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.travis.yml

@@ -14,7 +14,7 @@ install:
 - pip freeze
 
 script:
-# test without cython but with ujson and email-validator
+# test without cython but with ujson, email-validator, and typing-extensions
 - python -c "import sys, pydantic; print('compiled:', pydantic.compiled); sys.exit(1 if pydantic.compiled else 0)"
 - make test
 
@@ -40,7 +40,7 @@ jobs:
     python: 3.6
     name: 'Cython: 3.6'
     script:
-    # test with cython, ujson and email-validator
+    # test with cython, ujson, email-validator, and typing-extensions
     - make build-cython-trace
     - python -c "import sys, pydantic; print('compiled:', pydantic.compiled); sys.exit(0 if pydantic.compiled else 1)"
     - make test
@@ -50,7 +50,7 @@ jobs:
     python: 3.7
     name: 'Cython: 3.7'
     script:
-    # test with cython, ujson and email-validator
+    # test with cython, ujson, email-validator, and typing-extensions
     - make build-cython-trace
     - python -c "import sys, pydantic; print('compiled:', pydantic.compiled); sys.exit(0 if pydantic.compiled else 1)"
     - make test
@@ -61,17 +61,17 @@ jobs:
     python: 3.6
     name: 'Without Deps 3.6'
     script:
-    # test without cython, ujson and email-validator
-    - pip uninstall -y ujson email-validator
+    # test without cython, ujson, email-validator, and typing-extensions
+    - pip uninstall -y ujson email-validator typing-extensions
     - make test
     env:
     - 'DEPS=no'
   - stage: test
     python: 3.7
     name: 'Without Deps 3.7'
     script:
-    # test without cython, ujson and email-validator
-    - pip uninstall -y ujson email-validator cython
+    # test without cython, ujson, email-validator, and typing-extensions
+    - pip uninstall -y ujson email-validator cython typing-extensions
     - make test
     env:
     - 'DEPS=no'

pydantic/errors.py

@@ -49,7 +49,11 @@ class NoneIsAllowedError(PydanticTypeError):
 
 class WrongConstantError(PydanticValueError):
     code = 'const'
-    msg_template = 'unexpected constant value; permitted values: {allowed!r}'
+
+    def __str__(self):
+        given = self.ctx['given']
+        permitted = ', '.join(repr(v) for v in self.ctx['permitted'])
+        return f'unexpected value; given: {given!r}; permitted: {permitted}'
 
 
 class BytesError(PydanticTypeError):

pydantic/fields.py

@@ -23,14 +23,18 @@
 from .class_validators import Validator, make_generic_validator
 from .error_wrappers import ErrorWrapper
 from .types import Json, JsonWrapper
-from .utils import AnyCallable, AnyType, Callable, ForwardRef, display_as_type, lenient_issubclass, sequence_like
+from .utils import (
+    AnyCallable,
+    AnyType,
+    Callable,
+    ForwardRef,
+    Literal,
+    display_as_type,
+    lenient_issubclass,
+    sequence_like,
+)
 from .validators import NoneType, constant_validator, dict_validator, find_validators
 
-try:
-    from typing_extensions import Literal
-except ImportError:
-    Literal = object()  # type: ignore
-
 Required: Any = Ellipsis
 
 if TYPE_CHECKING:  # pragma: no cover
@@ -192,7 +196,7 @@ def _populate_sub_fields(self) -> None:  # noqa: C901 (ignore complexity)
             return
         if origin is Callable:
             return
-        if origin is Literal:
+        if Literal is not None and origin is Literal:
             return
         if origin is Union:
             types_ = []

pydantic/utils.py

@@ -27,7 +27,7 @@
 try:
     from typing_extensions import Literal
 except ImportError:
-    Literal = object()  # type: ignore
+    Literal = None  # type: ignore
 
 try:
     import email_validator
@@ -291,7 +291,7 @@ def is_callable_type(type_: AnyType) -> bool:
 
