diff --git a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PythonClientCodegen.java b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PythonClientCodegen.java
index 92c24648aa9d..054005cfc515 100755
--- a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PythonClientCodegen.java
+++ b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PythonClientCodegen.java
@@ -196,6 +196,13 @@ public class PythonClientCodegen extends DefaultCodegen implements CodegenConfig
         return str.replaceAll("\\.", "_");
     }
 
+    
+    @Override
+    public Map<String, Object> postProcessModels(Map<String, Object> objs) {
+        // process enum in models
+        return postProcessModelsEnum(objs);
+    }
+    
     @Override
     public void postProcessParameter(CodegenParameter parameter){
         postProcessPattern(parameter.pattern, parameter.vendorExtensions);
diff --git a/modules/swagger-codegen/src/main/resources/python/model.mustache b/modules/swagger-codegen/src/main/resources/python/model.mustache
index dcf3c755020a..44504dbfb6b8 100644
--- a/modules/swagger-codegen/src/main/resources/python/model.mustache
+++ b/modules/swagger-codegen/src/main/resources/python/model.mustache
@@ -14,6 +14,13 @@ class {{classname}}(object):
     NOTE: This class is auto generated by the swagger code generator program.
     Do not edit the class manually.
     """
+{{#allowableValues}}
+
+{{#enumVars}}
+    {{name}} = {{{value}}}
+{{/enumVars}}
+
+{{/allowableValues}}
     def __init__(self{{#vars}}, {{name}}={{#defaultValue}}{{{defaultValue}}}{{/defaultValue}}{{^defaultValue}}None{{/defaultValue}}{{/vars}}):
         """
         {{classname}} - a model defined in Swagger
@@ -34,7 +41,15 @@ class {{classname}}(object):
         }
 
 {{#vars}}
-        self._{{name}} = {{name}}
+        self._{{name}} = None
+{{/vars}}
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+{{#vars}}
+        if {{name}} is not None:
+          self.{{name}} = {{name}}
 {{/vars}}
 
 {{#vars}}
@@ -62,9 +77,13 @@ class {{classname}}(object):
         :param {{name}}: The {{name}} of this {{classname}}.
         :type: {{datatype}}
         """
+{{#required}}
+        if {{name}} is None:
+            raise ValueError("Invalid value for `{{name}}`, must not be `None`")
+{{/required}}
 {{#isEnum}}
-        allowed_values = [{{#allowableValues}}{{#values}}"{{{this}}}"{{^-last}}, {{/-last}}{{/values}}{{/allowableValues}}]
 {{#isContainer}}
+        allowed_values = [{{#allowableValues}}{{#values}}{{#items.isString}}"{{/items.isString}}{{{this}}}{{#items.isString}}"{{/items.isString}}{{^-last}}, {{/-last}}{{/values}}{{/allowableValues}}]
 {{#isListContainer}}
         if not set({{{name}}}).issubset(set(allowed_values)):
             raise ValueError(
@@ -83,6 +102,7 @@ class {{classname}}(object):
 {{/isMapContainer}}
 {{/isContainer}}
 {{^isContainer}}
+        allowed_values = [{{#allowableValues}}{{#values}}{{#isString}}"{{/isString}}{{{this}}}{{#isString}}"{{/isString}}{{^-last}}, {{/-last}}{{/values}}{{/allowableValues}}]
         if {{{name}}} not in allowed_values:
             raise ValueError(
                 "Invalid value for `{{{name}}}` ({0}), must be one of {1}"
@@ -91,10 +111,6 @@ class {{classname}}(object):
 {{/isContainer}}
 {{/isEnum}}
 {{^isEnum}}
-{{#required}}
-        if {{name}} is None:
-            raise ValueError("Invalid value for `{{name}}`, must not be `None`")
-{{/required}}
 {{#hasValidation}}
 {{#maxLength}}
         if {{name}} is not None and len({{name}}) > {{maxLength}}:
diff --git a/modules/swagger-codegen/src/main/resources/python/tox.mustache b/modules/swagger-codegen/src/main/resources/python/tox.mustache
index d99517b76b6b..1cf0829dc930 100644
--- a/modules/swagger-codegen/src/main/resources/python/tox.mustache
+++ b/modules/swagger-codegen/src/main/resources/python/tox.mustache
@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34
+envlist = py27, py3
 
