diff --git a/ChangeLog.md b/ChangeLog.md
index 7ac23fb8a42..708857a83f7 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,16 @@
 # Change Log
 
+### 2021-06-16 - 5.8.1
+
+min Autorest core version: 3.3.0
+
+min Modelerfour version: 4.19.1
+
+**Bug Fixes**
+
+- Fix optional properties with constant schemas. Now, properties that have constant schemas but are optional will not have the hardcoded constant value,
+but will default to its `x-ms-client-default` or `None`  #952
+
 ### 2021-05-17 - 5.8.0
 
 min Autorest core version: 3.3.0
diff --git a/autorest/codegen/models/object_schema.py b/autorest/codegen/models/object_schema.py
index 200e8c13963..996c759e7b8 100644
--- a/autorest/codegen/models/object_schema.py
+++ b/autorest/codegen/models/object_schema.py
@@ -129,7 +129,6 @@ def fill_instance_from_yaml(self, namespace: str, yaml_data: Dict[str, Any], **k
             # map of discriminator value to child's name
             for children_yaml in yaml_data["discriminator"]["immediate"].values():
                 subtype_map[children_yaml["discriminatorValue"]] = children_yaml["language"]["python"]["name"]
-
         if yaml_data.get("properties"):
             properties += [
                 Property.from_yaml(p, has_additional_properties=len(properties) > 0, **kwargs)
diff --git a/autorest/codegen/models/parameter.py b/autorest/codegen/models/parameter.py
index c3d041c1344..4dff6d3302b 100644
--- a/autorest/codegen/models/parameter.py
+++ b/autorest/codegen/models/parameter.py
@@ -173,7 +173,7 @@ def in_method_signature(self) -> bool:
 
     @property
     def in_method_code(self) -> bool:
-        return not (isinstance(self.schema, ConstantSchema) and self.location == ParameterLocation.Other)
+        return not (self.constant and self.location == ParameterLocation.Other)
 
     @property
     def implementation(self) -> str:
diff --git a/autorest/codegen/models/property.py b/autorest/codegen/models/property.py
index 426435cab46..43c616fad78 100644
--- a/autorest/codegen/models/property.py
+++ b/autorest/codegen/models/property.py
@@ -32,26 +32,36 @@ def __init__(
         self.required: bool = yaml_data.get("required", False)
         self.readonly: bool = yaml_data.get("readOnly", False)
         self.is_discriminator: bool = yaml_data.get("isDiscriminator", False)
-        # this bool doesn't consider you to be constant if you are a discriminator
-        self.constant: bool = isinstance(self.schema, ConstantSchema) and not self.is_discriminator
-
         if description:
             self.description = description
         else:
             self.description = yaml_data["language"]["python"]["description"]
 
-        validation_map: Dict[str, Union[bool, int, str]] = {}
+        self.client_default_value = client_default_value
+
+    @property
+    def constant(self) -> bool:
+        # this bool doesn't consider you to be constant if you are a discriminator
+        # you also have to be required to be considered a constant
+        return (
+            isinstance(self.schema, ConstantSchema) and
+            self.required and
+            not self.is_discriminator
+        )
+
+    @property
+    def validation_map(self) -> Optional[Dict[str, Union[bool, int, str]]]:
+        retval: Dict[str, Union[bool, int, str]] = {}
         if self.required:
-            validation_map["required"] = True
+            retval["required"] = True
         if self.readonly:
-            validation_map["readonly"] = True
+            retval["readonly"] = True
         if self.constant:
-            validation_map["constant"] = True
+            retval["constant"] = True
         if self.schema.validation_map:
             validation_map_from_schema = cast(Dict[str, Union[bool, int, str]], self.schema.validation_map)
-            validation_map.update(validation_map_from_schema)
-        self.validation_map = validation_map or None
-        self.client_default_value = client_default_value
+            retval.update(validation_map_from_schema)
+        return retval or None
 
