Generate a hash function when allow_mutation is False

The generated hash is the hash of the dict_value
of the BaseModel.__dict__ attribute.

closes pydantic#1880

docs/usage/model_config.md

@@ -24,7 +24,7 @@ Options:
   and `'allow'` will assign the attributes to the model.
 
 **`allow_mutation`**
-: whether or not models are faux-immutable, i.e. whether `__setattr__` is allowed (default: `True`)
+: whether or not models are faux-immutable, i.e. whether `__setattr__` is allowed (default: `True`). `False` will make the model immutable and hashable i.e. a default `__hash__` function is generated for you, but you can still overwrite it.
 
 **`use_enum_values`**
 : whether to populate models with the `value` property of enums, rather than the raw enum.

pydantic/main.py

@@ -313,6 +313,7 @@ def __new__(mcs, name, bases, namespace, **kwargs):  # noqa C901
             '__schema_cache__': {},
             '__json_encoder__': staticmethod(json_encoder),
             '__custom_root_type__': _custom_root_type,
+            '__hash__': (lambda self: hash(self.__dict__.values())) if not config.allow_mutation else None,
             **{n: v for n, v in namespace.items() if n not in fields},
         }
 

tests/test_main.py

@@ -355,6 +355,19 @@ class Config:
     assert '"TestModel" object has no field "b"' in exc_info.value.args[0]
 
 
+def test_mutable_are_not_hashable():
+    class TestModel(BaseModel):
+        a: int = 10
+
+        class Config:
+            allow_mutation = True
+
+    m = TestModel()
+    with pytest.raises(TypeError) as exc_info:
+        hash(m)
+    assert "unhashable type: 'TestModel'" in exc_info.value.args[0]
+
+
 def test_immutability():
     class TestModel(BaseModel):
         a: int = 10
@@ -373,6 +386,18 @@ class Config:
     assert '"TestModel" object has no field "b"' in exc_info.value.args[0]
 
 
+def test_immutable_are_hashable():
+    class TestModel(BaseModel):
+        a: int = 10
+
+        class Config:
+            allow_mutation = False
+
+    m = TestModel()
+    assert m.__hash__ is not None
+    assert isinstance(hash(m), int)
+
+
 def test_const_validates():
     class Model(BaseModel):
         a: int = Field(3, const=True)