 [testenv]
 deps=-r{toxinidir}/requirements.txt
diff --git a/samples/client/petstore-security-test/python/petstore_api/models/model_return.py b/samples/client/petstore-security-test/python/petstore_api/models/model_return.py
index 2b6f2a451721..80673374e7aa 100644
--- a/samples/client/petstore-security-test/python/petstore_api/models/model_return.py
+++ b/samples/client/petstore-security-test/python/petstore_api/models/model_return.py
@@ -38,7 +38,13 @@ class ModelReturn(object):
             '_return': 'return'
         }
 
-        self.__return = _return
+        self.__return = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if _return is not None:
+          self._return = _return
 
     @property
     def _return(self):
diff --git a/samples/client/petstore-security-test/python/tox.ini b/samples/client/petstore-security-test/python/tox.ini
index d99517b76b6b..1cf0829dc930 100644
--- a/samples/client/petstore-security-test/python/tox.ini
+++ b/samples/client/petstore-security-test/python/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34
+envlist = py27, py3
 
 [testenv]
 deps=-r{toxinidir}/requirements.txt
diff --git a/samples/client/petstore/python/petstore_api/models/additional_properties_class.py b/samples/client/petstore/python/petstore_api/models/additional_properties_class.py
index 5873f70643f3..4d8206aa0132 100644
--- a/samples/client/petstore/python/petstore_api/models/additional_properties_class.py
+++ b/samples/client/petstore/python/petstore_api/models/additional_properties_class.py
@@ -40,8 +40,16 @@ class AdditionalPropertiesClass(object):
             'map_of_map_property': 'map_of_map_property'
         }
 
-        self._map_property = map_property
-        self._map_of_map_property = map_of_map_property
+        self._map_property = None
+        self._map_of_map_property = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if map_property is not None:
+          self.map_property = map_property
+        if map_of_map_property is not None:
+          self.map_of_map_property = map_of_map_property
 
     @property
     def map_property(self):
diff --git a/samples/client/petstore/python/petstore_api/models/animal.py b/samples/client/petstore/python/petstore_api/models/animal.py
index c46cfb28ddb4..db1febb58eca 100644
--- a/samples/client/petstore/python/petstore_api/models/animal.py
+++ b/samples/client/petstore/python/petstore_api/models/animal.py
@@ -40,8 +40,16 @@ class Animal(object):
             'color': 'color'
         }
 
-        self._class_name = class_name
-        self._color = color
+        self._class_name = None
+        self._color = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if class_name is not None:
+          self.class_name = class_name
+        if color is not None:
+          self.color = color
 
     @property
     def class_name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/animal_farm.py b/samples/client/petstore/python/petstore_api/models/animal_farm.py
index ff8a3794e904..3766a4f39a48 100644
--- a/samples/client/petstore/python/petstore_api/models/animal_farm.py
+++ b/samples/client/petstore/python/petstore_api/models/animal_farm.py
@@ -39,6 +39,10 @@ class AnimalFarm(object):
         }
 
 
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+
     def to_dict(self):
         """
         Returns the model properties as a dict
diff --git a/samples/client/petstore/python/petstore_api/models/api_response.py b/samples/client/petstore/python/petstore_api/models/api_response.py
index ba5c434abc74..bc072ca025ab 100644
--- a/samples/client/petstore/python/petstore_api/models/api_response.py
+++ b/samples/client/petstore/python/petstore_api/models/api_response.py
@@ -42,9 +42,19 @@ class ApiResponse(object):
             'message': 'message'
         }
 
-        self._code = code
-        self._type = type
-        self._message = message
+        self._code = None
+        self._type = None
+        self._message = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if code is not None:
+          self.code = code
+        if type is not None:
+          self.type = type
+        if message is not None:
+          self.message = message
 
     @property
     def code(self):
diff --git a/samples/client/petstore/python/petstore_api/models/array_of_array_of_number_only.py b/samples/client/petstore/python/petstore_api/models/array_of_array_of_number_only.py
index 16c6345baadf..a06f0528433e 100644
--- a/samples/client/petstore/python/petstore_api/models/array_of_array_of_number_only.py
+++ b/samples/client/petstore/python/petstore_api/models/array_of_array_of_number_only.py
@@ -38,7 +38,13 @@ class ArrayOfArrayOfNumberOnly(object):
             'array_array_number': 'ArrayArrayNumber'
         }
 
-        self._array_array_number = array_array_number
+        self._array_array_number = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if array_array_number is not None:
+          self.array_array_number = array_array_number
 
