diff --git a/modules/openapi-generator/src/main/resources/python-nextgen/model_anyof.mustache b/modules/openapi-generator/src/main/resources/python-nextgen/model_anyof.mustache
index f963021e087..c1e67ffb1a4 100644
--- a/modules/openapi-generator/src/main/resources/python-nextgen/model_anyof.mustache
+++ b/modules/openapi-generator/src/main/resources/python-nextgen/model_anyof.mustache
@@ -37,6 +37,16 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
     }
 {{/discriminator}}
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_anyof(cls, v):
         {{#isNullable}}
@@ -44,7 +54,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
             return v
 
         {{/isNullable}}
-        instance = cls()
+        instance = {{{classname}}}.construct()
         error_messages = []
         {{#composedSchemas.anyOf}}
         # validate data type: {{{dataType}}}
@@ -74,7 +84,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
         {{/composedSchemas.anyOf}}
         if error_messages:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into {{{classname}}} with anyOf schemas: {{#anyOf}}{{{.}}}{{^-last}}, {{/-last}}{{/anyOf}}. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting the actual_instance in {{{classname}}} with anyOf schemas: {{#anyOf}}{{{.}}}{{^-last}}, {{/-last}}{{/anyOf}}. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -85,7 +95,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
     @classmethod
     def from_json(cls, json_str: str) -> {{{classname}}}:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = {{{classname}}}.construct()
         {{#isNullable}}
         if json_str is None:
             return instance
diff --git a/modules/openapi-generator/src/main/resources/python-nextgen/model_oneof.mustache b/modules/openapi-generator/src/main/resources/python-nextgen/model_oneof.mustache
index 9bba34bad02..9c3463ba5ec 100644
--- a/modules/openapi-generator/src/main/resources/python-nextgen/model_oneof.mustache
+++ b/modules/openapi-generator/src/main/resources/python-nextgen/model_oneof.mustache
@@ -36,6 +36,16 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
     }
 {{/discriminator}}
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
         {{#isNullable}}
@@ -43,7 +53,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
             return v
 