     @property
     def escaped_swagger_name(self) -> str:
diff --git a/autorest/codegen/serializers/model_base_serializer.py b/autorest/codegen/serializers/model_base_serializer.py
index 01e827c7e17..8a798d86542 100644
--- a/autorest/codegen/serializers/model_base_serializer.py
+++ b/autorest/codegen/serializers/model_base_serializer.py
@@ -62,14 +62,19 @@ def prop_documentation_string(prop: Property) -> str:
             description += "."
         if prop.name == "tags":
             description = "A set of tags. " + description
-        if prop.required:
+
+        if prop.constant:
+            description += f' Has constant value: {prop.constant_declaration}.'
+        elif prop.required:
             if description:
                 description = "Required. " + description
             else:
                 description = "Required. "
-        if prop.constant:
-            constant_prop = cast(ConstantSchema, prop.schema)
-            description += f' Default value: "{constant_prop.value}".'
+        elif isinstance(prop.schema, ConstantSchema):
+            description += (
+                f" The only acceptable values to pass in are None and {prop.constant_declaration}. " +
+                f"The default value is {prop.default_value_declaration}."
+            )
         if prop.is_discriminator:
             description += "Constant filled by server. "
         if isinstance(prop.schema, EnumSchema):
diff --git a/dev_requirements.txt b/dev_requirements.txt
index 6c769311074..60f41d2031a 100644
--- a/dev_requirements.txt
+++ b/dev_requirements.txt
@@ -7,3 +7,4 @@ pytest
 ptvsd
 mypy
 black
+types-PyYAML
\ No newline at end of file
diff --git a/package.json b/package.json
index d0e0e936ac3..846fa4f0754 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
     "name": "@autorest/python",
-    "version": "5.8.0",
+    "version": "5.8.1",
     "description": "The Python extension for generators in AutoRest.",
     "scripts": {
         "prepare": "node run-python3.js prepare.py",
@@ -28,7 +28,7 @@
     },
     "devDependencies": {
         "@autorest/autorest": "^3.0.0",
-        "@microsoft.azure/autorest.testserver": "^3.0.23"
+        "@microsoft.azure/autorest.testserver": "^3.0.24"
     },
     "files": [
         "autorest/**/*.py",
diff --git a/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models.py b/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models.py
index 15c69f1847d..7aaa9669255 100644
--- a/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models.py
+++ b/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models.py
@@ -19,7 +19,7 @@ class Error(msrest.serialization.Model):
 
     :param status:
     :type status: int
-    :ivar constant_id: Required.  Default value: "1".
+    :ivar constant_id:  Has constant value: 1.
     :vartype constant_id: int
     :param message:
     :type message: str
diff --git a/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models_py3.py b/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models_py3.py
index c8380361c06..806724b999b 100644
--- a/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models_py3.py
+++ b/test/azure/Expected/AcceptanceTests/AzureSpecials/azurespecialproperties/models/_models_py3.py
@@ -21,7 +21,7 @@ class Error(msrest.serialization.Model):
 
     :param status:
     :type status: int
-    :ivar constant_id: Required.  Default value: "1".
+    :ivar constant_id:  Has constant value: 1.
     :vartype constant_id: int
     :param message:
     :type message: str
diff --git a/test/vanilla/AcceptanceTests/asynctests/test_model_flattening.py b/test/vanilla/AcceptanceTests/asynctests/test_model_flattening.py
index 0d160557814..bd2de5fe53c 100644
--- a/test/vanilla/AcceptanceTests/asynctests/test_model_flattening.py
+++ b/test/vanilla/AcceptanceTests/asynctests/test_model_flattening.py
@@ -214,6 +214,7 @@ async def test_model_flattening_simple(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo",
             generic_value = "https://generic"
         )
@@ -230,6 +231,7 @@ async def test_model_flattening_with_parameter_flattening(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo"
         )
         simple_product.additional_properties = {} # Not the purpose of this test. This enables the ==.
@@ -238,6 +240,7 @@ async def test_model_flattening_with_parameter_flattening(self, client):
             "123", # product_id
             "product description", # description
             "max name", # max_product_display_name
+            "Large",  # capacity
             None, # generic_value
             "http://foo", # odata_value
         )
@@ -252,6 +255,7 @@ async def test_model_flattening_with_grouping(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo"
         )
         simple_product.additional_properties = {} # Not the purpose of this test. This enables the ==.
@@ -260,6 +264,7 @@ async def test_model_flattening_with_grouping(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name="max name",
+            capacity="Large",
             odata_value="http://foo",
             name="groupproduct")
 