     @property
     def array_array_number(self):
diff --git a/samples/client/petstore/python/petstore_api/models/array_of_number_only.py b/samples/client/petstore/python/petstore_api/models/array_of_number_only.py
index 0db1d9591d52..c166ebc49012 100644
--- a/samples/client/petstore/python/petstore_api/models/array_of_number_only.py
+++ b/samples/client/petstore/python/petstore_api/models/array_of_number_only.py
@@ -38,7 +38,13 @@ class ArrayOfNumberOnly(object):
             'array_number': 'ArrayNumber'
         }
 
-        self._array_number = array_number
+        self._array_number = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if array_number is not None:
+          self.array_number = array_number
 
     @property
     def array_number(self):
diff --git a/samples/client/petstore/python/petstore_api/models/array_test.py b/samples/client/petstore/python/petstore_api/models/array_test.py
index bd1858faa0da..8959dd96a8e3 100644
--- a/samples/client/petstore/python/petstore_api/models/array_test.py
+++ b/samples/client/petstore/python/petstore_api/models/array_test.py
@@ -42,9 +42,19 @@ class ArrayTest(object):
             'array_array_of_model': 'array_array_of_model'
         }
 
-        self._array_of_string = array_of_string
-        self._array_array_of_integer = array_array_of_integer
-        self._array_array_of_model = array_array_of_model
+        self._array_of_string = None
+        self._array_array_of_integer = None
+        self._array_array_of_model = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if array_of_string is not None:
+          self.array_of_string = array_of_string
+        if array_array_of_integer is not None:
+          self.array_array_of_integer = array_array_of_integer
+        if array_array_of_model is not None:
+          self.array_array_of_model = array_array_of_model
 
     @property
     def array_of_string(self):
diff --git a/samples/client/petstore/python/petstore_api/models/capitalization.py b/samples/client/petstore/python/petstore_api/models/capitalization.py
index b9030b5b76dc..9b329d456c59 100644
--- a/samples/client/petstore/python/petstore_api/models/capitalization.py
+++ b/samples/client/petstore/python/petstore_api/models/capitalization.py
@@ -48,12 +48,28 @@ class Capitalization(object):
             'att_name': 'ATT_NAME'
         }
 
-        self._small_camel = small_camel
-        self._capital_camel = capital_camel
-        self._small_snake = small_snake
-        self._capital_snake = capital_snake
-        self._sca_eth_flow_points = sca_eth_flow_points
-        self._att_name = att_name
+        self._small_camel = None
+        self._capital_camel = None
+        self._small_snake = None
+        self._capital_snake = None
+        self._sca_eth_flow_points = None
+        self._att_name = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if small_camel is not None:
+          self.small_camel = small_camel
+        if capital_camel is not None:
+          self.capital_camel = capital_camel
+        if small_snake is not None:
+          self.small_snake = small_snake
+        if capital_snake is not None:
+          self.capital_snake = capital_snake
+        if sca_eth_flow_points is not None:
+          self.sca_eth_flow_points = sca_eth_flow_points
+        if att_name is not None:
+          self.att_name = att_name
 
     @property
     def small_camel(self):
diff --git a/samples/client/petstore/python/petstore_api/models/cat.py b/samples/client/petstore/python/petstore_api/models/cat.py
index 67cae27ae8ac..c0f522f3a5eb 100644
--- a/samples/client/petstore/python/petstore_api/models/cat.py
+++ b/samples/client/petstore/python/petstore_api/models/cat.py
@@ -42,9 +42,19 @@ class Cat(object):
             'declawed': 'declawed'
         }
 
-        self._class_name = class_name
-        self._color = color
-        self._declawed = declawed
+        self._class_name = None
+        self._color = None
+        self._declawed = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if class_name is not None:
+          self.class_name = class_name
+        if color is not None:
+          self.color = color
+        if declawed is not None:
+          self.declawed = declawed
 
     @property
     def class_name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/category.py b/samples/client/petstore/python/petstore_api/models/category.py
index 2cdc9911d4ec..956d3416c349 100644
--- a/samples/client/petstore/python/petstore_api/models/category.py
+++ b/samples/client/petstore/python/petstore_api/models/category.py
@@ -40,8 +40,16 @@ class Category(object):
             'name': 'name'
         }
 