         {{/isNullable}}
-        instance = cls()
+        instance = {{{classname}}}.construct()
         error_messages = []
         match = 0
         {{#composedSchemas.oneOf}}
@@ -68,16 +78,15 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
             error_messages.append(f"Error! Input type `{type(v)}` is not `{{{dataType}}}`")
         else:
             match += 1
-
         {{/isPrimitiveType}}
         {{/isContainer}}
         {{/composedSchemas.oneOf}}
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into {{{classname}}} with oneOf schemas: {{#oneOf}}{{{.}}}{{^-last}}, {{/-last}}{{/oneOf}}. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in {{{classname}}} with oneOf schemas: {{#oneOf}}{{{.}}}{{^-last}}, {{/-last}}{{/oneOf}}. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into {{{classname}}} with oneOf schemas: {{#oneOf}}{{{.}}}{{^-last}}, {{/-last}}{{/oneOf}}. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in {{{classname}}} with oneOf schemas: {{#oneOf}}{{{.}}}{{^-last}}, {{/-last}}{{/oneOf}}. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -88,7 +97,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
     @classmethod
     def from_json(cls, json_str: str) -> {{{classname}}}:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = {{{classname}}}.construct()
         {{#isNullable}}
         if json_str is None:
             return instance
diff --git a/modules/openapi-generator/src/test/resources/3_0/python/petstore-with-fake-endpoints-models-for-testing.yaml b/modules/openapi-generator/src/test/resources/3_0/python/petstore-with-fake-endpoints-models-for-testing.yaml
index d9792ff1b18..c7ea2b82a87 100644
--- a/modules/openapi-generator/src/test/resources/3_0/python/petstore-with-fake-endpoints-models-for-testing.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/python/petstore-with-fake-endpoints-models-for-testing.yaml
@@ -2194,3 +2194,8 @@ components:
       properties:
         optionalDict:
           $ref: "#/components/schemas/DictWithAdditionalProperties"
+    IntOrString:
+      oneOf:
+      - type: integer
+        minimum: 10
+      - type: string
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/.openapi-generator/FILES b/samples/openapi3/client/petstore/python-nextgen-aiohttp/.openapi-generator/FILES
index 63a87c86fdf..e17417e7c4b 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/.openapi-generator/FILES
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/.openapi-generator/FILES
@@ -44,6 +44,7 @@ docs/FormatTest.md
 docs/HasOnlyReadOnly.md
 docs/HealthCheckResult.md
 docs/InnerDictWithProperty.md
+docs/IntOrString.md
 docs/List.md
 docs/MapTest.md
 docs/MixedPropertiesAndAdditionalPropertiesClass.md
@@ -130,6 +131,7 @@ petstore_api/models/format_test.py
 petstore_api/models/has_only_read_only.py
 petstore_api/models/health_check_result.py
 petstore_api/models/inner_dict_with_property.py
+petstore_api/models/int_or_string.py
 petstore_api/models/list.py
 petstore_api/models/map_test.py
 petstore_api/models/mixed_properties_and_additional_properties_class.py
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/README.md b/samples/openapi3/client/petstore/python-nextgen-aiohttp/README.md
index 338c15e81db..bbf76b9e2f6 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/README.md
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/README.md
@@ -169,6 +169,7 @@ Class | Method | HTTP request | Description
  - [HasOnlyReadOnly](docs/HasOnlyReadOnly.md)
  - [HealthCheckResult](docs/HealthCheckResult.md)
  - [InnerDictWithProperty](docs/InnerDictWithProperty.md)
+ - [IntOrString](docs/IntOrString.md)
  - [List](docs/List.md)
  - [MapTest](docs/MapTest.md)
  - [MixedPropertiesAndAdditionalPropertiesClass](docs/MixedPropertiesAndAdditionalPropertiesClass.md)
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/docs/IntOrString.md b/samples/openapi3/client/petstore/python-nextgen-aiohttp/docs/IntOrString.md
new file mode 100644
index 00000000000..0a441f186db
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/docs/IntOrString.md
@@ -0,0 +1,27 @@
+# IntOrString
+
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+
+## Example
+
+```python
+from petstore_api.models.int_or_string import IntOrString
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of IntOrString from a JSON string
+int_or_string_instance = IntOrString.from_json(json)
+# print the JSON string representation of the object
+print IntOrString.to_json()
+
+# convert the object into a dict
+int_or_string_dict = int_or_string_instance.to_dict()
+# create an instance of IntOrString from a dict
+int_or_string_form_dict = int_or_string.from_dict(int_or_string_dict)
+```
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/__init__.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/__init__.py
index 9e08911cc56..d4bbb4133f7 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/__init__.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/__init__.py
@@ -75,6 +75,7 @@ from petstore_api.models.format_test import FormatTest
 from petstore_api.models.has_only_read_only import HasOnlyReadOnly
 from petstore_api.models.health_check_result import HealthCheckResult
 from petstore_api.models.inner_dict_with_property import InnerDictWithProperty
+from petstore_api.models.int_or_string import IntOrString
 from petstore_api.models.list import List
 from petstore_api.models.map_test import MapTest
 from petstore_api.models.mixed_properties_and_additional_properties_class import MixedPropertiesAndAdditionalPropertiesClass
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/__init__.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/__init__.py
index 1e9aae863bc..8412641c064 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/__init__.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/__init__.py
@@ -51,6 +51,7 @@ from petstore_api.models.format_test import FormatTest
 from petstore_api.models.has_only_read_only import HasOnlyReadOnly
 from petstore_api.models.health_check_result import HealthCheckResult
 from petstore_api.models.inner_dict_with_property import InnerDictWithProperty
+from petstore_api.models.int_or_string import IntOrString
 from petstore_api.models.list import List
 from petstore_api.models.map_test import MapTest
 from petstore_api.models.mixed_properties_and_additional_properties_class import MixedPropertiesAndAdditionalPropertiesClass
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_color.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_color.py
index 70353dcd305..a48b11b5926 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_color.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_color.py
@@ -42,9 +42,19 @@ class AnyOfColor(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_anyof(cls, v):
-        instance = cls()
+        instance = AnyOfColor.construct()
         error_messages = []
         # validate data type: List[int]
         try:
@@ -66,7 +76,7 @@ class AnyOfColor(BaseModel):
             error_messages.append(str(e))
         if error_messages:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into AnyOfColor with anyOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting the actual_instance in AnyOfColor with anyOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -77,7 +87,7 @@ class AnyOfColor(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> AnyOfColor:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = AnyOfColor.construct()
         error_messages = []
         # deserialize data into List[int]
         try:
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_pig.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_pig.py
index 03db208b03f..909d95d4d43 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_pig.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/any_of_pig.py
@@ -42,9 +42,19 @@ class AnyOfPig(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_anyof(cls, v):
-        instance = cls()
+        instance = AnyOfPig.construct()
         error_messages = []
         # validate data type: BasquePig
         if not isinstance(v, BasquePig):
@@ -60,7 +70,7 @@ class AnyOfPig(BaseModel):
 
         if error_messages:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into AnyOfPig with anyOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting the actual_instance in AnyOfPig with anyOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -71,7 +81,7 @@ class AnyOfPig(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> AnyOfPig:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = AnyOfPig.construct()
         error_messages = []
         # anyof_schema_1_validator: Optional[BasquePig] = None
         try:
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/color.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/color.py
index 99de5867c48..97049e5811c 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/color.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/color.py
@@ -41,12 +41,22 @@ class Color(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
         if v is None:
             return v
 