diff --git a/test/vanilla/AcceptanceTests/test_model_flattening.py b/test/vanilla/AcceptanceTests/test_model_flattening.py
index 0fd477f5c1e..f07085f72e9 100644
--- a/test/vanilla/AcceptanceTests/test_model_flattening.py
+++ b/test/vanilla/AcceptanceTests/test_model_flattening.py
@@ -208,6 +208,7 @@ def test_model_flattening_simple(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo",
             generic_value = "https://generic"
         )
@@ -223,6 +224,7 @@ def test_model_flattening_with_parameter_flattening(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo"
         )
         simple_product.additional_properties = {} # Not the purpose of this test. This enables the ==.
@@ -231,6 +233,7 @@ def test_model_flattening_with_parameter_flattening(self, client):
             "123", # product_id
             "product description", # description
             "max name", # max_product_display_name
+            "Large",  # capacity
             None, # generic_value
             "http://foo", # odata_value
         )
@@ -244,6 +247,7 @@ def test_model_flattening_with_grouping(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name = "max name",
+            capacity="Large",
             odata_value = "http://foo"
         )
         simple_product.additional_properties = {} # Not the purpose of this test. This enables the ==.
@@ -252,6 +256,7 @@ def test_model_flattening_with_grouping(self, client):
             product_id = "123",
             description = "product description",
             max_product_display_name="max name",
+            capacity="Large",
             odata_value="http://foo",
             name="groupproduct")
 
diff --git a/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models.py b/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models.py
index f378cc31f39..50bae4d73a0 100644
--- a/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models.py
+++ b/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models.py
@@ -37,8 +37,7 @@ class RefColorConstant(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar color_constant: Required. Referenced Color Constant Description. Default value:
-     "green-color".
+    :ivar color_constant: Referenced Color Constant Description. Has constant value: "green-color".
     :vartype color_constant: str
     :param field1: Sample string.
     :type field1: str
diff --git a/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models_py3.py b/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models_py3.py
index 9fa4e50bc63..236df42ec85 100644
--- a/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models_py3.py
+++ b/test/vanilla/Expected/AcceptanceTests/BodyString/bodystring/models/_models_py3.py
@@ -39,8 +39,7 @@ class RefColorConstant(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar color_constant: Required. Referenced Color Constant Description. Default value:
-     "green-color".
+    :ivar color_constant: Referenced Color Constant Description. Has constant value: "green-color".
     :vartype color_constant: str
     :param field1: Sample string.
     :type field1: str
diff --git a/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models.py b/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models.py
index 49053a757a8..2c36e865ba5 100644
--- a/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models.py
+++ b/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models.py
@@ -164,47 +164,35 @@ def __init__(self, **kwargs):
 class NoModelAsStringNoRequiredOneValueDefault(msrest.serialization.Model):
     """NoModelAsStringNoRequiredOneValueDefault.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
-    :ivar parameter:  Default value: "value1".
-    :vartype parameter: str
+    :param parameter:  The only acceptable values to pass in are None and "value1". The default
+     value is "value1".
+    :type parameter: str
     """
 
-    _validation = {
-        "parameter": {"constant": True},
-    }
-
     _attribute_map = {
         "parameter": {"key": "parameter", "type": "str"},
     }
 
-    parameter = "value1"
-
     def __init__(self, **kwargs):
         super(NoModelAsStringNoRequiredOneValueDefault, self).__init__(**kwargs)
+        self.parameter = kwargs.get("parameter", "value1")
 
 
 class NoModelAsStringNoRequiredOneValueNoDefault(msrest.serialization.Model):
     """NoModelAsStringNoRequiredOneValueNoDefault.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
-    :ivar parameter:  Default value: "value1".
-    :vartype parameter: str
+    :param parameter:  The only acceptable values to pass in are None and "value1". The default
+     value is None.
+    :type parameter: str
     """
 