-        self._id = id
-        self._name = name
+        self._id = None
+        self._name = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if id is not None:
+          self.id = id
+        if name is not None:
+          self.name = name
 
     @property
     def id(self):
diff --git a/samples/client/petstore/python/petstore_api/models/class_model.py b/samples/client/petstore/python/petstore_api/models/class_model.py
index cf93b27c7606..65dd54337710 100644
--- a/samples/client/petstore/python/petstore_api/models/class_model.py
+++ b/samples/client/petstore/python/petstore_api/models/class_model.py
@@ -38,7 +38,13 @@ class ClassModel(object):
             '_class': '_class'
         }
 
-        self.__class = _class
+        self.__class = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if _class is not None:
+          self._class = _class
 
     @property
     def _class(self):
diff --git a/samples/client/petstore/python/petstore_api/models/client.py b/samples/client/petstore/python/petstore_api/models/client.py
index 2d572bfbeb2c..5b0ed44c49c3 100644
--- a/samples/client/petstore/python/petstore_api/models/client.py
+++ b/samples/client/petstore/python/petstore_api/models/client.py
@@ -38,7 +38,13 @@ class Client(object):
             'client': 'client'
         }
 
-        self._client = client
+        self._client = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if client is not None:
+          self.client = client
 
     @property
     def client(self):
diff --git a/samples/client/petstore/python/petstore_api/models/dog.py b/samples/client/petstore/python/petstore_api/models/dog.py
index fc1d1fbf30c6..f8dfde1c3c40 100644
--- a/samples/client/petstore/python/petstore_api/models/dog.py
+++ b/samples/client/petstore/python/petstore_api/models/dog.py
@@ -42,9 +42,19 @@ class Dog(object):
             'breed': 'breed'
         }
 
-        self._class_name = class_name
-        self._color = color
-        self._breed = breed
+        self._class_name = None
+        self._color = None
+        self._breed = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if class_name is not None:
+          self.class_name = class_name
+        if color is not None:
+          self.color = color
+        if breed is not None:
+          self.breed = breed
 
     @property
     def class_name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/enum_arrays.py b/samples/client/petstore/python/petstore_api/models/enum_arrays.py
index 46b34bd41f70..1e8f6d4b8f0c 100644
--- a/samples/client/petstore/python/petstore_api/models/enum_arrays.py
+++ b/samples/client/petstore/python/petstore_api/models/enum_arrays.py
@@ -40,8 +40,16 @@ class EnumArrays(object):
             'array_enum': 'array_enum'
         }
 
-        self._just_symbol = just_symbol
-        self._array_enum = array_enum
+        self._just_symbol = None
+        self._array_enum = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if just_symbol is not None:
+          self.just_symbol = just_symbol
+        if array_enum is not None:
+          self.array_enum = array_enum
 
     @property
     def just_symbol(self):
diff --git a/samples/client/petstore/python/petstore_api/models/enum_class.py b/samples/client/petstore/python/petstore_api/models/enum_class.py
index 45f6b0b45781..4544ad58528b 100644
--- a/samples/client/petstore/python/petstore_api/models/enum_class.py
+++ b/samples/client/petstore/python/petstore_api/models/enum_class.py
@@ -21,6 +21,11 @@ class EnumClass(object):
     NOTE: This class is auto generated by the swagger code generator program.
     Do not edit the class manually.
     """
+
+    _ABC = "_abc"
+    _EFG = "-efg"
+    _XYZ_ = "(xyz)"
+
     def __init__(self):
         """
         EnumClass - a model defined in Swagger
@@ -39,6 +44,10 @@ class EnumClass(object):
         }
 
 
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+
     def to_dict(self):
         """
         Returns the model properties as a dict
diff --git a/samples/client/petstore/python/petstore_api/models/enum_test.py b/samples/client/petstore/python/petstore_api/models/enum_test.py
index e32934250494..de3c8d5a4fe5 100644
--- a/samples/client/petstore/python/petstore_api/models/enum_test.py
+++ b/samples/client/petstore/python/petstore_api/models/enum_test.py
@@ -44,10 +44,22 @@ class EnumTest(object):
             'outer_enum': 'outerEnum'
         }
 
-        self._enum_string = enum_string
-        self._enum_integer = enum_integer
-        self._enum_number = enum_number
-        self._outer_enum = outer_enum
+        self._enum_string = None
+        self._enum_integer = None
+        self._enum_number = None
+        self._outer_enum = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if enum_string is not None:
+          self.enum_string = enum_string
+        if enum_integer is not None:
+          self.enum_integer = enum_integer
+        if enum_number is not None:
+          self.enum_number = enum_number
+        if outer_enum is not None:
+          self.outer_enum = outer_enum
 
