pydantic · samuelcolvin · Oct 31, 2017 · Oct 24, 2017 · Oct 26, 2017 · Oct 28, 2017
diff --git a/docs/index.rst b/docs/index.rst
@@ -171,6 +171,7 @@ Options:
 :allow_extra: whether or not too allow (and include on the model) any extra values in input data (default: ``False``)
 :allow_mutation: whether or not models are faux-immutable, e.g. __setattr__ fails (default: ``True``)
 :fields: extra information on each field, currently just "alias" is allowed (default: ``None``)
+:validate_assignment: whether to perform validation on assignment to attributes or not (default: ``False``)
 
 .. literalinclude:: examples/config.py
 

diff --git a/pydantic/main.py b/pydantic/main.py
@@ -21,6 +21,7 @@ class BaseConfig:
     allow_extra = False
     allow_mutation = True
     fields = {}
+    validate_assignment = False
 
 
 def inherit_config(self_config, parent_config) -> BaseConfig:
@@ -111,7 +112,14 @@ def __setattr__(self, name, value):
             raise ValueError(f'"{self.__class__.__name__}" object has no field "{name}"')
         elif not self.config.allow_mutation:
             raise TypeError(f'"{self.__class__.__name__}" is immutable and does not support item assignment')
-        self.__values__[name] = value
+        elif self.config.validate_assignment:
+            value_, error_ = self.fields[name].validate(value, self.values(exclude={name}))
+            if error_:
+                raise ValidationError({name: error_})
+            else:
+                self.__values__[name] = value_
+        else:
+            self.__values__[name] = value
 
     def __getstate__(self):
         return self.__values__

diff --git a/tests/test_main.py b/tests/test_main.py
@@ -3,7 +3,7 @@
 
 import pytest
 
-from pydantic import BaseModel, ConfigError, NoneBytes, NoneStr, Required, ValidationError
+from pydantic import BaseModel, ConfigError, NoneBytes, NoneStr, Required, ValidationError, constr
 from pydantic.exceptions import pretty_errors
 
 
@@ -337,3 +337,35 @@ class Config:
     with pytest.raises(ValueError) as exc_info:
         m.b = 11
     assert '"TestModel" object has no field "b"' in str(exc_info)
+
+
+class ValidateAssignmentModel(BaseModel):
+    a: int = 2
+    b: constr(min_length=1)
+
+    class Config:
+        validate_assignment = True
+
+
+def test_validating_assignment_pass():
+    p = ValidateAssignmentModel(a=5, b='hello')
+    p.a = 2
+    assert p.a == 2
+    assert p.values() == {'a': 2, 'b': 'hello'}
+    p.b = 'hi'
+    assert p.b == 'hi'
+    assert p.values() == {'a': 2, 'b': 'hi'}
+
+
+def test_validating_assignment_fail():
+    p = ValidateAssignmentModel(a=5, b='hello')
+    with pytest.raises(ValidationError) as exc_info:
+        p.a = 'b'
+    assert """error validating input
+a:
+  invalid literal for int() with base 10: 'b' (error_type=ValueError track=int)""" == str(exc_info.value)
+    with pytest.raises(ValidationError) as exc_info:
+        p.b = ''
+    assert """error validating input
+b:
+  length less than minimum allowed: 1 (error_type=ValueError track=ConstrainedStrValue)""" == str(exc_info.value)