-    _validation = {
-        "parameter": {"constant": True},
-    }
-
     _attribute_map = {
         "parameter": {"key": "parameter", "type": "str"},
     }
 
-    parameter = "value1"
-
     def __init__(self, **kwargs):
         super(NoModelAsStringNoRequiredOneValueNoDefault, self).__init__(**kwargs)
+        self.parameter = kwargs.get("parameter", None)
 
 
 class NoModelAsStringNoRequiredTwoValueDefault(msrest.serialization.Model):
@@ -246,7 +234,7 @@ class NoModelAsStringRequiredOneValueDefault(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar parameter: Required.  Default value: "value1".
+    :ivar parameter:  Has constant value: "value1".
     :vartype parameter: str
     """
 
@@ -271,7 +259,7 @@ class NoModelAsStringRequiredOneValueNoDefault(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar parameter: Required.  Default value: "value1".
+    :ivar parameter:  Has constant value: "value1".
     :vartype parameter: str
     """
 
diff --git a/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models_py3.py b/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models_py3.py
index 9faaff8c9db..c62571056ea 100644
--- a/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models_py3.py
+++ b/test/vanilla/Expected/AcceptanceTests/Constants/constants/models/_models_py3.py
@@ -176,47 +176,35 @@ def __init__(self, *, parameter: Union[str, "ModelAsStringRequiredTwoValueNoDefa
 class NoModelAsStringNoRequiredOneValueDefault(msrest.serialization.Model):
     """NoModelAsStringNoRequiredOneValueDefault.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
-    :ivar parameter:  Default value: "value1".
-    :vartype parameter: str
+    :param parameter:  The only acceptable values to pass in are None and "value1". The default
+     value is "value1".
+    :type parameter: str
     """
 
-    _validation = {
-        "parameter": {"constant": True},
-    }
-
     _attribute_map = {
         "parameter": {"key": "parameter", "type": "str"},
     }
 
-    parameter = "value1"
-
-    def __init__(self, **kwargs):
+    def __init__(self, *, parameter: Optional[str] = "value1", **kwargs):
         super(NoModelAsStringNoRequiredOneValueDefault, self).__init__(**kwargs)
+        self.parameter = parameter
 
 
 class NoModelAsStringNoRequiredOneValueNoDefault(msrest.serialization.Model):
     """NoModelAsStringNoRequiredOneValueNoDefault.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
-    :ivar parameter:  Default value: "value1".
-    :vartype parameter: str
+    :param parameter:  The only acceptable values to pass in are None and "value1". The default
+     value is None.
+    :type parameter: str
     """
 
-    _validation = {
-        "parameter": {"constant": True},
-    }
-
     _attribute_map = {
         "parameter": {"key": "parameter", "type": "str"},
     }
 
-    parameter = "value1"
-
-    def __init__(self, **kwargs):
+    def __init__(self, *, parameter: Optional[str] = None, **kwargs):
         super(NoModelAsStringNoRequiredOneValueNoDefault, self).__init__(**kwargs)
+        self.parameter = parameter
 
 
 class NoModelAsStringNoRequiredTwoValueDefault(msrest.serialization.Model):
@@ -262,7 +250,7 @@ class NoModelAsStringRequiredOneValueDefault(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar parameter: Required.  Default value: "value1".
+    :ivar parameter:  Has constant value: "value1".
     :vartype parameter: str
     """
 