     @property
     def enum_string(self):
@@ -94,7 +106,7 @@ class EnumTest(object):
         :param enum_integer: The enum_integer of this EnumTest.
         :type: int
         """
-        allowed_values = ["1", "-1"]
+        allowed_values = [1, -1]
         if enum_integer not in allowed_values:
             raise ValueError(
                 "Invalid value for `enum_integer` ({0}), must be one of {1}"
@@ -121,7 +133,7 @@ class EnumTest(object):
         :param enum_number: The enum_number of this EnumTest.
         :type: float
         """
-        allowed_values = ["1.1", "-1.2"]
+        allowed_values = [1.1, -1.2]
         if enum_number not in allowed_values:
             raise ValueError(
                 "Invalid value for `enum_number` ({0}), must be one of {1}"
diff --git a/samples/client/petstore/python/petstore_api/models/format_test.py b/samples/client/petstore/python/petstore_api/models/format_test.py
index cde65b89a177..cc5af78fd0cf 100644
--- a/samples/client/petstore/python/petstore_api/models/format_test.py
+++ b/samples/client/petstore/python/petstore_api/models/format_test.py
@@ -62,19 +62,49 @@ class FormatTest(object):
             'password': 'password'
         }
 
-        self._integer = integer
-        self._int32 = int32
-        self._int64 = int64
-        self._number = number
-        self._float = float
-        self._double = double
-        self._string = string
-        self._byte = byte
-        self._binary = binary
-        self._date = date
-        self._date_time = date_time
-        self._uuid = uuid
-        self._password = password
+        self._integer = None
+        self._int32 = None
+        self._int64 = None
+        self._number = None
+        self._float = None
+        self._double = None
+        self._string = None
+        self._byte = None
+        self._binary = None
+        self._date = None
+        self._date_time = None
+        self._uuid = None
+        self._password = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if integer is not None:
+          self.integer = integer
+        if int32 is not None:
+          self.int32 = int32
+        if int64 is not None:
+          self.int64 = int64
+        if number is not None:
+          self.number = number
+        if float is not None:
+          self.float = float
+        if double is not None:
+          self.double = double
+        if string is not None:
+          self.string = string
+        if byte is not None:
+          self.byte = byte
+        if binary is not None:
+          self.binary = binary
+        if date is not None:
+          self.date = date
+        if date_time is not None:
+          self.date_time = date_time
+        if uuid is not None:
+          self.uuid = uuid
+        if password is not None:
+          self.password = password
 
     @property
     def integer(self):
diff --git a/samples/client/petstore/python/petstore_api/models/has_only_read_only.py b/samples/client/petstore/python/petstore_api/models/has_only_read_only.py
index c374793e6178..f528c3e3e88d 100644
--- a/samples/client/petstore/python/petstore_api/models/has_only_read_only.py
+++ b/samples/client/petstore/python/petstore_api/models/has_only_read_only.py
@@ -40,8 +40,16 @@ class HasOnlyReadOnly(object):
             'foo': 'foo'
         }
 
-        self._bar = bar
-        self._foo = foo
+        self._bar = None
+        self._foo = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if bar is not None:
+          self.bar = bar
+        if foo is not None:
+          self.foo = foo
 
     @property
     def bar(self):
diff --git a/samples/client/petstore/python/petstore_api/models/list.py b/samples/client/petstore/python/petstore_api/models/list.py
index f1393a4b6a3b..f18bd004dbe9 100644
--- a/samples/client/petstore/python/petstore_api/models/list.py
+++ b/samples/client/petstore/python/petstore_api/models/list.py
@@ -38,7 +38,13 @@ class List(object):
             '_123_list': '123-list'
         }
 
-        self.__123_list = _123_list
+        self.__123_list = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if _123_list is not None:
+          self._123_list = _123_list
 
     @property
     def _123_list(self):
diff --git a/samples/client/petstore/python/petstore_api/models/map_test.py b/samples/client/petstore/python/petstore_api/models/map_test.py
index e6ece72af81a..180c4208d892 100644
--- a/samples/client/petstore/python/petstore_api/models/map_test.py
+++ b/samples/client/petstore/python/petstore_api/models/map_test.py
@@ -40,8 +40,16 @@ class MapTest(object):
             'map_of_enum_string': 'map_of_enum_string'
         }
 