-        instance = cls()
+        instance = Color.construct()
         error_messages = []
         match = 0
         # validate data type: List[int]
@@ -69,10 +79,10 @@ class Color(BaseModel):
             error_messages.append(str(e))
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -83,7 +93,7 @@ class Color(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> Color:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = Color.construct()
         if json_str is None:
             return instance
 
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/int_or_string.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/int_or_string.py
new file mode 100644
index 00000000000..63293ee016d
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/int_or_string.py
@@ -0,0 +1,143 @@
+# coding: utf-8
+
+"""
+    OpenAPI Petstore
+
+    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by OpenAPI Generator (https://openapi-generator.tech)
+
+    Do not edit the class manually.
+"""
+
+
+from __future__ import annotations
+from inspect import getfullargspec
+import json
+import pprint
+import re  # noqa: F401
+
+from typing import Any, List, Optional
+from pydantic import BaseModel, Field, StrictStr, ValidationError, conint, validator
+from typing import Any, List
+from pydantic import StrictStr, Field
+
+INTORSTRING_ONE_OF_SCHEMAS = ["int", "str"]
+
+class IntOrString(BaseModel):
+    """
+    IntOrString
+    """
+    # data type: int
+    oneof_schema_1_validator: Optional[conint(strict=True, ge=10)] = None
+    # data type: str
+    oneof_schema_2_validator: Optional[StrictStr] = None
+    actual_instance: Any
+    one_of_schemas: List[str] = Field(INTORSTRING_ONE_OF_SCHEMAS, const=True)
+
+    class Config:
+        validate_assignment = True
+
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
+    @validator('actual_instance')
+    def actual_instance_must_validate_oneof(cls, v):
+        instance = IntOrString.construct()
+        error_messages = []
+        match = 0
+        # validate data type: int
+        try:
+            instance.oneof_schema_1_validator = v
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        # validate data type: str
+        try:
+            instance.oneof_schema_2_validator = v
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        if match > 1:
+            # more than 1 match
+            raise ValueError("Multiple matches found when setting `actual_instance` in IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        elif match == 0:
+            # no match
+            raise ValueError("No match found when setting `actual_instance` in IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        else:
+            return v
+
+    @classmethod
+    def from_dict(cls, obj: dict) -> IntOrString:
+        return cls.from_json(json.dumps(obj))
+
+    @classmethod
+    def from_json(cls, json_str: str) -> IntOrString:
+        """Returns the object represented by the json string"""
+        instance = IntOrString.construct()
+        error_messages = []
+        match = 0
+
+        # deserialize data into int
+        try:
+            # validation
+            instance.oneof_schema_1_validator = json.loads(json_str)
+            # assign value to actual_instance
+            instance.actual_instance = instance.oneof_schema_1_validator
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        # deserialize data into str
+        try:
+            # validation
+            instance.oneof_schema_2_validator = json.loads(json_str)
+            # assign value to actual_instance
+            instance.actual_instance = instance.oneof_schema_2_validator
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+
+        if match > 1:
+            # more than 1 match
+            raise ValueError("Multiple matches found when deserializing the JSON string into IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        elif match == 0:
+            # no match
+            raise ValueError("No match found when deserializing the JSON string into IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        else:
+            return instance
+
+    def to_json(self) -> str:
+        """Returns the JSON representation of the actual instance"""
+        if self.actual_instance is None:
+            return "null"
+
+        to_json = getattr(self.actual_instance, "to_json", None)
+        if callable(to_json):
+            return self.actual_instance.to_json()
+        else:
+            return json.dumps(self.actual_instance)
+
+    def to_dict(self) -> dict:
+        """Returns the dict representation of the actual instance"""
+        if self.actual_instance is None:
+            return None
+
+        to_dict = getattr(self.actual_instance, "to_dict", None)
+        if callable(to_dict):
+            return self.actual_instance.to_dict()
+        else:
+            # primitive type
+            return self.actual_instance
+
+    def to_str(self) -> str:
+        """Returns the string representation of the actual instance"""
+        return pprint.pformat(self.dict())
+
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/one_of_enum_string.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/one_of_enum_string.py
index 04a9440e0d6..ae674028175 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/one_of_enum_string.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/one_of_enum_string.py
@@ -41,9 +41,19 @@ class OneOfEnumString(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
-        instance = cls()
+        instance = OneOfEnumString.construct()
         error_messages = []
         match = 0
         # validate data type: EnumString1
@@ -51,19 +61,17 @@ class OneOfEnumString(BaseModel):
             error_messages.append(f"Error! Input type `{type(v)}` is not `EnumString1`")
         else:
             match += 1
-
         # validate data type: EnumString2
         if not isinstance(v, EnumString2):
             error_messages.append(f"Error! Input type `{type(v)}` is not `EnumString2`")
         else:
             match += 1
-
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -74,7 +82,7 @@ class OneOfEnumString(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> OneOfEnumString:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = OneOfEnumString.construct()
         error_messages = []
         match = 0
 