@@ -287,7 +275,7 @@ class NoModelAsStringRequiredOneValueNoDefault(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar parameter: Required.  Default value: "value1".
+    :ivar parameter:  Has constant value: "value1".
     :vartype parameter: str
     """
 
diff --git a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/aio/operations/_auto_rest_resource_flattening_test_service_operations.py b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/aio/operations/_auto_rest_resource_flattening_test_service_operations.py
index 1d4f8693d5a..a5f2c554752 100644
--- a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/aio/operations/_auto_rest_resource_flattening_test_service_operations.py
+++ b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/aio/operations/_auto_rest_resource_flattening_test_service_operations.py
@@ -454,6 +454,7 @@ async def post_flattened_simple_product(
         product_id: str,
         description: Optional[str] = None,
         max_product_display_name: Optional[str] = None,
+        capacity: Optional[str] = "Large",
         generic_value: Optional[str] = None,
         odata_value: Optional[str] = None,
         **kwargs: Any
@@ -468,6 +469,8 @@ async def post_flattened_simple_product(
         :type description: str
         :param max_product_display_name: Display name of product.
         :type max_product_display_name: str
+        :param capacity: Capacity of product. For example, 4 people.
+        :type capacity: str
         :param generic_value: Generic URL value.
         :type generic_value: str
         :param odata_value: URL value.
@@ -485,6 +488,7 @@ async def post_flattened_simple_product(
             product_id=product_id,
             description=description,
             max_product_display_name=max_product_display_name,
+            capacity=capacity,
             generic_value=generic_value,
             odata_value=odata_value,
         )
@@ -565,6 +569,7 @@ async def put_simple_product_with_grouping(
             product_id=_product_id,
             description=_description,
             max_product_display_name=_max_product_display_name,
+            capacity=capacity,
             generic_value=_generic_value,
             odata_value=_odata_value,
         )
diff --git a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models.py b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models.py
index b485c619526..1db7dc4c8bf 100644
--- a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models.py
+++ b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models.py
@@ -159,8 +159,6 @@ def __init__(self, **kwargs):
 class FlattenParameterGroup(msrest.serialization.Model):
     """Parameter group.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
     All required parameters must be populated in order to send to Azure.
 
     :param name: Required. Product name with value 'groupproduct'.
@@ -175,8 +173,9 @@ class FlattenParameterGroup(msrest.serialization.Model):
     :type description: str
     :param max_product_display_name: Display name of product.
     :type max_product_display_name: str
-    :ivar capacity: Capacity of product. For example, 4 people. Default value: "Large".
-    :vartype capacity: str
+    :param capacity: Capacity of product. For example, 4 people. The only acceptable values to pass
+     in are None and "Large". The default value is None.
+    :type capacity: str
     :param generic_value: Generic URL value.
     :type generic_value: str
     :param odata_value: URL value.
@@ -186,7 +185,6 @@ class FlattenParameterGroup(msrest.serialization.Model):
     _validation = {
         "name": {"required": True},
         "product_id": {"required": True},
-        "capacity": {"constant": True},
     }
 
     _attribute_map = {
@@ -200,8 +198,6 @@ class FlattenParameterGroup(msrest.serialization.Model):
         "odata_value": {"key": "@odata\\.value", "type": "str"},
     }
 
-    capacity = "Large"
-
     def __init__(self, **kwargs):
         super(FlattenParameterGroup, self).__init__(**kwargs)
         self.name = kwargs["name"]
@@ -209,6 +205,7 @@ def __init__(self, **kwargs):
         self.product_id = kwargs["product_id"]
         self.description = kwargs.get("description", None)
         self.max_product_display_name = kwargs.get("max_product_display_name", None)
+        self.capacity = kwargs.get("capacity", None)
         self.generic_value = kwargs.get("generic_value", None)
         self.odata_value = kwargs.get("odata_value", None)
 
@@ -291,8 +288,6 @@ def __init__(self, **kwargs):
 class SimpleProduct(BaseProduct):
     """The product documentation.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
     All required parameters must be populated in order to send to Azure.
 
