better support for allow_mutation

pydantic/fields.py

@@ -106,7 +106,8 @@ class FieldInfo(Representation):
         'extra',
     )
 
-    __field_constraints__ = {  # field constraints with the default value
+    # field constraints with the default value, it's also used in update_from_config below
+    __field_constraints__ = {
         'min_length': None,
         'max_length': None,
         'regex': None,
@@ -164,7 +165,7 @@ def update_from_config(self, from_config: Dict[str, Any]) -> None:
                 # attr_name is not an attribute of FieldInfo, it should therefore be added to extra
                 self.extra[attr_name] = value
             else:
-                if current_value is None:
+                if current_value is self.__field_constraints__.get(attr_name, None):
                     setattr(self, attr_name, value)
 
     def _validate(self) -> None:

tests/test_edge_cases.py

@@ -1814,13 +1814,28 @@ class Config:
 
 
 def test_config_field_info_allow_mutation():
-    """
-    allow_mutation cannot be customised via Config.field because it has a default which is not None
-    """
     class Foo(BaseModel):
         a: str = Field(...)
 
         class Config:
-            fields = {'a': {'allow_mutation': False}}
+            validate_assignment = True
 
     assert Foo.__fields__['a'].field_info.allow_mutation is True
+
+    f = Foo(a='x')
+    f.a = 'y'
+    assert f.dict() == {'a': 'y'}
+
+    class Bar(BaseModel):
+        a: str = Field(...)
+
+        class Config:
+            fields = {'a': {'allow_mutation': False}}
+            validate_assignment = True
+
+    assert Bar.__fields__['a'].field_info.allow_mutation is False
+
+    b = Bar(a='x')
+    with pytest.raises(TypeError):
+        b.a = 'y'
+    assert b.dict() == {'a': 'x'}