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pig.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pig.py
index 96d60d12e8d..859c2dc9475 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pig.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/petstore_api/models/pig.py
@@ -44,9 +44,19 @@ class Pig(BaseModel):
     discriminator_value_class_map = {
     }
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
-        instance = cls()
+        instance = Pig.construct()
         error_messages = []
         match = 0
         # validate data type: BasquePig
@@ -54,19 +64,17 @@ class Pig(BaseModel):
             error_messages.append(f"Error! Input type `{type(v)}` is not `BasquePig`")
         else:
             match += 1
-
         # validate data type: DanishPig
         if not isinstance(v, DanishPig):
             error_messages.append(f"Error! Input type `{type(v)}` is not `DanishPig`")
         else:
             match += 1
-
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -77,7 +85,7 @@ class Pig(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> Pig:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = Pig.construct()
         error_messages = []
         match = 0
 
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/test/test_int_or_string.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/test/test_int_or_string.py
new file mode 100644
index 00000000000..0de76602cb6
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/test/test_int_or_string.py
@@ -0,0 +1,53 @@
+# coding: utf-8
+
+"""
+    OpenAPI Petstore
+
+    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by OpenAPI Generator (https://openapi-generator.tech)
+
+    Do not edit the class manually.
+"""
+
+
+import unittest
+import datetime
+
+import petstore_api
+from petstore_api.models.int_or_string import IntOrString  # noqa: E501
+from petstore_api.rest import ApiException
+
+class TestIntOrString(unittest.TestCase):
+    """IntOrString unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def make_instance(self, include_optional):
+        """Test IntOrString
+            include_option is a boolean, when False only required
+            params are included, when True both required and
+            optional params are included """
+        # uncomment below to create an instance of `IntOrString`
+        """
+        model = petstore_api.models.int_or_string.IntOrString()  # noqa: E501
+        if include_optional :
+            return IntOrString(
+            )
+        else :
+            return IntOrString(
+        )
+        """
+
+    def testIntOrString(self):
+        """Test IntOrString"""
+        # inst_req_only = self.make_instance(include_optional=False)
+        # inst_req_and_optional = self.make_instance(include_optional=True)
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/samples/openapi3/client/petstore/python-nextgen-aiohttp/tests/test_model.py b/samples/openapi3/client/petstore/python-nextgen-aiohttp/tests/test_model.py
index c9dc0681baf..615f50732e1 100644
--- a/samples/openapi3/client/petstore/python-nextgen-aiohttp/tests/test_model.py
+++ b/samples/openapi3/client/petstore/python-nextgen-aiohttp/tests/test_model.py
@@ -91,8 +91,7 @@ class ModelTests(unittest.TestCase):
             self.assertTrue(False)  # this line shouldn't execute
         except ValueError as e:
             self.assertTrue(
-                "No match found when deserializing the JSON string into Pig with oneOf schemas: "
-                "BasquePig, DanishPig" in str(e))
+                "No match found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig" in str(e))
 
         # failure
         try:
@@ -141,7 +140,7 @@ class ModelTests(unittest.TestCase):
             self.assertTrue(False)  # this line shouldn't execute
         except ValueError as e:
             self.assertTrue(
-                "No match found when deserializing the JSON string into AnyOfPig with anyOf schemas: BasquePig, "
+                "No match found when setting the actual_instance in AnyOfPig with anyOf schemas: BasquePig, "
                 "DanishPig" in str(e))
 