     :param product_id: Required. Unique identifier representing a specific product for a given
@@ -303,8 +298,9 @@ class SimpleProduct(BaseProduct):
     :type description: str
     :param max_product_display_name: Display name of product.
     :type max_product_display_name: str
-    :ivar capacity: Capacity of product. For example, 4 people. Default value: "Large".
-    :vartype capacity: str
+    :param capacity: Capacity of product. For example, 4 people. The only acceptable values to pass
+     in are None and "Large". The default value is None.
+    :type capacity: str
     :param generic_value: Generic URL value.
     :type generic_value: str
     :param odata_value: URL value.
@@ -313,7 +309,6 @@ class SimpleProduct(BaseProduct):
 
     _validation = {
         "product_id": {"required": True},
-        "capacity": {"constant": True},
     }
 
     _attribute_map = {
@@ -325,11 +320,10 @@ class SimpleProduct(BaseProduct):
         "odata_value": {"key": "details.max_product_image.@odata\\.value", "type": "str"},
     }
 
-    capacity = "Large"
-
     def __init__(self, **kwargs):
         super(SimpleProduct, self).__init__(**kwargs)
         self.max_product_display_name = kwargs.get("max_product_display_name", None)
+        self.capacity = kwargs.get("capacity", None)
         self.generic_value = kwargs.get("generic_value", None)
         self.odata_value = kwargs.get("odata_value", None)
 
diff --git a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models_py3.py b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models_py3.py
index 369cf4238f6..356b1ea37bf 100644
--- a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models_py3.py
+++ b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/models/_models_py3.py
@@ -177,8 +177,6 @@ def __init__(
 class FlattenParameterGroup(msrest.serialization.Model):
     """Parameter group.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
     All required parameters must be populated in order to send to Azure.
 
     :param name: Required. Product name with value 'groupproduct'.
@@ -193,8 +191,9 @@ class FlattenParameterGroup(msrest.serialization.Model):
     :type description: str
     :param max_product_display_name: Display name of product.
     :type max_product_display_name: str
-    :ivar capacity: Capacity of product. For example, 4 people. Default value: "Large".
-    :vartype capacity: str
+    :param capacity: Capacity of product. For example, 4 people. The only acceptable values to pass
+     in are None and "Large". The default value is None.
+    :type capacity: str
     :param generic_value: Generic URL value.
     :type generic_value: str
     :param odata_value: URL value.
@@ -204,7 +203,6 @@ class FlattenParameterGroup(msrest.serialization.Model):
     _validation = {
         "name": {"required": True},
         "product_id": {"required": True},
-        "capacity": {"constant": True},
     }
 
     _attribute_map = {
@@ -218,8 +216,6 @@ class FlattenParameterGroup(msrest.serialization.Model):
         "odata_value": {"key": "@odata\\.value", "type": "str"},
     }
 
-    capacity = "Large"
-
     def __init__(
         self,
         *,
@@ -228,6 +224,7 @@ def __init__(
         simple_body_product: Optional["SimpleProduct"] = None,
         description: Optional[str] = None,
         max_product_display_name: Optional[str] = None,
+        capacity: Optional[str] = None,
         generic_value: Optional[str] = None,
         odata_value: Optional[str] = None,
         **kwargs
@@ -238,6 +235,7 @@ def __init__(
         self.product_id = product_id
         self.description = description
         self.max_product_display_name = max_product_display_name
+        self.capacity = capacity
         self.generic_value = generic_value
         self.odata_value = odata_value
 
@@ -327,8 +325,6 @@ def __init__(
 class SimpleProduct(BaseProduct):
     """The product documentation.
 
-    Variables are only populated by the server, and will be ignored when sending a request.
-
     All required parameters must be populated in order to send to Azure.
 
     :param product_id: Required. Unique identifier representing a specific product for a given
@@ -339,8 +335,9 @@ class SimpleProduct(BaseProduct):
     :type description: str
     :param max_product_display_name: Display name of product.
     :type max_product_display_name: str
-    :ivar capacity: Capacity of product. For example, 4 people. Default value: "Large".
-    :vartype capacity: str
+    :param capacity: Capacity of product. For example, 4 people. The only acceptable values to pass
+     in are None and "Large". The default value is None.
+    :type capacity: str
     :param generic_value: Generic URL value.
     :type generic_value: str
     :param odata_value: URL value.
@@ -349,7 +346,6 @@ class SimpleProduct(BaseProduct):
 
