_protocols']:\n                if not kwargs['certificates']:\n                    message = 'Certificates should be specified when using HTTPS for {0}'\n                    matches.append(RuleMatch(path, message.format(('/'.join(map(str, path))))))\n\n        return matches\n\n    def match(self, cfn):\n        \"\"\"Check ELB Resource Parameters\"\"\"\n\n        matches = []\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::Listener'])\n        for result in results:\n            matches.extend(\n                cfn.check_value(\n                    result['Value'], 'Protocol', result['Path'],\n                    check_value=self.check_protocol_value,\n                    accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'TLS'],\n                    certificate_protocols=['HTTPS', 'TLS'],\n                    certificates=result['Value'].get('Certificates')))\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancing::LoadBalancer', 'Listeners'])\n        for result in results:\n            if isinstance(result['Value'], list):\n                for index, listener in enumerate(result['Value']):\n                    matches.extend(\n                        cfn.check_value(\n                            listener, 'Protocol', result['Path'] + [index],\n                            check_value=self.check_protocol_value,\n                            accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'SSL'],\n                            certificate_protocols=['HTTPS', 'SSL'],\n                            certificates=listener.get('SSLCertificateId')))\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::LoadBalancer'])\n        for result in results:\n            properties = result['Value']\n            if 'Type' in properties and properties['Type'] == 'network':\n                if 'SecurityGroups' in properties:\n                    path = result['Path'] + ['SecurityGroups']\n                    matches.append(RuleMatch(path, 'Security groups are not supported for load balancers with type \"network\"'))\n\n        return matches\n", "path": "src/cfnlint/rules/resources/elb/Elb.py"}], "after_files": [{"content": "\"\"\"\n  Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.\n\n  Permission is hereby granted, free of charge, to any person obtaining a copy of this\n  software and associated documentation files (the \"Software\"), to deal in the Software\n  without restriction, including without limitation the rights to use, copy, modify,\n  merge, publish, distribute, sublicense, and/or sell copies of the Software, and to\n  permit persons to whom the Software is furnished to do so.\n\n  THE SOFTWARE IS PROVIDED \"AS IS\", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,\n  INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A\n  PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT\n  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION\n  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE\n  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.\n\"\"\"\nimport six\nfrom cfnlint import CloudFormationLintRule\nfrom cfnlint import RuleMatch\n\n\nclass Elb(CloudFormationLintRule):\n    \"\"\"Check if Elb Resource Properties\"\"\"\n    id = 'E2503'\n    shortdesc = 'Resource ELB Properties'\n    description = 'See if Elb Resource Properties are set correctly \\\nHTTPS has certificate HTTP has no certificate'\n    source_url = 'https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-elb-listener.html'\n    tags = ['properties', 'elb']\n\n    def check_protocol_value(self, value, path, **kwargs):\n        \"\"\"\n            Check Protocol Value\n        \"\"\"\n        matches = []\n        if isinstance(value, six.string_types):\n            if value.upper() not in kwargs['accepted_protocols']:\n                message = 'Protocol must be {0} is invalid at {1}'\n                matches.append(RuleMatch(path, message.format((', '.join(kwargs['accepted_protocols'])), ('/'.join(map(str, path))))))\n            elif value.upper() in kwargs['certificate_protocols']:\n                if not kwargs['certificates']:\n                    message = 'Certificates should be specified when using HTTPS for {0}'\n                    matches.append(RuleMatch(path, message.format(('/'.join(map(str, path))))))\n\n        return matches\n\n    def check_alb_subnets(self, props, path):\n        \"\"\" Validate at least two subnets with ALBs\"\"\"\n        matches = []\n        elb_type = props.get('Type')\n        if elb_type == 'application':\n            subnets = props.get('Subnets')\n            if isinstance(subnets, list):\n                if len(subnets) < 2:\n                    path = path + ['Subnets']\n                    matches.append(RuleMatch(path, 'You must specify at least two Subnets for load balancers with type \"application\"'))\n            subnet_mappings = props.get('SubnetMappings')\n            if isinstance(subnet_mappings, list):\n                if len(subnet_mappings) < 2:\n                    path = path + ['SubnetMappings']\n                    matches.append(RuleMatch(path, 'You must specify at least two SubnetMappings for load balancers with type \"application\"'))\n\n        return matches\n\n    def match(self, cfn):\n        \"\"\"Check ELB Resource Parameters\"\"\"\n\n        matches = []\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::Listener'])\n        for result in results:\n            matches.extend(\n                cfn.check_value(\n                    result['Value'], 'Protocol', result['Path'],\n                    check_value=self.check_protocol_value,\n                    accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'TLS'],\n                    certificate_protocols=['HTTPS', 'TLS'],\n                    certificates=result['Value'].get('Certificates')))\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancing::LoadBalancer', 'Listeners'])\n        for result in results:\n            if isinstance(result['Value'], list):\n                for index, listener in enumerate(result['Value']):\n                    matches.extend(\n                        cfn.check_value(\n                            listener, 'Protocol', result['Path'] + [index],\n                            check_value=self.check_protocol_value,\n                            accepted_protocols=['HTTP', 'HTTPS', 'TCP', 'SSL'],\n                            certificate_protocols=['HTTPS', 'SSL'],\n                            certificates=listener.get('SSLCertificateId')))\n\n        results = cfn.get_resource_properties(['AWS::ElasticLoadBalancingV2::LoadBalancer'])\n        for result in results:\n            properties = result['Value']\n            elb_type = properties.get('Type')\n            if elb_type == 'network':\n                if 'SecurityGroups' in properties:\n                    path = result['Path'] + ['SecurityGroups']\n                    matches.append(RuleMatch(path, 'Security groups are not supported for load balancers with type \"network\"'))\n\n            matches.extend(self.check_alb_subnets(properties, result['Path']))\n\n        return matches\n", "path": "src/cfnlint/rules/resources/elb/Elb.py"}]}