-        self._map_map_of_string = map_map_of_string
-        self._map_of_enum_string = map_of_enum_string
+        self._map_map_of_string = None
+        self._map_of_enum_string = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if map_map_of_string is not None:
+          self.map_map_of_string = map_map_of_string
+        if map_of_enum_string is not None:
+          self.map_of_enum_string = map_of_enum_string
 
     @property
     def map_map_of_string(self):
diff --git a/samples/client/petstore/python/petstore_api/models/mixed_properties_and_additional_properties_class.py b/samples/client/petstore/python/petstore_api/models/mixed_properties_and_additional_properties_class.py
index ee9b1e641e44..8d0cde871fea 100644
--- a/samples/client/petstore/python/petstore_api/models/mixed_properties_and_additional_properties_class.py
+++ b/samples/client/petstore/python/petstore_api/models/mixed_properties_and_additional_properties_class.py
@@ -42,9 +42,19 @@ class MixedPropertiesAndAdditionalPropertiesClass(object):
             'map': 'map'
         }
 
-        self._uuid = uuid
-        self._date_time = date_time
-        self._map = map
+        self._uuid = None
+        self._date_time = None
+        self._map = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if uuid is not None:
+          self.uuid = uuid
+        if date_time is not None:
+          self.date_time = date_time
+        if map is not None:
+          self.map = map
 
     @property
     def uuid(self):
diff --git a/samples/client/petstore/python/petstore_api/models/model_200_response.py b/samples/client/petstore/python/petstore_api/models/model_200_response.py
index 73b6dccc050d..bbdbfe00aeb1 100644
--- a/samples/client/petstore/python/petstore_api/models/model_200_response.py
+++ b/samples/client/petstore/python/petstore_api/models/model_200_response.py
@@ -40,8 +40,16 @@ class Model200Response(object):
             '_class': 'class'
         }
 
-        self._name = name
-        self.__class = _class
+        self._name = None
+        self.__class = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if name is not None:
+          self.name = name
+        if _class is not None:
+          self._class = _class
 
     @property
     def name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/model_return.py b/samples/client/petstore/python/petstore_api/models/model_return.py
index d99df7402b0f..0321a42ff086 100644
--- a/samples/client/petstore/python/petstore_api/models/model_return.py
+++ b/samples/client/petstore/python/petstore_api/models/model_return.py
@@ -38,7 +38,13 @@ class ModelReturn(object):
             '_return': 'return'
         }
 
-        self.__return = _return
+        self.__return = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if _return is not None:
+          self._return = _return
 
     @property
     def _return(self):
diff --git a/samples/client/petstore/python/petstore_api/models/name.py b/samples/client/petstore/python/petstore_api/models/name.py
index af281208f7a5..b7dc266bc18f 100644
--- a/samples/client/petstore/python/petstore_api/models/name.py
+++ b/samples/client/petstore/python/petstore_api/models/name.py
@@ -44,10 +44,22 @@ class Name(object):
             '_123_number': '123Number'
         }
 
-        self._name = name
-        self._snake_case = snake_case
-        self.__property = _property
-        self.__123_number = _123_number
+        self._name = None
+        self._snake_case = None
+        self.__property = None
+        self.__123_number = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if name is not None:
+          self.name = name
+        if snake_case is not None:
+          self.snake_case = snake_case
+        if _property is not None:
+          self._property = _property
+        if _123_number is not None:
+          self._123_number = _123_number
 
     @property
     def name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/number_only.py b/samples/client/petstore/python/petstore_api/models/number_only.py
index 48fea0685099..9d927edc0b16 100644
--- a/samples/client/petstore/python/petstore_api/models/number_only.py
+++ b/samples/client/petstore/python/petstore_api/models/number_only.py
@@ -38,7 +38,13 @@ class NumberOnly(object):
             'just_number': 'JustNumber'
         }
 
-        self._just_number = just_number
+        self._just_number = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if just_number is not None:
+          self.just_number = just_number
 
     @property
     def just_number(self):
diff --git a/samples/client/petstore/python/petstore_api/models/order.py b/samples/client/petstore/python/petstore_api/models/order.py
index d0ed63cd4826..0445e56b4e94 100644
--- a/samples/client/petstore/python/petstore_api/models/order.py
+++ b/samples/client/petstore/python/petstore_api/models/order.py
@@ -48,12 +48,28 @@ class Order(object):
             'complete': 'complete'
         }
 