     _validation = {
         "product_id": {"required": True},
-        "capacity": {"constant": True},
     }
 
     _attribute_map = {
@@ -361,20 +357,20 @@ class SimpleProduct(BaseProduct):
         "odata_value": {"key": "details.max_product_image.@odata\\.value", "type": "str"},
     }
 
-    capacity = "Large"
-
     def __init__(
         self,
         *,
         product_id: str,
         description: Optional[str] = None,
         max_product_display_name: Optional[str] = None,
+        capacity: Optional[str] = None,
         generic_value: Optional[str] = None,
         odata_value: Optional[str] = None,
         **kwargs
     ):
         super(SimpleProduct, self).__init__(product_id=product_id, description=description, **kwargs)
         self.max_product_display_name = max_product_display_name
+        self.capacity = capacity
         self.generic_value = generic_value
         self.odata_value = odata_value
 
diff --git a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/operations/_auto_rest_resource_flattening_test_service_operations.py b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/operations/_auto_rest_resource_flattening_test_service_operations.py
index a0c8577211c..0017fddaf60 100644
--- a/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/operations/_auto_rest_resource_flattening_test_service_operations.py
+++ b/test/vanilla/Expected/AcceptanceTests/ModelFlattening/modelflattening/operations/_auto_rest_resource_flattening_test_service_operations.py
@@ -487,6 +487,7 @@ def post_flattened_simple_product(
         product_id,  # type: str
         description=None,  # type: Optional[str]
         max_product_display_name=None,  # type: Optional[str]
+        capacity="Large",  # type: Optional[str]
         generic_value=None,  # type: Optional[str]
         odata_value=None,  # type: Optional[str]
         **kwargs  # type: Any
@@ -502,6 +503,8 @@ def post_flattened_simple_product(
         :type description: str
         :param max_product_display_name: Display name of product.
         :type max_product_display_name: str
+        :param capacity: Capacity of product. For example, 4 people.
+        :type capacity: str
         :param generic_value: Generic URL value.
         :type generic_value: str
         :param odata_value: URL value.
@@ -519,6 +522,7 @@ def post_flattened_simple_product(
             product_id=product_id,
             description=description,
             max_product_display_name=max_product_display_name,
+            capacity=capacity,
             generic_value=generic_value,
             odata_value=odata_value,
         )
@@ -602,6 +606,7 @@ def put_simple_product_with_grouping(
             product_id=_product_id,
             description=_description,
             max_product_display_name=_max_product_display_name,
+            capacity=capacity,
             generic_value=_generic_value,
             odata_value=_odata_value,
         )
diff --git a/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models.py b/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models.py
index 650d73eba0f..b517ba12c17 100644
--- a/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models.py
+++ b/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models.py
@@ -17,7 +17,7 @@ class ChildProduct(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar const_property: Required. Constant string. Default value: "constant".
+    :ivar const_property: Constant string. Has constant value: "constant".
     :vartype const_property: str
     :param count: Count.
     :type count: int
@@ -46,9 +46,9 @@ class ConstantProduct(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar const_property: Required. Constant string. Default value: "constant".
+    :ivar const_property: Constant string. Has constant value: "constant".
     :vartype const_property: str
-    :ivar const_property2: Required. Constant string2. Default value: "constant2".
+    :ivar const_property2: Constant string2. Has constant value: "constant2".
     :vartype const_property2: str
     """
 
@@ -110,12 +110,13 @@ class Product(msrest.serialization.Model):
     :type child: ~validation.models.ChildProduct
     :param const_child: Required. The product documentation.
     :type const_child: ~validation.models.ConstantProduct
-    :ivar const_int: Required. Constant int. Default value: "0".
+    :ivar const_int: Constant int. Has constant value: 0.
     :vartype const_int: int
-    :ivar const_string: Required. Constant string. Default value: "constant".
+    :ivar const_string: Constant string. Has constant value: "constant".
     :vartype const_string: str
-    :ivar const_string_as_enum: Constant string as Enum. Default value: "constant_string_as_enum".
-    :vartype const_string_as_enum: str
+    :param const_string_as_enum: Constant string as Enum. The only acceptable values to pass in are
+     None and "constant_string_as_enum". The default value is None.
+    :type const_string_as_enum: str
     """
 