         # failure
diff --git a/samples/openapi3/client/petstore/python-nextgen/.openapi-generator/FILES b/samples/openapi3/client/petstore/python-nextgen/.openapi-generator/FILES
index cb19ba4a03d..dcecb210387 100755
--- a/samples/openapi3/client/petstore/python-nextgen/.openapi-generator/FILES
+++ b/samples/openapi3/client/petstore/python-nextgen/.openapi-generator/FILES
@@ -44,6 +44,7 @@ docs/FormatTest.md
 docs/HasOnlyReadOnly.md
 docs/HealthCheckResult.md
 docs/InnerDictWithProperty.md
+docs/IntOrString.md
 docs/List.md
 docs/MapTest.md
 docs/MixedPropertiesAndAdditionalPropertiesClass.md
@@ -130,6 +131,7 @@ petstore_api/models/format_test.py
 petstore_api/models/has_only_read_only.py
 petstore_api/models/health_check_result.py
 petstore_api/models/inner_dict_with_property.py
+petstore_api/models/int_or_string.py
 petstore_api/models/list.py
 petstore_api/models/map_test.py
 petstore_api/models/mixed_properties_and_additional_properties_class.py
diff --git a/samples/openapi3/client/petstore/python-nextgen/README.md b/samples/openapi3/client/petstore/python-nextgen/README.md
index ccafe47182a..b6df197925b 100755
--- a/samples/openapi3/client/petstore/python-nextgen/README.md
+++ b/samples/openapi3/client/petstore/python-nextgen/README.md
@@ -169,6 +169,7 @@ Class | Method | HTTP request | Description
  - [HasOnlyReadOnly](docs/HasOnlyReadOnly.md)
  - [HealthCheckResult](docs/HealthCheckResult.md)
  - [InnerDictWithProperty](docs/InnerDictWithProperty.md)
+ - [IntOrString](docs/IntOrString.md)
  - [List](docs/List.md)
  - [MapTest](docs/MapTest.md)
  - [MixedPropertiesAndAdditionalPropertiesClass](docs/MixedPropertiesAndAdditionalPropertiesClass.md)
diff --git a/samples/openapi3/client/petstore/python-nextgen/docs/IntOrString.md b/samples/openapi3/client/petstore/python-nextgen/docs/IntOrString.md
new file mode 100644
index 00000000000..0a441f186db
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen/docs/IntOrString.md
@@ -0,0 +1,27 @@
+# IntOrString
+
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+
+## Example
+
+```python
+from petstore_api.models.int_or_string import IntOrString
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of IntOrString from a JSON string
+int_or_string_instance = IntOrString.from_json(json)
+# print the JSON string representation of the object
+print IntOrString.to_json()
+
+# convert the object into a dict
+int_or_string_dict = int_or_string_instance.to_dict()
+# create an instance of IntOrString from a dict
+int_or_string_form_dict = int_or_string.from_dict(int_or_string_dict)
+```
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/__init__.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/__init__.py
index 9e08911cc56..d4bbb4133f7 100755
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/__init__.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/__init__.py
@@ -75,6 +75,7 @@ from petstore_api.models.format_test import FormatTest
 from petstore_api.models.has_only_read_only import HasOnlyReadOnly
 from petstore_api.models.health_check_result import HealthCheckResult
 from petstore_api.models.inner_dict_with_property import InnerDictWithProperty
+from petstore_api.models.int_or_string import IntOrString
 from petstore_api.models.list import List
 from petstore_api.models.map_test import MapTest
 from petstore_api.models.mixed_properties_and_additional_properties_class import MixedPropertiesAndAdditionalPropertiesClass
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/__init__.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/__init__.py
index 1e9aae863bc..8412641c064 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/__init__.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/__init__.py
@@ -51,6 +51,7 @@ from petstore_api.models.format_test import FormatTest
 from petstore_api.models.has_only_read_only import HasOnlyReadOnly
 from petstore_api.models.health_check_result import HealthCheckResult
 from petstore_api.models.inner_dict_with_property import InnerDictWithProperty
+from petstore_api.models.int_or_string import IntOrString
 from petstore_api.models.list import List
 from petstore_api.models.map_test import MapTest
 from petstore_api.models.mixed_properties_and_additional_properties_class import MixedPropertiesAndAdditionalPropertiesClass
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_color.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_color.py
index 70353dcd305..a48b11b5926 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_color.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_color.py
@@ -42,9 +42,19 @@ class AnyOfColor(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_anyof(cls, v):
-        instance = cls()
+        instance = AnyOfColor.construct()
         error_messages = []
         # validate data type: List[int]
         try:
@@ -66,7 +76,7 @@ class AnyOfColor(BaseModel):
             error_messages.append(str(e))
         if error_messages:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into AnyOfColor with anyOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting the actual_instance in AnyOfColor with anyOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -77,7 +87,7 @@ class AnyOfColor(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> AnyOfColor:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = AnyOfColor.construct()
         error_messages = []
         # deserialize data into List[int]
         try:
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_pig.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_pig.py
index 03db208b03f..909d95d4d43 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_pig.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/any_of_pig.py
@@ -42,9 +42,19 @@ class AnyOfPig(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_anyof(cls, v):
-        instance = cls()
+        instance = AnyOfPig.construct()
         error_messages = []
         # validate data type: BasquePig
         if not isinstance(v, BasquePig):
@@ -60,7 +70,7 @@ class AnyOfPig(BaseModel):
 
