Handle AWS list functions in template generator (#2331)

The template generator would introduce a list wrapping for AWS
Functions (Cidr, GetAZs, Split) that return a list resulting in
the function results wrapped in a list. This detects and corrects
that behavior.

Author: markpeek

tests/test_template_generator.py

@@ -1,7 +1,7 @@
 import json
 import unittest
 
-from troposphere import AWSObject, Template
+from troposphere import AWSObject, Cidr, GetAZs, Split, Template
 from troposphere.template_generator import (
     ResourceTypeNotDefined,
     ResourceTypeNotFound,
@@ -117,6 +117,34 @@ def test_no_nested_name(self):
         name = d["Outputs"]["TestOutput"]["Export"]["Name"]
         self.assertIn("Fn::Sub", name)
 
+    def test_list_functions(self):
+        """
+        Ensures that list functions are handled properly..
+        """
+        template = Template()
+        template.add_resource(
+            MyListResource(
+                "foo",
+                AZs=GetAZs(""),
+                Cidr=Cidr("192.168.0.0/24", "6", "5"),
+                Split=Split(",", "a,b,c"),
+            )
+        )
+        generated = TemplateGenerator(
+            json.loads(template.to_json()), CustomMembers=[MyListResource]
+        )
+
+        # validated that the templates are equal to each other
+        self.assertDictEqual(template.to_dict(), generated.to_dict())
+
+        # Further validate the list types are correct
+        generated_azs = generated.resources["foo"].properties["AZs"]
+        assert isinstance(generated_azs, GetAZs)
+        generated_cidr = generated.resources["foo"].properties["Cidr"]
+        assert isinstance(generated_cidr, Cidr)
+        generated_split = generated.resources["foo"].properties["Split"]
+        assert isinstance(generated_split, Split)
+
 
 class MyCustomResource(AWSObject):
     resource_type = "Custom::Resource"
@@ -135,5 +163,15 @@ class MyMacroResource(AWSObject):
     }
 
 
+class MyListResource(AWSObject):
+    resource_type = "Some::Special::Resource"
+
+    props = {
+        "AZs": ([str], True),
+        "Cidr": ([str], True),
+        "Split": ([str], True),
+    }
+
+
 if __name__ == "__main__":
     unittest.main()

troposphere/template_generator.py

@@ -24,16 +24,21 @@
 from troposphere import Parameter  # AWSDeclarations
 from troposphere import (
     AWSHelperFn,
+    Cidr,
     Export,
+    GetAZs,
     Output,
     Ref,
+    Split,
     Tags,
     Template,
     autoscaling,
     cloudformation,
 )
 from troposphere.policies import CreationPolicy, UpdatePolicy
 
+AWS_LIST_RETURN_FUNCTIONS = (Cidr, GetAZs, Split)
+
 
 class TemplateGenerator(Template):
     DEPRECATED_MODULES = ["troposphere.dynamodb2"]
@@ -221,7 +226,17 @@ def _create_instance(self, cls, args, ref=None):
                 # a list of 1 type means we must provide a list of such objects
                 if isinstance(args, str) or not isinstance(args, Sequence):
                     args = [args]
-                return [self._create_instance(cls[0], v) for v in args]
+
+                result = [self._create_instance(cls[0], v) for v in args]
+
+                # AWS functions that return lists should not be wrapped
+                # in another list
+                if len(result) == 1 and isinstance(
+                    result[0], AWS_LIST_RETURN_FUNCTIONS
+                ):
+                    return result[0]
+
+                return result
 
         if isinstance(cls, Sequence) or cls not in self.inspect_members.union(
             self._custom_members