 
 def is_literal_type(type_: AnyType) -> bool:
-    return getattr(type_, '__origin__', None) is Literal
+    return Literal is not None and getattr(type_, '__origin__', None)
 
 
 def _check_classvar(v: AnyType) -> bool:

pydantic/validators.py

@@ -149,7 +149,7 @@ def constant_validator(v: 'Any', field: 'Field') -> 'Any':
     Schema.
     """
     if v != field.default:
-        raise errors.WrongConstantError(given=v, allowed=[field.default])
+        raise errors.WrongConstantError(given=v, permitted=[field.default])
 
     return v
 
@@ -344,9 +344,11 @@ def callable_validator(v: Any) -> AnyCallable:
 
 
 def make_literal_validator(allowed_choices: Tuple[Any, ...]) -> Callable[[Any], Any]:
+    allowed_choices_set = set(allowed_choices)
+
     def literal_validator(v: Any) -> Any:
-        if v not in allowed_choices:
-            raise errors.WrongConstantError(allowed=list(allowed_choices))
+        if v not in allowed_choices_set:
+            raise errors.WrongConstantError(given=v, permitted=list(allowed_choices))
         return v
 
     return literal_validator

setup.py

@@ -100,7 +100,7 @@ def extra(self):
     extras_require={
         'ujson': ['ujson>=1.35'],
         'email': ['email-validator>=1.0.3'],
-        'typing_extensions': ['typing_extensions']
+        'typing_extensions': ['typing-extensions>=3.7.2']
     },
     ext_modules=ext_modules,
 )

tests/test_main.py

@@ -391,9 +391,9 @@ class Model(BaseModel):
     assert exc_info.value.errors() == [
         {
             'loc': ('a',),
-            'msg': 'unexpected constant value; permitted values: [3]',
+            'msg': 'unexpected value; given: 4; permitted: 3',
             'type': 'value_error.const',
-            'ctx': {'given': 4, 'allowed': [3]},
+            'ctx': {'given': 4, 'permitted': [3]},
         }
     ]
 

tests/test_types.py

@@ -52,6 +52,7 @@
 except ImportError:
     typing_extensions = None
 
+
 class ConBytesModel(BaseModel):
     v: conbytes(max_length=10) = b'foobar'
 
@@ -1575,25 +1576,25 @@ class Model(BaseModel):
 @pytest.mark.skipif(not typing_extensions, reason='typing_extensions not installed')
 def test_literal_single():
     class Model(BaseModel):
-        a: Literal['a']
+        a: typing_extensions.Literal['a']
 
     Model(a='a')
     with pytest.raises(ValidationError) as exc_info:
         Model(a='b')
     assert exc_info.value.errors() == [
         {
             'loc': ('a',),
-            'msg': 'unexpected constant value; permitted values: [\'a\']',
+            'msg': 'unexpected value; given: \'b\'; permitted: \'a\'',
             'type': 'value_error.const',
-            'ctx': {'allowed': ['a']},
+            'ctx': {'given': 'b', 'permitted': ['a']},
         }
     ]
 
 
 @pytest.mark.skipif(not typing_extensions, reason='typing_extensions not installed')
 def test_literal_multiple():
     class Model(BaseModel):
-        a_or_b: Literal['a', 'b']
+        a_or_b: typing_extensions.Literal['a', 'b']
 
     Model(a_or_b='a')
     Model(a_or_b='b')
@@ -1602,8 +1603,8 @@ class Model(BaseModel):
     assert exc_info.value.errors() == [
         {
             'loc': ('a_or_b',),
-            'msg': 'unexpected constant value; permitted values: [\'a\', \'b\']',
+            'msg': 'unexpected value; given: \'c\'; permitted: \'a\', \'b\'',
             'type': 'value_error.const',
-            'ctx': {'allowed': ['a', 'b']},
+            'ctx': {'given': 'c', 'permitted': ['a', 'b']},
         }
     ]