From 30ee8e2203267d3ea86ca2a51472738d98d05cd5 Mon Sep 17 00:00:00 2001
From: Davis Kirkendall <davis.e.kirkendall@gmail.com>
Date: Fri, 9 Oct 2020 14:25:24 +0200
Subject: [PATCH] Force fields.Undefined to be a singleton objectIn various
 places of the code, we compare directly to `fields.Undefined`since we assume
 it to be constant.When new models get created however, the object is
 deepcopied andis no longer identical with the original object.We therefore
 add `__copy__` and `__deepcopy__` methods to ensurethat the copied objects
 are actually the same original object. (#1981)

---
 changes/1981-daviskirk.md |  1 +
 pydantic/fields.py        |  8 ++++++++
 tests/test_generics.py    | 20 ++++++++++++++++++++
 tests/test_utils.py       |  7 ++++++-
 4 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 changes/1981-daviskirk.md

diff --git a/changes/1981-daviskirk.md b/changes/1981-daviskirk.md
new file mode 100644
index 0000000..ef52cb1
--- /dev/null
+++ b/changes/1981-daviskirk.md
@@ -0,0 +1 @@
+Force `fields.Undefined` to be a singleton object, fixing inherited generic model schemas
diff --git a/pydantic/fields.py b/pydantic/fields.py
index f9114a1..d251237 100644
--- a/pydantic/fields.py
+++ b/pydantic/fields.py
@@ -40,11 +40,19 @@ from .validators import constant_validator, dict_validator, find_validators, val
 
 Required: Any = Ellipsis
 
+T = TypeVar('T')
+
 
 class UndefinedType:
     def __repr__(self) -> str:
         return 'PydanticUndefined'
 
+    def __copy__(self: T) -> T:
+        return self
+
+    def __deepcopy__(self: T, _: Any) -> T:
+        return self
+
 
 Undefined = UndefinedType()
 
diff --git a/tests/test_generics.py b/tests/test_generics.py
index b1821e7..452fe3b 100644
--- a/tests/test_generics.py
+++ b/tests/test_generics.py
@@ -410,6 +410,26 @@ def test_custom_schema():
     assert schema['properties']['a'].get('description') == 'Custom'
 
 
+@skip_36
+def test_child_schema():
+
+    T = TypeVar('T')
+
+    class Model(GenericModel, Generic[T]):
+        a: T
+
+    class Child(Model[T], Generic[T]):
+        pass
+
+    schema = Child[int].schema()
+    assert schema == {
+        'title': 'Child[int]',
+        'type': 'object',
+        'properties': {'a': {'title': 'A', 'type': 'integer'}},
+        'required': ['a'],
+    }
+
+
 @skip_36
 def test_custom_generic_naming():
     T = TypeVar('T')
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 935038f..989176c 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,7 +1,7 @@
 import os
 import re
 import string
-from copy import deepcopy
+from copy import copy, deepcopy
 from distutils.version import StrictVersion
 from enum import Enum
 from typing import NewType, Union
@@ -296,6 +296,11 @@ def test_undefined_repr():
     assert repr(Undefined) == 'PydanticUndefined'
 
 
+def test_undefined_copy():
+    copy(Undefined) is Undefined
+    deepcopy(Undefined) is Undefined
+
+
 def test_get_model():
     class A(BaseModel):
         a: str