     _validation = {
@@ -126,7 +127,6 @@ class Product(msrest.serialization.Model):
         "const_child": {"required": True},
         "const_int": {"required": True, "constant": True},
         "const_string": {"required": True, "constant": True},
-        "const_string_as_enum": {"constant": True},
     }
 
     _attribute_map = {
@@ -142,7 +142,6 @@ class Product(msrest.serialization.Model):
 
     const_int = 0
     const_string = "constant"
-    const_string_as_enum = "constant_string_as_enum"
 
     def __init__(self, **kwargs):
         super(Product, self).__init__(**kwargs)
@@ -151,3 +150,4 @@ def __init__(self, **kwargs):
         self.image = kwargs.get("image", None)
         self.child = kwargs["child"]
         self.const_child = kwargs["const_child"]
+        self.const_string_as_enum = kwargs.get("const_string_as_enum", None)
diff --git a/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models_py3.py b/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models_py3.py
index e8ed1d89950..0e77bb9c137 100644
--- a/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models_py3.py
+++ b/test/vanilla/Expected/AcceptanceTests/Validation/validation/models/_models_py3.py
@@ -19,7 +19,7 @@ class ChildProduct(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar const_property: Required. Constant string. Default value: "constant".
+    :ivar const_property: Constant string. Has constant value: "constant".
     :vartype const_property: str
     :param count: Count.
     :type count: int
@@ -48,9 +48,9 @@ class ConstantProduct(msrest.serialization.Model):
 
     All required parameters must be populated in order to send to Azure.
 
-    :ivar const_property: Required. Constant string. Default value: "constant".
+    :ivar const_property: Constant string. Has constant value: "constant".
     :vartype const_property: str
-    :ivar const_property2: Required. Constant string2. Default value: "constant2".
+    :ivar const_property2: Constant string2. Has constant value: "constant2".
     :vartype const_property2: str
     """
 
@@ -114,12 +114,13 @@ class Product(msrest.serialization.Model):
     :type child: ~validation.models.ChildProduct
     :param const_child: Required. The product documentation.
     :type const_child: ~validation.models.ConstantProduct
-    :ivar const_int: Required. Constant int. Default value: "0".
+    :ivar const_int: Constant int. Has constant value: 0.
     :vartype const_int: int
-    :ivar const_string: Required. Constant string. Default value: "constant".
+    :ivar const_string: Constant string. Has constant value: "constant".
     :vartype const_string: str
-    :ivar const_string_as_enum: Constant string as Enum. Default value: "constant_string_as_enum".
-    :vartype const_string_as_enum: str
+    :param const_string_as_enum: Constant string as Enum. The only acceptable values to pass in are
+     None and "constant_string_as_enum". The default value is None.
+    :type const_string_as_enum: str
     """
 
     _validation = {
@@ -130,7 +131,6 @@ class Product(msrest.serialization.Model):
         "const_child": {"required": True},
         "const_int": {"required": True, "constant": True},
         "const_string": {"required": True, "constant": True},
-        "const_string_as_enum": {"constant": True},
     }
 
     _attribute_map = {
@@ -146,7 +146,6 @@ class Product(msrest.serialization.Model):
 
     const_int = 0
     const_string = "constant"
-    const_string_as_enum = "constant_string_as_enum"
 
     def __init__(
         self,
@@ -156,6 +155,7 @@ def __init__(
         display_names: Optional[List[str]] = None,
         capacity: Optional[int] = None,
         image: Optional[str] = None,
+        const_string_as_enum: Optional[str] = None,
         **kwargs
     ):
         super(Product, self).__init__(**kwargs)
@@ -164,3 +164,4 @@ def __init__(
         self.image = image
         self.child = child
         self.const_child = const_child
+        self.const_string_as_enum = const_string_as_enum