         if error_messages:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into AnyOfPig with anyOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting the actual_instance in AnyOfPig with anyOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -71,7 +81,7 @@ class AnyOfPig(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> AnyOfPig:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = AnyOfPig.construct()
         error_messages = []
         # anyof_schema_1_validator: Optional[BasquePig] = None
         try:
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/color.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/color.py
index 99de5867c48..97049e5811c 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/color.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/color.py
@@ -41,12 +41,22 @@ class Color(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
         if v is None:
             return v
 
-        instance = cls()
+        instance = Color.construct()
         error_messages = []
         match = 0
         # validate data type: List[int]
@@ -69,10 +79,10 @@ class Color(BaseModel):
             error_messages.append(str(e))
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in Color with oneOf schemas: List[int], str. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -83,7 +93,7 @@ class Color(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> Color:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = Color.construct()
         if json_str is None:
             return instance
 
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/int_or_string.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/int_or_string.py
new file mode 100644
index 00000000000..63293ee016d
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/int_or_string.py
@@ -0,0 +1,143 @@
+# coding: utf-8
+
+"""
+    OpenAPI Petstore
+
+    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by OpenAPI Generator (https://openapi-generator.tech)
+
+    Do not edit the class manually.
+"""
+
+
+from __future__ import annotations
+from inspect import getfullargspec
+import json
+import pprint
+import re  # noqa: F401
+
+from typing import Any, List, Optional
+from pydantic import BaseModel, Field, StrictStr, ValidationError, conint, validator
+from typing import Any, List
+from pydantic import StrictStr, Field
+
+INTORSTRING_ONE_OF_SCHEMAS = ["int", "str"]
+
+class IntOrString(BaseModel):
+    """
+    IntOrString
+    """
+    # data type: int
+    oneof_schema_1_validator: Optional[conint(strict=True, ge=10)] = None
+    # data type: str
+    oneof_schema_2_validator: Optional[StrictStr] = None
+    actual_instance: Any
+    one_of_schemas: List[str] = Field(INTORSTRING_ONE_OF_SCHEMAS, const=True)
+
+    class Config:
+        validate_assignment = True
+
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
+    @validator('actual_instance')
+    def actual_instance_must_validate_oneof(cls, v):
+        instance = IntOrString.construct()
+        error_messages = []
+        match = 0
+        # validate data type: int
+        try:
+            instance.oneof_schema_1_validator = v
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        # validate data type: str
+        try:
+            instance.oneof_schema_2_validator = v
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        if match > 1:
+            # more than 1 match
+            raise ValueError("Multiple matches found when setting `actual_instance` in IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        elif match == 0:
+            # no match
+            raise ValueError("No match found when setting `actual_instance` in IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        else:
+            return v
+
+    @classmethod
+    def from_dict(cls, obj: dict) -> IntOrString:
+        return cls.from_json(json.dumps(obj))
+
+    @classmethod
+    def from_json(cls, json_str: str) -> IntOrString:
+        """Returns the object represented by the json string"""
+        instance = IntOrString.construct()
+        error_messages = []
+        match = 0
+
+        # deserialize data into int
+        try:
+            # validation
+            instance.oneof_schema_1_validator = json.loads(json_str)
+            # assign value to actual_instance
+            instance.actual_instance = instance.oneof_schema_1_validator
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+        # deserialize data into str
+        try:
+            # validation
+            instance.oneof_schema_2_validator = json.loads(json_str)
+            # assign value to actual_instance
+            instance.actual_instance = instance.oneof_schema_2_validator
+            match += 1
+        except (ValidationError, ValueError) as e:
+            error_messages.append(str(e))
+
+        if match > 1:
+            # more than 1 match
+            raise ValueError("Multiple matches found when deserializing the JSON string into IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        elif match == 0:
+            # no match
+            raise ValueError("No match found when deserializing the JSON string into IntOrString with oneOf schemas: int, str. Details: " + ", ".join(error_messages))
+        else:
+            return instance
+
+    def to_json(self) -> str:
+        """Returns the JSON representation of the actual instance"""
+        if self.actual_instance is None:
+            return "null"
+
+        to_json = getattr(self.actual_instance, "to_json", None)
+        if callable(to_json):
+            return self.actual_instance.to_json()
+        else:
+            return json.dumps(self.actual_instance)
+
+    def to_dict(self) -> dict:
+        """Returns the dict representation of the actual instance"""
+        if self.actual_instance is None:
+            return None
+
+        to_dict = getattr(self.actual_instance, "to_dict", None)
+        if callable(to_dict):
+            return self.actual_instance.to_dict()
+        else:
+            # primitive type
+            return self.actual_instance
+
+    def to_str(self) -> str:
+        """Returns the string representation of the actual instance"""
+        return pprint.pformat(self.dict())
+
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/one_of_enum_string.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/one_of_enum_string.py
index 04a9440e0d6..ae674028175 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/one_of_enum_string.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/one_of_enum_string.py
@@ -41,9 +41,19 @@ class OneOfEnumString(BaseModel):
     class Config:
         validate_assignment = True
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
-        instance = cls()
+        instance = OneOfEnumString.construct()
         error_messages = []
         match = 0
         # validate data type: EnumString1
@@ -51,19 +61,17 @@ class OneOfEnumString(BaseModel):
             error_messages.append(f"Error! Input type `{type(v)}` is not `EnumString1`")
         else:
             match += 1
-
         # validate data type: EnumString2
         if not isinstance(v, EnumString2):
             error_messages.append(f"Error! Input type `{type(v)}` is not `EnumString2`")
         else:
             match += 1
-
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in OneOfEnumString with oneOf schemas: EnumString1, EnumString2. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -74,7 +82,7 @@ class OneOfEnumString(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> OneOfEnumString:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = OneOfEnumString.construct()
         error_messages = []
         match = 0
 