-        self._id = id
-        self._pet_id = pet_id
-        self._quantity = quantity
-        self._ship_date = ship_date
-        self._status = status
-        self._complete = complete
+        self._id = None
+        self._pet_id = None
+        self._quantity = None
+        self._ship_date = None
+        self._status = None
+        self._complete = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if id is not None:
+          self.id = id
+        if pet_id is not None:
+          self.pet_id = pet_id
+        if quantity is not None:
+          self.quantity = quantity
+        if ship_date is not None:
+          self.ship_date = ship_date
+        if status is not None:
+          self.status = status
+        if complete is not None:
+          self.complete = complete
 
     @property
     def id(self):
diff --git a/samples/client/petstore/python/petstore_api/models/outer_enum.py b/samples/client/petstore/python/petstore_api/models/outer_enum.py
index 8c732caee816..3e8cb6f23871 100644
--- a/samples/client/petstore/python/petstore_api/models/outer_enum.py
+++ b/samples/client/petstore/python/petstore_api/models/outer_enum.py
@@ -21,6 +21,11 @@ class OuterEnum(object):
     NOTE: This class is auto generated by the swagger code generator program.
     Do not edit the class manually.
     """
+
+    PLACED = "placed"
+    APPROVED = "approved"
+    DELIVERED = "delivered"
+
     def __init__(self):
         """
         OuterEnum - a model defined in Swagger
@@ -39,6 +44,10 @@ class OuterEnum(object):
         }
 
 
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+
     def to_dict(self):
         """
         Returns the model properties as a dict
diff --git a/samples/client/petstore/python/petstore_api/models/pet.py b/samples/client/petstore/python/petstore_api/models/pet.py
index 7dea8afd1b23..e32e779fbd4d 100644
--- a/samples/client/petstore/python/petstore_api/models/pet.py
+++ b/samples/client/petstore/python/petstore_api/models/pet.py
@@ -48,12 +48,28 @@ class Pet(object):
             'status': 'status'
         }
 
-        self._id = id
-        self._category = category
-        self._name = name
-        self._photo_urls = photo_urls
-        self._tags = tags
-        self._status = status
+        self._id = None
+        self._category = None
+        self._name = None
+        self._photo_urls = None
+        self._tags = None
+        self._status = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if id is not None:
+          self.id = id
+        if category is not None:
+          self.category = category
+        if name is not None:
+          self.name = name
+        if photo_urls is not None:
+          self.photo_urls = photo_urls
+        if tags is not None:
+          self.tags = tags
+        if status is not None:
+          self.status = status
 
     @property
     def id(self):
diff --git a/samples/client/petstore/python/petstore_api/models/read_only_first.py b/samples/client/petstore/python/petstore_api/models/read_only_first.py
index 2b1dfda420c4..a3446564d326 100644
--- a/samples/client/petstore/python/petstore_api/models/read_only_first.py
+++ b/samples/client/petstore/python/petstore_api/models/read_only_first.py
@@ -40,8 +40,16 @@ class ReadOnlyFirst(object):
             'baz': 'baz'
         }
 
-        self._bar = bar
-        self._baz = baz
+        self._bar = None
+        self._baz = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if bar is not None:
+          self.bar = bar
+        if baz is not None:
+          self.baz = baz
 
     @property
     def bar(self):
diff --git a/samples/client/petstore/python/petstore_api/models/special_model_name.py b/samples/client/petstore/python/petstore_api/models/special_model_name.py
index 651c51e8e56e..e4893b92d453 100644
--- a/samples/client/petstore/python/petstore_api/models/special_model_name.py
+++ b/samples/client/petstore/python/petstore_api/models/special_model_name.py
@@ -38,7 +38,13 @@ class SpecialModelName(object):
             'special_property_name': '$special[property.name]'
         }
 
-        self._special_property_name = special_property_name
+        self._special_property_name = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if special_property_name is not None:
+          self.special_property_name = special_property_name
 
     @property
     def special_property_name(self):
diff --git a/samples/client/petstore/python/petstore_api/models/tag.py b/samples/client/petstore/python/petstore_api/models/tag.py
index 4bd6bfedfb62..986769a3df39 100644
--- a/samples/client/petstore/python/petstore_api/models/tag.py
+++ b/samples/client/petstore/python/petstore_api/models/tag.py
@@ -40,8 +40,16 @@ class Tag(object):
             'name': 'name'
         }
 