diff --git a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pig.py b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pig.py
index 398f5ef2f70..dd7887ed432 100644
--- a/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pig.py
+++ b/samples/openapi3/client/petstore/python-nextgen/petstore_api/models/pig.py
@@ -44,9 +44,19 @@ class Pig(BaseModel):
     discriminator_value_class_map = {
     }
 
+    def __init__(self, *args, **kwargs):
+        if args:
+            if len(args) > 1:
+                raise ValueError("If a position argument is used, only 1 is allowed to set `actual_instance`")
+            if kwargs:
+                raise ValueError("If a position argument is used, keyword arguments cannot be used.")
+            super().__init__(actual_instance=args[0])
+        else:
+            super().__init__(**kwargs)
+
     @validator('actual_instance')
     def actual_instance_must_validate_oneof(cls, v):
-        instance = cls()
+        instance = Pig.construct()
         error_messages = []
         match = 0
         # validate data type: BasquePig
@@ -54,19 +64,17 @@ class Pig(BaseModel):
             error_messages.append(f"Error! Input type `{type(v)}` is not `BasquePig`")
         else:
             match += 1
-
         # validate data type: DanishPig
         if not isinstance(v, DanishPig):
             error_messages.append(f"Error! Input type `{type(v)}` is not `DanishPig`")
         else:
             match += 1
-
         if match > 1:
             # more than 1 match
-            raise ValueError("Multiple matches found when deserializing the JSON string into Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("Multiple matches found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         elif match == 0:
             # no match
-            raise ValueError("No match found when deserializing the JSON string into Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
+            raise ValueError("No match found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig. Details: " + ", ".join(error_messages))
         else:
             return v
 
@@ -77,7 +85,7 @@ class Pig(BaseModel):
     @classmethod
     def from_json(cls, json_str: str) -> Pig:
         """Returns the object represented by the json string"""
-        instance = cls()
+        instance = Pig.construct()
         error_messages = []
         match = 0
 
diff --git a/samples/openapi3/client/petstore/python-nextgen/test/test_int_or_string.py b/samples/openapi3/client/petstore/python-nextgen/test/test_int_or_string.py
new file mode 100644
index 00000000000..0de76602cb6
--- /dev/null
+++ b/samples/openapi3/client/petstore/python-nextgen/test/test_int_or_string.py
@@ -0,0 +1,53 @@
+# coding: utf-8
+
+"""
+    OpenAPI Petstore
+
+    This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\  # noqa: E501
+
+    The version of the OpenAPI document: 1.0.0
+    Generated by OpenAPI Generator (https://openapi-generator.tech)
+
+    Do not edit the class manually.
+"""
+
+
+import unittest
+import datetime
+
+import petstore_api
+from petstore_api.models.int_or_string import IntOrString  # noqa: E501
+from petstore_api.rest import ApiException
+
+class TestIntOrString(unittest.TestCase):
+    """IntOrString unit test stubs"""
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def make_instance(self, include_optional):
+        """Test IntOrString
+            include_option is a boolean, when False only required
+            params are included, when True both required and
+            optional params are included """
+        # uncomment below to create an instance of `IntOrString`
+        """
+        model = petstore_api.models.int_or_string.IntOrString()  # noqa: E501
+        if include_optional :
+            return IntOrString(
+            )
+        else :
+            return IntOrString(
+        )
+        """
+
+    def testIntOrString(self):
+        """Test IntOrString"""
+        # inst_req_only = self.make_instance(include_optional=False)
+        # inst_req_and_optional = self.make_instance(include_optional=True)
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py b/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
index 2abca389ea6..3934cd2b41c 100644
--- a/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
+++ b/samples/openapi3/client/petstore/python-nextgen/tests/test_model.py
@@ -142,6 +142,15 @@ class ModelTests(unittest.TestCase):
         except ValueError as e:
             self.assertTrue("'e' is not a valid EnumString1, 'e' is not a valid EnumString" in str(e))
 
+        # test the constructor
+        enum_string1 = petstore_api.EnumString1('a')
+        constructor1 = petstore_api.OneOfEnumString(actual_instance=enum_string1)
+        self.assertEqual(constructor1.actual_instance, enum_string1)
+        constructor2 = petstore_api.OneOfEnumString(enum_string1)
+        self.assertEqual(constructor2.actual_instance, enum_string1)
+        constructor3 = petstore_api.OneOfEnumString()
+        self.assertEqual(constructor3.actual_instance, None)
+
     def test_anyOf_array_of_integers(self):
         # test new Color 
         new_color = petstore_api.AnyOfColor()
@@ -180,9 +189,22 @@ class ModelTests(unittest.TestCase):
 
     def test_oneOf(self):
         # test new Pig
+        bp = petstore_api.BasquePig.from_dict({"className": "BasquePig", "color": "red"})
         new_pig = petstore_api.Pig()
         self.assertEqual("null", new_pig.to_json())
         self.assertEqual(None, new_pig.actual_instance)
+        new_pig2 = petstore_api.Pig(actual_instance=bp)
+        self.assertEqual('{"className": "BasquePig", "color": "red"}', new_pig2.to_json())
+        new_pig3 = petstore_api.Pig(bp)
+        self.assertEqual('{"className": "BasquePig", "color": "red"}', new_pig3.to_json())
+        try:
+            new_pig4 = petstore_api.Pig(bp, actual_instance=bp)
+        except ValueError as e:
+            self.assertTrue("If position argument is used, keyword argument cannot be used.", str(e))
+        try:
+            new_pig5 = petstore_api.Pig(bp, bp)
+        except ValueError as e:
+            self.assertTrue("If position argument is used, only 1 is allowed to set `actual_instance`", str(e))
 
         # test from_json
         json_str = '{"className": "BasquePig", "color": "red"}'
@@ -204,9 +226,7 @@ class ModelTests(unittest.TestCase):
             pig3 = petstore_api.Pig(actual_instance="123")
             self.assertTrue(False)  # this line shouldn't execute
         except ValueError as e:
-            self.assertTrue(
-                "No match found when deserializing the JSON string into Pig with oneOf schemas: "
-                "BasquePig, DanishPig" in str(e))
+            self.assertTrue("No match found when setting `actual_instance` in Pig with oneOf schemas: BasquePig, DanishPig" in str(e))
 
         # failure
         try:
@@ -254,7 +274,7 @@ class ModelTests(unittest.TestCase):
 
         # test init
         basque_pig = p.actual_instance
-        pig2 = petstore_api.Pig(actual_instance=basque_pig)
+        pig2 = petstore_api.AnyOfPig(actual_instance=basque_pig)
         self.assertIsInstance(pig2.actual_instance, petstore_api.BasquePig)
 
         # test failed init
@@ -263,7 +283,7 @@ class ModelTests(unittest.TestCase):
             self.assertTrue(False)  # this line shouldn't execute
         except ValueError as e:
             self.assertTrue(
-                "No match found when deserializing the JSON string into AnyOfPig with anyOf schemas: BasquePig, "
+                "No match found when setting the actual_instance in AnyOfPig with anyOf schemas: BasquePig, "
                 "DanishPig" in str(e))
 
         # failure
@@ -447,3 +467,14 @@ class ModelTests(unittest.TestCase):
         self.assertEqual(a.value, "-efg")
         self.assertEqual(a.name, "MINUS_EFG")
         self.assertEqual(a, "-efg")
+
+    def test_int_or_string_oneof(self):
+        a = petstore_api.IntOrString("-efg")
+        self.assertEqual(a.actual_instance, "-efg")
+        a = petstore_api.IntOrString(100)
+        self.assertEqual(a.actual_instance, 100)
+
+        try:
+            a = petstore_api.IntOrString(1)
+        except ValueError as e:
+            self.assertTrue("ensure this value is greater than or equal to 10" in str(e))