-        self._id = id
-        self._name = name
+        self._id = None
+        self._name = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if id is not None:
+          self.id = id
+        if name is not None:
+          self.name = name
 
     @property
     def id(self):
diff --git a/samples/client/petstore/python/petstore_api/models/user.py b/samples/client/petstore/python/petstore_api/models/user.py
index ed3af1ce20d5..dd3602637564 100644
--- a/samples/client/petstore/python/petstore_api/models/user.py
+++ b/samples/client/petstore/python/petstore_api/models/user.py
@@ -52,14 +52,34 @@ class User(object):
             'user_status': 'userStatus'
         }
 
-        self._id = id
-        self._username = username
-        self._first_name = first_name
-        self._last_name = last_name
-        self._email = email
-        self._password = password
-        self._phone = phone
-        self._user_status = user_status
+        self._id = None
+        self._username = None
+        self._first_name = None
+        self._last_name = None
+        self._email = None
+        self._password = None
+        self._phone = None
+        self._user_status = None
+
+        # TODO: let required properties as mandatory parameter in the constructor.
+        #       - to check if required property is not None (e.g. by calling setter)
+        #       - ApiClient.__deserialize_model has to be adapted as well
+        if id is not None:
+          self.id = id
+        if username is not None:
+          self.username = username
+        if first_name is not None:
+          self.first_name = first_name
+        if last_name is not None:
+          self.last_name = last_name
+        if email is not None:
+          self.email = email
+        if password is not None:
+          self.password = password
+        if phone is not None:
+          self.phone = phone
+        if user_status is not None:
+          self.user_status = user_status
 
     @property
     def id(self):
diff --git a/samples/client/petstore/python/tests/test_enum_arrays.py b/samples/client/petstore/python/tests/test_enum_arrays.py
index aa373339a60a..41e7dcb0a74a 100644
--- a/samples/client/petstore/python/tests/test_enum_arrays.py
+++ b/samples/client/petstore/python/tests/test_enum_arrays.py
@@ -42,22 +42,21 @@ class EnumArraysTests(unittest.TestCase):
       #
       try:
         fish_or_crab = petstore_api.EnumArrays(just_symbol="<=")
+        self.assertTrue(0)
       except ValueError:
-        self.assertEqual(fish_or_crab.just_symbol, None)
-        self.assertEqual(fish_or_crab.array_enum, None)
+        self.assertTrue(1)
 
       try:
         fish_or_crab = petstore_api.EnumArrays(just_symbol="$", array_enum=["dog"])
+        self.assertTrue(0)
       except ValueError:
-        self.assertEqual(fish_or_crab.just_symbol, None)
-        self.assertEqual(fish_or_crab.array_enum, None)
+        self.assertTrue(1)
 
-      
       try:
         fish_or_crab = petstore_api.EnumArrays(just_symbol=["$"], array_enum=["dog"])
+        self.assertTrue(0)
       except ValueError:
-        self.assertEqual(fish_or_crab.just_symbol, None)
-        self.assertEqual(fish_or_crab.array_enum, None)
+        self.assertTrue(1)
 
 
     def test_enumarrays_setter(self):
diff --git a/samples/client/petstore/python/tests/test_map_test.py b/samples/client/petstore/python/tests/test_map_test.py
index 19903c4e6308..012b9254b471 100644
--- a/samples/client/petstore/python/tests/test_map_test.py
+++ b/samples/client/petstore/python/tests/test_map_test.py
@@ -46,8 +46,9 @@ class MapTestTests(unittest.TestCase):
       }
       try:
         map_enum_test = petstore_api.MapTest(map_of_enum_string=black_or_white_dict)
+        self.assertTrue(0)
       except ValueError:
-        self.assertEqual(map_enum_test, None)
+        self.assertTrue(1)
 
 
     def test_maptest_setter(self):
diff --git a/samples/client/petstore/python/tox.ini b/samples/client/petstore/python/tox.ini
index d99517b76b6b..1cf0829dc930 100644
--- a/samples/client/petstore/python/tox.ini
+++ b/samples/client/petstore/python/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py34
+envlist = py27, py3
 
 [testenv]
 deps=-r{toxinidir}/requirements.txt